The Arcon-IRAF Interface

A Preliminary User's Guide for Direct Imaging

S. Heathcote
31 March 1994

1. Introduction

This is a preliminary guide to using the IRAF based user interface with CTIO's new CCD controller, Arcon, to obtain direct imaging data. It should be read in conjunction with the user's manual for the instrument you are using:

cfccd: "CTIO 0.9-m, 1.5-m, 4.0-m Telescopes, Cassegrain Focus CCD Direct using Arcon"

pfcdd: "CTIO 4.0-m Telescope, Prime Focus CCD Direct using Arcon"

nfccd: "CCD Observations at The Schmidt Telescope using Arcon".

Both the hardware and software for Arcon are still under development and are consequently a little buggy. This manual is pretty buggy too. The mountain Observer Support staff, have, as yet, little experience with the system and thus will be less able to help with problems than is usual, and will often have to refer to the "experts" in La Serena. Please bear with us during this transition phase. But please also be diligent about reporting any problems and bugs you may encounter and please feel free to make any suggestions or criticisms you may have.

The IRAF based user interface allows observing commands to be sent to Arcon from within the IRAF cl. This results in a single uniform user interface for data taking and data reduction and allows the Arcon user to employ features of the cl such as the parameter mechanism and the history editor. It also allows advanced users to write cl scripts which freely mix data acquisition and data reduction operations. It is nonetheless simple enough, that users need very little knowledge of IRAF in order to obtain their data. As far as possible the user interface is modeled on the ICE software in use at KPNO; the present manual also owes more than a little to the KPNO ICE manual. However, users should be aware that the two systems are NOT identical, so that parameter lists may differ and data acquisition scripts written for ICE will require some modification.

2 Logging in and Logging Out

2.1 Computers and Peripherals

Figure 1 shows a block diagram of the various computers and peripherals which make up the data acquisition system at a typical CTIO telescope. You should find a version of this diagram for your specific telescope, showing the names of the actual computers and peripherals involved, posted in the console room. At least some of the key pieces of hardware are labeled; if you have trouble locating, for instance, the exabyte drive, at your telescope ask observer support.

In the console room of each telescope, except the Schmidt, you will find two Sun Computers. One of these, a SPARC-station 10/41, is the Arcon data acquisition computer. You must log on to this machine in order to use Arcon and, in general, will also want to use it for all your data reduction. The other machine is an older VME-bus based Sun 4/330; currently peripherals such as the 9-track and exabyte tape drives and laser printer are attached to this second Sun. If more than one observer is present at the telescope, or if you find a single Sun screen to be too restrictive, you may wish to use this second machine for your reductions. However, if you do so we recommend you treat it as an "intelligent terminal" from which you log on to the data acquisition machine. Accessing images across the network imposes a very stiff penalty in the form of increased I/O overheads when reducing your data.

2.2 Starting From Scratch

When you first log on to the data acquisition computer using your visitor account, you will be asked which windowing system you prefer % both Sunview and Openwindows are available % as follows:

- What windowing system do you want?

0 -- No windowing system

1 -- SunView

2 -- OpenWindows

Enter selection (1): 1

Pick whichever you are most comfortable with, or select SunView if you know neither (in this case you may want to refer to Appendix C). The system remembers your answer, so you will only be asked to make this choice the first time you log on to a given machine. You will then be asked if you want to start up the data acquisition program, or just to reduce data as follows:

- What type of IRAF setup do you want?

1 -- IRAF for data reduction

2 -- IRAF with Arcon

Enter selection (1): 2

If you select "2", as in the example, your chosen window system will start up and several windows will open so that your screen should look something like Figure 1; the function of these various windows will be explained in Section 2.2. An automatic start-up procedure will also be run which initializes the controller hardware and software. This takes some time; when it finishes the message "Array Controller ready for user commands ..." will appear along with a beep. Wait until you see this message and hear the beep before continuing.

At present there are a few additional steps which you must carry out manually by typing thefollowing in the specified windows (as a reminder, the necessary commands are shown in the parentheses in the title bars of most of these windows):

*In the window labeled Arcon STATUS: type astro1_up

*In the very small green window: type countdown

*The IRAF cl will automatically start up in the blue window labeled "IRAF, Acquisition (arcon......)". You must first load the arcon package, and then the specific package for the instrument you are using % cfccd for the Cassegrain focus CCD (4.0-m, 1.5-m and 0.9-m telescopes); pfccd for the prime focus CCD (4.0-m) and nfccd for the Newtonian focus CCD (Curtis Schmidt). To do so proceed as follows:

cl> arcon

astronomer. contributed. dtools. laboratory. pfccd.

cfccd. csccd. fpccd. nfccd. quad.

ar> cfccd

Connecting to the Nexus...

abort disconnect movie preview tchange zero

connect focus object resume teloffset

dark instrpars@ observe sflat telpars@

detpars@ instrument obspars@ shutter wheel1@

dflat more pause stop wheel2@


Loading the instrument package automatically establishes the connection to the controller, as indicated by the "Connecting to the Nexus...." message hidden amongst the package menus. Exiting from the package with bye or (usually inadvertently!) ctrl-z breaks the connection.

You are now ready to start taking data. However, before you begin doing so in earnest, you should check that everything is set up and working properly: (i) Check you know where your home directory is ( show home ); (ii) Check where your imdir is ( show imdir ), that it really exists ( cd imdir ) and that you have the necessary privileges ( copy home$ imdir$junk ; delete imdir$junk ); (iii) check there is plenty of disk space ( disks ); (iv) carry out the basic tests of the operation of the detector and controller described in the instrument manual. If there are problems with any of this seek help from observer support.

A further word about image directories is in order at this point. Each of the data acquisition machines has several 2-3Gb capacity disks for bulk data storage. On each of these there will be a directory, /iraf, containing sub-directories for each visitor account. At the start of your run all these disks will be cleared. However, especially if you are using the Tek2048 CCD, you will have no trouble filling the available space in a few nights of typical observing. From time to time you should use the command disks to see how much space is free on each disk. Arcon uses the value of the IRAF environment variable imdir in order to decide where to write the pixel files for newly created images.

You can change the value of this variable "on the fly" with the command

cl> reset imdir = /ua42/iraf/v19/

Note that the trailing "/" character is necessary. This variable will be restored to its original default value when you log out of IRAF. Also the values in the acquisition and reduction windows are independent and must be set separately. To change imdir permanently and insure the two windows use the same value, you must edit your .cshrc file as follows,

cl> edit home$.cshrc



# Image pixel directory definition. This is the ONLY place where this

# variable should be defined and changed.

setenv imdir "/ua42/iraf/$USER/" <----- Change this line as required

You must then log out of IRAF and back in again, for the change to take effect.

2.3 A Guided Tour of the Windows

Once you have completed the initialization procedure, your screen should appear similar to Figure 1. The key windows for taking data with Arcon , identified by the names given in their title bars, are as follows:

Arcon CONSOLE % As its name implies this window serves as a console for Arcon . While you are observing you will see many messages appear here, only some of which will be repeated in the IRAF acquisition window. This window takes up a lot of screen space so you will probably prefer to close it. But don't quit from it. In the event that something goes wrong, the diagnostic messages appearing in this window may tell you (or at least us) what happened.

Arcon STATUS % This very important window gives several lines of information summarizing the status of the controller, the instrument, and any ongoing exposures (see Figure 2). The first line shows what the controller is currently doing. if you have just brought the system up, it should read "CONTINUOUSLY_ERASING" indicating that the CCD is idle and is continuously running the erase cycle; if it doesn't, chances are something went wrong in the initialization and you should seek help. During exposures this line should read "INTEGRATING", and should change to "READING" as the CCD is read out. Other messages which may occur will be described later as appropriate. Just below the status line are counters showing the number of seconds left in the current exposure, the number of exposures left in the current sequence, and, during read out, the number of buffers of data successfully transferred to the Sun. Also shown are parameters of the current exposure such as the title, picture name etc. Finally the bottom three lines of the status window show, during readout, the values of various statistics of the image. These numbers are generated on the fly by the real time display program (see Section 5) and are used in setting the display look up table. You, however, may find this information useful when adjusting exposure times during sequences of sky flats, etc.

COUNTDOWN %This is the very small window with the very large font. It provides a copy of the exposure time counter for the visually handicapped.

IRAF Acquisition % This blue colored window is one of two running the IRAF cl. We recommendyou type all data taking commands here, so that your data taking and data reduction activities are well separated and don't interfere with one another.

IRAF Reductions % This window (colored a dirty brick red) is the other IRAF window. We suggest that normal IRAF commands, used to examine or reduce your data are entered in this window.

IRAF Display Window % depending on your choice of window environment there will be either an

"SAO Image" (Open Windows) or "Imtool" (Sun View) window which will be used when displaying your images from IRAF.

2.4 Shutting Down and Logging Out

Before you log out, for whatever reason, you must first stop the various processes, related to the controller running on the Sun. If you don't do this they will continue running and cause all sorts of entertainment for you or whoever else next tries to bring up the system. To do so first, in the blue "IRAF, Acquisition" window, break the connection to the controller by either typing disconnect or by exiting from the instrument package by typing bye . Then in the "Arcon CONSOLE" window type:

arsh> arsh stop

ctioa1% kleenex

This last command should clean-up all the processes related to Arcon ; unfortunately it doesn't always. To see if it worked type:

% NexUp

No match.

No match.

No match.

If you get "No match." three times in a row, as above, all is well. If instead you see something like

ctioa1% NexUp

root 2227 0.0 3.0 2664 844 p6 S 17:32 0:05 muxnex

arcon 2229 0.0 .6 172 160 p2 S 17:32 0:00 arsh -c 0 -e 20

arcon 2228 0.0 .0 144 0 p2 IW 17:32 0:00 arsh -c 0-e20



-rw--rw- 1 arcon 0 Jul 13 17:32 /tmp/xpim2229.1

-rw-rw-rw- 1 arcon 0 Jul 13 17:33 /tmp/xpim2245.1

then there are some leftover processes which you must kill by hand as follows,

ctioa1% kill -9 2227 2229 2228

In this command the list of numbers after the -9 are the pid's of the leftover processes as displayed by NexUp . The files with names like /tmp/xpim2229.1 are the spool files used by Arcon to transfer data to the Sun. If any of these are owned by you and have a size other than zero (the number just before that date), then they may contain your missing data! See section 3.1.2 for information on how toretrieve this. It is common to see a few zero length files as shown in the example, but these can be ignored.

Now you can exit from the windowing system and log out completely, by moving the mouse to a blank area of the screen, then hold down the right mouse button, and select " exit " from the menu which will appear.

2.5 Warm Starts

Rather more often than we would like, currently about once a night, something or other happens which causes the system to hang requiring that the software be reloaded.

Before doing so it is worth testing to see if the problem is confined to the IRAF interface layer, by proceeding as follows:

co> flpr

co> disconnect

Disconnecting from Nexus ....

co> connect

Connecting to Nexus ....

co> flpr

This re-establishes and initializes the connection to the Arcon and, for good measure, flushes out any brain-damaged executables locked into the IRAF process cache. Having done this, test to see if the problem has gone away by taking a "zero" frame.

If this fails, or if the status window or real time display have stopped functioning, then the problem is probably in Arcon itself and it is best to reload the software from scratch. This takes very little time. First follow the steps in Section 2.3 for shutting down the system, but do not perform the very last step of exiting from the window system. Instead restart the Arcon software as follows:

*In Arcon CONSOLE type arcon_visitor , then wait until you see the message "Array Controller ready for user commands ..." and hear a beep before continuing.

*In Arcon STATUS type astro1_up or simply " !! "

*In green font window type countdown or simply " !! "

*In the blue IRAF window type connect or reload the instrument package ( cfccd , pfccd , nfccd ).

*Also in the IRAF window type setdetector force+ to ensure that all parameters of the detector match the values you have selected in the detpars pset (see Section 4.3)

2.6 If All Else Fails

Just once in a while a problem will occur which just refuses to go away even when you reload the Arcon software. This may be due to a hardware failure. However, it may also be that the Sun has got irremediably confused, in which case the rather drastic step of rebooting it may be called for. Should this prove necessary contact observer support. They can show you how to reboot the Sun safely, cleanly, and without having to know the superuser password.

3.Taking Data

3.1 Observe - the only command you really need to know

All data taking can be done by using a single command: observe . This command takes one or more ccd exposures, as in the following example:

cf> observe

Exposure type (|zero|dark|object|dflat|sflat|focus) (zero): object

Number of exposures to take (1:) (1):

Exposure time (0.:) (0.): 1000

Filter in wheel one (dia):

picture mechanism filter1 move 8 dia

Old Position: 4

New position 8

Filter in wheel two (r): sii

picture mechanism filter2 move 4 r

Old Position: 3

New position 4

Title of picture (bias 16/07/93): HH80/81 [SII]

cf> observe sequence 1 "object" 1073 "HH80/81 [SII]" 1000.

4 amplifiers in total

Image obj107 written to disk

Observation finished...

You will be prompted for all the information required which includes:

exposure type: can be "zero" (sometimes referred to as bias), "dark", "object", "dflat", "sflat" or "focus". Note that when selecting from a list of options like this you may enter any unique abbreviation. Focus exposures are somewhat special; we will return to them in section 6.1.

number of exposures to take: a sequence of this number of pictures, all having the same parameters, will be taken.

exposure time: is in seconds, and will not be requested in the case of exposures of type "zero" for which it is 0.0 by definition.

filter in wheel one/two: the required filter(s). Note that filters can be specified by name, rather than by their position in the filter wheel; the way this translation is set up will be described in section 4.2. The filter wheels will be moved to the selected positions before continuing. The filters used will also be noted in the image header. This information is not requested for exposures of types "zero" and "dark" for which the filter is irrelevant. It is also (by default) not requested at the Schmidt telescope where the filter slide must currently be moved manually. Warning: If the filter fails to move to the correct position a warning message will be printed. However, currently the observe command cannot trap such errors in order to take corrective action, and the filter position recorded in the image header is the requested position, not the actual position. If such an error occurs you should abort the observe command by typing ctrl-c ; having done so you should issue the IRAF command flpr to clean up any residual mess. You are then ready to run the observe command again.

picture title will be included as the title in the IRAF image header.

Note that in each parameter query you will be supplied with a default value, which you can accept by simply hitting ; these values are just the previous entries.

If you make a mistake, or change your mind, you can abort the command during the parameter entry stage by typing ctrl-c ; having done so you should always enter the command flpr, as a means of warding off the evil eye. Once the exposure has started it can be terminated using the abort command (see Section 3.2).

As soon as you enter the title, the CCD will be prepared, and then the exposure will begin. The first line in the status window will change from "CONTINUOUSLY_ERASING" to "INTEGRATING" and the status window will also show parameters of the exposure such as the picture title. A counter in the status window, and more legibly the countdown window will begin counting down the time remaining in the exposure. At present these counters are not exactly synchronized with the internal clock in the controller so for very long exposures they may indicate that a few seconds remain when the exposure is in fact complete. A further counter in the status window will count up the dark time % the time since the CCD stopped being erased. This may be slightly greater than the elapsed exposure time due to overheads in the controller, and will of course be very much longer if you paused the exposure.

Note that the observe command terminates as soon as the exposure starts and you can enter other commands in the IRAF acquisition window. While you could type any IRAF command you like, we suggest you keep this window free for entering the special exposure control commands described in Section 3.2

When the exposure finishes the CCD will be read out. The first line in the status window will change to "READING" and the "buffers read" counter will indicate the number of buffers of datasuccessfully transferred to the Sun. The data is initially written in the controllers internal format to a spool file on /tmp, but it is automatically converted into an IRAF image within a few seconds of the exposure finishing. The message "Image ...... written to disk" appears as soon as this process is complete and shows you the name of the new IRAF image. This name is derived from the exposure type by appending a running number (see section 4.1 for how to adjust this number) which is automatically incremented after each exposure. The image header will be in the current directory (at the time the observe command was issued) and the pixel file will be located in your imdir .

During readout the image will also be displayed on the real time display (see Section 5). This occurs independently from and in parallel with the transfer of the data to disk on the Sun. You need not wait for the real time display to finish before starting another exposure.

If you requested that observe take only a single exposure, the message "observation finished ....." will appear in the IRAF interface window as soon as the readout is complete; things are then ready for you to start another exposure. If, instead, you requested a sequence of several pictures, the next exposure will start automatically. You may immediately examine or process the resulting image even though the sequence is not complete. Note that the "pictures remaining" counter in the status window shows how many exposures remain in the sequence. Once the final picture has been readout the message "sequence finished ......" will appear in the IRAF interface window. Should you miss the end of sequence or end of exposure message, note that the CCD is idle and things are ready for you to initiate new exposures, whenever the top line of the status display reads "continuously_erasing".

3.1.2 What to do if Your Picture Doesn't Show Up % Recover

Sometimes Arcon will successfully transfer your data to the spool file on /tmp but the picread program which converts this to an IRAF image will fail. Usually there is an error message, but you should be suspicious that this has happened if the exposure completes normally, but you can't find the output image. The command recover will assist you to retrieve your valuable images in such cases. It searches /tmp for any spool files owned by you and for any of these that are complete will show the FITS header and ask if you want to recover the image or not. If you reply yes then picread will be run to convert the xpim file to an IRAF image. A single spool file occasionally contains more than one image. In this case recover will list how many images are present, but will only show you the header of the first. All images will be recovered if you tell the program to go ahead. Spool files are not removed from /tmp until successfully converted by picread. However, we suggest you run recover immediately if you encounter this problem, so that you don't forget later, and so that /tmp doesn't get filled to overflowing with unprocessed spool files.

Sometimes a failure occurs during the transfer of the data from the controller to the Sun. In this case a partial spool file results which cannot be used to resurrect the data. Recover will give you the option of deleting any such files, and you should do so to avoid filling /tmp with debris.

But, don't give up hope yet! There is a 2.5M-byte internal buffer in the controller itself, so you can still rescue your data, provided the entire image fits in this space; a Tek1024 CCD image fits, a full unbinned Tek2048 image does not. To attempt this, reload the Arcon software by following the"warm start instructions" (See Section 2.5). Wait for the message

Array Controller ready for user commands ....

then type the following in the Arcon console window:

arsh> macro NoData

Now take a picture as you normally would. The picture will be read from

the memory buffer, NOT from the CCD. Once you have done this successfully


arsh> macro DetectorData

(again in the Arcon console window) to restore things to normal.

3.2 Exposure Control Commands

The following commands can be used to modify an ongoing exposure:

pause -Pause the exposure e.g. while waiting out passing clouds.

resume -Resume a paused exposure.

tchange -Change exposure time. You will be prompted for the amount by which to change the exposure which may be positive or negative. If used during a sequence the duration of the present exposure and all subsequent exposures is changed.

stop -Stop the exposure early, read out the CCD and save the data to disk. If used during a sequence, the sequence is also terminated.

abort -Abort the exposure. The CCD is not read out and any data collected during the exposure is irrevocably lost. If used during a sequence, the sequence is also terminated.

Note that, currently, abort does not work properly if issued while the CCD is being read out; the exposure is terminated correctly, but the CCD controller hangs up and has to be re-initialized. An important consequence of this is that abort cannot be used to terminate a sequence of zero-length, or very short exposures. You can use stop to do this, in which case you will have to wait while the current exposure reads out. For very large chips like the Tek2048, it may be faster to just re-initialize the controller (see Section 2.4).

3.3 Other Commands For Taking Data

In addition to observe, there are specific commands to take one or more pictures of each type:

dark -Take one or more exposures of type dark

dflat -Take one or more exposures of type dome flat

object -Take one or more exposures of type object

sflat -Take one or more exposures of type sky flat

zero -Take one or more exposures of type bias

Except, of course, for the exposure type these commands take the same parameters (and prompt for them in the same order) as does observe . Apart from saving you entering that one extra parameter, use of these commands allows one to set default parameter values, and also select which parameters are prompted for according to picture type.

Another useful command is:

more -Take one or more exposures exactly like the previous one

The more command is slightly unusual in the way it prompts for parameters (it is patterned after commands like directory and help ). If you type

cf> more

you will not be prompted for the number of exposures (as one might expect) but rather a single exposure will be taken (which more often than not is what you actually wanted to do). Conversely

cf> more 10

will take ten more exposures.

A very useful task when you want to take calibration exposures, or exposures of the same object, in various filters is doobs . For example,

cf> doobs

Exposure type (|object|dflat|sflat|): dflat

Number of exposures to take in each filter (1:) (1): 10

list of filters in wheel1: dia,cb

list of filters in wheel2: u,b,v,r,i

List of exposure times: 60,10,5

The following pictures will be taken:

Pictures Filter1 Filter2 Exposure

100 - 109 dia u 60

110 - 119 cb b 10

120 - 129 cb v 5

130 - 139 cb r 5

140 - 149 cb i 5

Title for pictures: Dome flats 30 sept 93

will take sequences of 10 dome flats each in u plus dia (60s exposures), b plus cb (10s) and v, r, and i also with the colour balance filter.

3.4 And Two That Take No Data - Preview and Movie

Two commands which may be useful when centering your field on the detector or for establishing rough focus, are:

preview -Take a CCD exposure and display it on the real time display but does not write any data to disk.

movie -Loop continuously taking and displaying preview exposures until terminated by stop .

Both commands prompt for a single parameter the exposure time; since the readout time in quad mode is about 12 seconds for the Tek1024 CCD some degree of real-time feedback can be obtained using movie with exposure times of a few seconds. In the near future preview and movie will optionally use special waveforms which reduce the readout time at the expense of increased noise &/or reduced spatial resolution.

4 The Parameter Files

As with ICE many of the nitty-gritty details of taking your data are hidden from your immediate view in four parameter files:

obspars This contains several parameters which you, the astronomer, can use to tailor the behavior of observe to your liking. It also contains a number of other infrequently changed parameters, in particular those needed when taking focus frames.

instrpars This parameter file contains information relating to the instrument being used % the shutter filter bolt, etc. in the case of direct imaging applications. Because this hardware differs somewhat from telescope-to-telescope, so does instrpars .

detpars This parameter file controls the fundamentals of how the CCD is readout % binning, gain, regions of interest, etc.

telpars Except at the Curtis Schmidt all CTIO telescopes are run by a control program which is interrogated by Arcon, at the start of each exposure, in order to obtain information such as the time, telescope coordinates, etc. for inclusion in the image header. This parameter file currently does nothing and has only been retained to maintain compatibility with ICE.

You should review these parameter files, and may want to change some values, at the start of your run, but will probably leave them alone thereafter.

These parameter files can be listed by using the lpar command, eg.,

cf> lpar obspars

and may be edited using the parameter editor, epar , or by simply typing the name of the parameter set e.g.,

cf> epar obspars -OR-

cf> obspars

To change a value, in either case, move the cursor up and down with the arrow keys until you are on the correct line and then simply type the new value followed by . When done editing the parameter file type cntrl-z

4.1 obspars

The parameter file obspars contains four distinct groups of parameters as shown bellow:


Image Reduction and Analysis Facility

PACKAGE= cfccd

TASK= obspars

ccdtype=zeroExposure type

npics=1 Number of exposures to take

picture=1 Picture number of first exposure

exposure=0. Exposure time

title=Title of picture

(autopic=yes)Generate picture number automatically ?


(setfilt=both)Query and set filters?

(filtype=instrument)Type of filters to use


(setfocu=yes)Query and set focus?

(foctype=telescope)Type of focus to use

(tempera=0.)Telescope temperature

(basefoc=INDEF)Focus base value

(reftemp=0.)Telescope temperature for base focus value

(tfrcoef=)Coefficients of Temperature Focus relationship


nfexpo=7 Number of focus exposures

(refis= middle)Reference is first, middle or last exposure?

freferen=5000 focus value

fdelta=30 Focus increment

(focmode=auto)Focus mode

(shtype=detector)Shift type

(fra_off=0.)Focus offset in RA

(fdec_of=30.)Focus offset in Declination

(nrvrows=30)Number of rows to reverse shift



The first group of parameters are used for all exposures. It is not necessary to set the values of most of these, since they are prompted for as needed. The values appearing in obspars are simply the values entered the last time observe was run. The parameter autopicnum determines whether observe will prompt you for the running picture number, picture , which forms part of the name of your images on disk. The value of picture is always incremented after each exposure. If autopicnum =yes (the default) the automatically derived value will always be used and you will not be prompted. If autopicnum =no you will be prompted for a new value of picture for every exposure, the automatically derived value being supplied as the default. In either case you can reset the sequence by just changing the value of picture in obspars. Note that picture will get out of step if you abort an exposure or sequence; the value used will be the one which would have been appropriate if the exposure or sequence had completed normally.

The second group of parameters controls the positioning of the filter wheels at the start of each exposure. For the pfccd and cfccd setfilt can be "none" to move neither wheel, "one" to move only wheel one, "two" to move only wheel two, or both to move both wheels. The nfccd system has only one manually controlled filter slide. In this case setfilt can be "no" or "yes". If you set it to "yes" you will be prompted for the filter position when appropriate and the value you enter will be saved in the image header, but you must still run up stairs to physically move the filter. Note that the filter position is never requested for exposures of type "zero" or "dark". The parameter filtype should always read "instrument" and you will find that you cannot change it to anything else.

The third group of parameters control the setting of the telescope focus for each exposure. If setfocus = yes, the focus will be adjusted whenever you take an exposure of type "object", "flat" or "sflat"; if setfocus = no this action is disabled. As described in section 6.2 the software can help you by keeping track of the changes to the focus needed when you change from filter to filter and as the temperature changes during the night. At the 4.0-m telescope the focus is under computer control and you should accordingly set foctype = telescope. At the start of each exposure, observe will then set the focus to the value appropriate for the filter in use, without any further action on your part. At the remaining telescopes you must change the focus manually using the buttons on the handpaddle. In these cases set foctype = manual; whenever the focus needs to be changed you will be prompted for the new focus, the value calculated by the focus correction algorithm being supplied as the default. Once you have manually set the focus, enter a new value if necessary, and hit ; the value you supply will be recorded in the image header. The remaining parameters in this group are used by the automatic focus correction algorithm; see Section 6.2 for full information.

The final group of parameters are used for focus exposures and will be described in Section 6.1.

4.2 instrpars

Because of slight differences between the shutter/filter assemblies used with the cfccd, pfccd and nfccd, different instrpars files are employed with each. That for the cfccd appears as follows,


Image Reduction and Analysis Facility

PACKAGE= cfccd

TASK= instrpars

instrfoc=300 Telescope focus

filter1=u Filter in wheel one

filter2=1 Filter in wheel two

(wheel1=)Filter info. pset for wheel one

(wheel2=)Filter info. pset for wheel two

(instrna=cfccd)Instrument name



Firstly, make absolutely sure that instrname = "cfccd" if you are using the cfccd (logically enough it should be pfccd or nfccd when using these instruments).

The first three parameters are used when setting the telescope focus and filter positions at the start of an exposure if this is enabled. You need not supply values for them since these are prompted for if needed. Note that these parameters are ignored if you disable focus setting or filter movement.

The parameters wheel1 and wheel2 are themselves parameter sets which are used to store information about the filters installed in each wheel. This allows filters to be referred to by name instead of having to remember which filter is in which slot, in which wheel. They also contain the corrections to the telescope focus which must be made when each filter is used. When editing instrpars you can access these parameter sets by using the arrow keys to move down to the correct line and then type :e . Alternatively you can edit these parameter files by typing e.g.

cf> epar wheel1 -OR-

cf> wheel1

When you have finished type ctrl-z . If you used :e you will return to editing instrpars. Make certain that no extraneous characters appear in the value field of wheel1 or wheel2 after exiting; if they do just enter a null string ("") as the value. Whichever method you choose you will see something like this,


Image Reduction and Analysis Facility

PACKAGE= cfccd

TASK= wheel1

(id1=u)Short identifier for filter in position 1

(name1=U Hamilton #1)Full name of filter in position 1

(focus1=-40)Focus value for filter 1

(id2=b)Short identifier for filter in position 2

(name2=B Harris set #3)Full name of filter in position 2

(focus2=10)Focus value for filter 2

(id3=clear)Short identifier for filter in position 3

(name3=)Full name of filter in position 3

(focus3=0)Focus value for filter 3

each group of three parameters corresponds to one slot in the filter wheel. The parameter id# is the short identification for the filter that you should type when prompted for a filter position by observe ; It should be short, should be unique amongst the filters in the wheel and must not contain any white space or other weird characters. One exception to the uniqueness requirement is that any empty positions may be given the same name e.g. "clear" (but not a null "" or blank " " name ) provided you don't care which one is actually used. The value of id# is also what will be recorded in the image header. The parameter name# provides for a longer filter designation including, for instance, the filter set, however, this is currently not used for anything. Finally, focus# , should contain the focus correction to be used for that filter (see section 6.2). Normally observer support will edit wheel1 and wheel2 appropriately when they install the filters.

For the nfccd instrpars is the same as for the cfccd, except that there is only a single filter bolt and hence it only contains the parameters filter1 and wheel1 . For the pfccd instrpars is the same as for the cfccd except that it contains additional entries for controlling the other motors in the LF-PFCCD as described in Section 4.2.2.

4.2.1 Setting Up the Instrument Without Taking an Exposure % Instrument

If you want to say, move the filter wheel, without taking a CCD exposure, use instrument . This task will prompt you for positions for each motor and move it to that position, just as observe does. However, instrument obeys the setfilters and setfocus parameters in obspars and hence will only move those motors you have enabled. More often than not it is precisely those motors you have disabled in obspars that you want to move. For instance you may normally have filter2 disabled ( setfilters=one ) , but want to switch between the diaphragm and color balance position before taking you dome flats and switch back at the end. If you want to override setfilters for a single invocation of instrument proceed as follows:

cf> instrument setfilters=both

Filter in wheel one (u): v

Filter in wheel two (cb): dia

The setfocus parameter can be overridden on the command line in the same way.

4.2.3 Control of the LF-PFCCD (pfccd only)

In addition to the filter wheels, the LF-PFCCD hardware includes motors for adjustment of the dewar tilt and rotation. It is also possible to select the filter in front of the guide TV and adjust the camera focus under remote control. Finally, the guide TV is mounted on a motorized scan-table intended for use in drift scanning. Although drift scanning is not yet implemented, you may want to reposition the scan table when searching for guide stars in sparsely populated fields. Accordingly, instrpars for the pfccd contains the following additional entries:


(setscan= no) Set TV scantable position

tvscanpo= 500 TV scan table position

(settv = no) change TV filter and focus

tvfilter= clear TV Filter position

(tvfbolt= ) TV filter info. pset

tvfocus = -500 TV focus position

(setdewa= no) Set dewar tilt and rotation

xdewarti= 0 Dewar tilt in X

ydewarti= 0 Dewar tilt in Y

dewarrot= 0 Dewar rotation

These extra motors are divided into three logically related groups. The boolean parameters setscan , settv and setdewar can be used to separately select movement (and prompting) for the motors in each group. Normally each of these parameters is set to no in which case the motors in the corresponding group will not be moved. On the first night of your run observer support will help you adjust the dewar tilt and rotation if necessary. If you want to set, say the tvfilter and tvfocus , on every exposure then set settv=yes . If you only want to move the motors from time to time this can be done with the instrument command (see Section 4.2.1), overriding the setdewar , settv and setscan parameters on the command line as required. In addition, the following special commands are available:

setdewar -Set dewar tilt and rotation.

settv -Set tv filter and focus.

setscantable -Set scan table position.

tvfocus -Set tv focus

Setdewar and settv simply prompt for and set a new position for the motors they control. Setscantable and tvfocus are interactive programs which allow you to iteratively adjust the scantable position and tvfocus respectively.

For example to set the scantable position:

pf> setscan

TV scan table position

(-10000:10000) (10000): 0

Interactively set scantable position:

c --> move to center of range

b --> move to bottom end of range

t --> move to top end of range

u --> move up one step

d --> move down one step

l --> set step size to large (1000) [default]

m --> set step size to medium (100)

s --> set step size to small (10)

h or ? --> show this help screen

q --> quit

> q

The program first prompts for an initial position, and then enters the interactive loop. Typing any of the single keys listed produces the corresponding action. The field of the CCD TV camera covers slightly more than one third of the range of motion of the scan table. Thus using the b , c and t keysin succession you can search the entire accessible range. Once you have located a guide star you can move it closer to the center of the field using the u and d keys. Finally type q to exit.

Tvfocus behaves in an analogous fashion. Note that telfocus and tvfocus are both measured in microns, however, currently run in the opposite direction (we plan to fix that). Thus, if you increase the telescope focus you must decrease tvfocus by the same amount in order to keep the camera parfocal with the telescope. The software will shortly be modified to automatically adjust tvfocus in step with the telescope focus.

4.3 detpars and the setdetector command

The detpars pset holds parameters related to the CCD detector itself such as the section of the chip to be readout, the binning factors and so on. The detpars file is slightly different for each detector, because each has its own personality. At the start of your run, and before you begin customising any parameters in detpars you should do the following

cf> unlearn detpars odetpars

(N.B. there is no coma). This ensures that the private copy of the pset stored in your uparm directory is appropriate for the detector you are using. You must repeat this command if you change detectors part way through your run.

As with any pset you can edit the contents of detpars by typing epar detpars or just detpars. However, any changes you make this way will not be immediately downloaded to the controller. To make your changes effective you must run the command setdetector

cf> setdetector

this will first bring up the parameter editor allowing you to review and make any additional changes to detpars . Once you are happy exit with cntrl-z and your changes will be sent down to the controller. If you change your mind or make a mistake, and want to exit without changing anything, type ctrl-c. For some parameters only a single command is sent to the controller to change the value stored internaly. Others require that the waveforms which control the clocking of the CCD are recompiled and downloaded into the controller, a process which takes some time and generates several dozen lines of output. Eventualy when all is done the package prompt (e.g. cf>) will be output.

Normaly setdetector only downloads new parameter values to the controller if these have changed since the last time it was run. When the controller software is reloaded (by running arcon_visitor see Sections 2.2 and 2.5) the internal variables in the controller are reset to their initial values. The information stored by setdetector about the parameter values in the controller is then incorrect. A similar confusion will arise if setdetector has been run from a different account (e.g. by observer support). To force setdetector to update all parameters ensuring that everything is in sync. type

cf> setdetector force+

IMPORTANT : If you are not using the default values for all the parameters in detpars then you must do this each time you reload the controller software .

The complete detpars pset for the Tek1024 CCD is shown below:


Image Reduction and Analysis Facility

PACKAGE = pfccd

TASK = detpars

(preflas= 0.) Preflash time (seconds)

(xsum = 1) pixels summed in X direction

(ysum = 1) pixels summed in Y direction

(xstart = 1) Start of ROI in X

(ystart = 1) Start of ROI in Y

(xsize = 1024) Size of ROI in X

(ysize = 1024) Size of ROI in Y

(extend = separate) Method of extending ROI to include overscan

(noversc= 18) Number of overscan pixels (physical)

(xskip1 = 3) X pixels to skip at start of overscan

(xskip2 = 0) X pixels to skip at end of overscan

(xtrim1 = 0) X pixels to trim at start of data

(xtrim2 = 0) X pixels to trim at end of data

(ytrim1 = 0) Y pixels to trim at start of data

(ytrim2 = 0) Y pixels to trim at end of data

(amplifi= quad) Readout amplifiers to be used

(pixsize= 24.) Pixel size in microns

(nxpixel= 1024) Detector size in X

(nypixel= 1024) Detector size in Y

(detname= Tek1024-2) Detector identification

the function of the various groups of parameters are described in the following sub-sections.

4.3.1 Selecting which amplifier(s) to use % amplifiers. Also a word or two on multiple readout images.

Many of CTIO's CCD's have more than one, typically four, working amplifiers. A major advance achieved with Arcon is the ability to read out the CCD using more than one of these amplifiers in parallel, leading to substantially faster read-out. Once properly reduced, such data is virtualy indistinguishable from that obtained when reading out through only a single amplifier. However, raw multi-readout images do look decidedly strange. Firstly, each read-out will typically have a slightly different, zero level, gain, and readout noise, and may differ slightly in its departures from perfect linearity. As a result both zero frames and uniformly illuminated exposures will show a characteristic chequer board pattern, the sections of the data read through each amplifier having different levels. Secondly, there will be a separate overscan strip, used to monitor the zero level, for each readout. The location of these overscan strips within the image depends on which amplifiers are selected as illustrated in Figure ?.

The combination of amplifiers to be used is set by the parameter amplifiers . The available choices are

quad %Use all four amplifiers. The resulting image is split horizontaly and verticaly into four equal quadrants the overscan regions forming a vertical stripe down the centre of the picture.

upper, lower %Use the upper or lower pair of amplifiers. The resulting image is split in two horizontaly, the overscan strips running side by side down the centre of the picture.

right, left %Use the right hand or left hand pair of amplifiers. The resulting image is split in two verticaly. The overscan strips lie one above the other along the side of the picture furthest from the amplifiers in use ( i.e. at the left if amplifiers = right and conversely).

ll, lr, ul, ur %Use single amplifier in the lower left, lower right, upper left and upper right corner respectively. The overscan strip runs verticaly down the edge of the picture farthest from the amplifier (i.e. down the right edge for ll and ul and the left edge for lr and ur ).

Theoreticaly there are another two cases, using the pairs of amplifiers at opposite corners of the chip, but we haven't had reason to impliment these yet. Not all choices are available with every CCD; enter ? as the value of amplifiers when editing detpars to see a list of the ones appropriate for the particular chip you are using.

The Thomson (Arcon 2.1 and Arcon 3.1) and Tek1024-2 (Arcon 3.2) chips each have four working amplifiers and you should in general choose quad for these detectors. Due to a broken bond wire, only the upper two amplifiers on the Tek 2048 (Arcon 3.3) can be used and one should in general select upper in this case. If the very small residual differences between the properties of the data in the segments that remain after data reduction would be a problem for your program you might wish to select a single amplifier. Consult Alistair Walker for advice on this and on which amplifier to use if you do.

4.3.2 preflash (pfccd only % the preflash hardware is not yet hooked up for the other cases)

This parameters sets the number of seconds for which the preflash leds should be illuminated at the start of each exposure. Currently none of the CCDs used with Arcon require a preflash, although the preflash leds are sometimes used when performing diagnostic tests on the CCD. You should in general just check that the preflash is set to 0.0 and leave it that way.

4.3.3 Binning % xsum, ysum

These parameters specify the number of adjacent detector pixels to be combined in the X and Y directions respectively. You should consult the instrument manuals for information on the pixel size on the sky and advice as to what binning factors are appropriate for your program.

4.3.4 Region of Interest % xstart, ystart, xsize, ysize, extend

These parameters specify the rectangular sub-section of the CCD which is to be readout (see Figure ?). For most direct imaging applications you will want to read out the entire chip and so should leave these parameters set to their default values. However, if you don't need the full field size, you can speed up the readout and save space on disk by only reading data within a specified region of interest (ROI). The coordinates of the bottom left hand corner of the region you want are given by xstart and ystart , while its size is given by xsize and ysize . These values must be specified in physical pixels on the CCD, not binned pixels. The parameter extend controls how the region is to be extended to include the overscan strip. The default value separate causes any pixels between the trailing edge of the ROI and the overscan strip to be skipped % this is usualy what you want. If you select extend the interveening pixels will be read out and included in the output image. If you select none there will be no overscan strip at all; this can be useful for test exposures but don't do this if you hope ever to reduced your data.

The actual portion of the CCD that will be read out will include the entire region you specify, however additional pixels may be included for a number of reasons. Firstly, if you are binning, the coordinates of the ROI will be automaticaly adjusted to be a whole number of binned pixels. Secondly, if the overscan is included by extend ing extra pixels will be read between the end of the region and the start of the overscan. Thirdly for some CCDs it is necessary to read a few extra gaurd pixels around the edges of the region to avoid contamination of the data by electronic transients. Finaly, if you are using more than one readout amplifier, additional pixels and/or "phantom regions" will be readout for reasons of symmetry (see Figure ?). The trimsection information recorded in the image header will be set so that all these extra pixels are excised when the image is reduced.

4.3.5 The overscan parameters % noverscan, xskip1, xskip2

The parameter noverscan sets the number of overscan pixels to be read for use in determining the DC-bias level of the CCD. This parameter should be given in binned pixels. There is an overscan strip for each amplifier used so that if the CCD is split in the horizontal direction there will be twice this number of overscan pixels in total. The parameters xskip1 and xskip2 are used in setting the biassection information in the image header (see Figure ??); xskip1 pixels will be ignored at the leading edge of the overscan strip and xskip2 pixels at the trailing edge. The default values will normaly be what you want. In any case you will have the opportunity to overide the header values when you actualy reduce the data.

4.3.6 The trim parameters % xtrim1, xtrim2, ytrim1, ytrim2

These parameters control the trimsection information written in the image header (see Figure ??) which is used when processing the data. These values must be given in physical CCD pixels, not binned pixels. The default values will normaly be what you want. In any case you will have the opportunityto overide the header values when you actualy reduce the data.

4.3.7 Some informational parameters % detname, nxpixels, nypixels, pixsize

The final group of parameters in detpars are purely for information and you will be unable to change their values. These are intended to be read by programs which need to know information about the characteristics of the detector, but currently this information is not being used anywhere.

4.3.8 Setting the Gain

In Arcon the parameter which determines the number of e-/ADU is the slope time of the "double-correlated-sampler" % usually erroneously referred to (by astronomers) as the gain. The readout noise and the readout time are also both affected by the choice of this parameter. Currently, this quantity is not adjusted via and entry in detpars, but rather by running a separate program gainchange (at least at CTIO, programmers do what astronomers tell them to do, even when its wrong), as follows:

cf> gainchange

Gain setting (0 for list) (0): 2

*** Regenerating waveforms ***

csh /pxp/run/macro/wdl Tek1K_1 -I..

WDL revision 2.18


*** Suspending the sequencer ***

*** Reloading new waveforms ***

You will be prompted for the gain setting, which must currently be one of a pre-defined list of values. The CCD readout waveforms are then edited, recompiled, and down loaded into the controller. This all takes about 12s. To get the list of acceptable gain settings, and also to find out the corresponding number of e-/ADU, readout noise, and readout time, enter a gain setting of 0, as follows:

as> gainchange

Gain setting (0 for list) (0):

dcsT Delay RON ADU/e- readout_time (quad)

(us)(clk) (e-) (seconds)


---- ---- -------------- -------------- -------------------

1: 11 2

2: 15 4

3: 20 2

4: 39 4

5: 80 2


* *** Select gain setting from the first column ***

For advice on what gain is right for you, refer to the appropriate users manual, or consult observersupport.

4.4 telpars

This parameter set is not used at this time.

5. The Real Time Display

Arcon includes a real-time display which automatically shows each picture as it is being read out on a separate Sun-style monitor next to the data Acquisition computer. This occurs independently from, and in parallel with, the transfer of the data to disk on the Sun and does not slow down this process. You need not wait for the real time display to finish before starting another exposure. The real time display offers a number of convenient features:

*Display of the picture begins substantially before all the data has been transferred to the Sun and converted into an IRAF image.

*Various picture statistics are accumulated on the fly and are used to optimally map the 16-bit CCD data into the 256 grey-levels shown by the display. These statistics are also shown in the status window (see Section x.x) and may be useful when estimating exposure levels for sky flats.

*Saturated pixels (data value % 65535) are shown in red.

*Quad readout pictures are automatically overscan subtracted and trimmed for the display.

*The cardinal directions are labeled on the display monitor. You will soon be able to specify any rotation and flipping necessary to have the display match your finding charts.

These features mean that you can always see the last picture taken to verify that the picture looks reasonable, is the field you want, and that no important objects are saturated.

The display itself takes place in two stages. A first fast pass keeps up with the readout but the data is shown at slightly reduced spatial resolution. A second, slower full resolution pass is then performed once the entire picture is available. Since the ideal mapping from 16 to 8 bits can't be known until the readout is finished the second pass may modify the look up table unless you specify otherwise (see Section 5.2).

5.1 Changing the look of the displayed picture

You can change the way the mapping from 16 to 8 bits is performed and also whether the picture is shown in normal, reversed or false color mode. You can also show the pixels above and below the mapped range in green and blue, respectively. The commands to control these settings are temporarily contained in the contributed package (a sub-package of arcon ):

lut -Change the look up table.


video = "reverse" (normal|reverse|falsecolor)

Show stars as white ("normal"), black ("reverse") or use false-color ("falsecolor").

colors = "nocolor"(nocolor|3color)

Disable ("nocolor") or enable ("3color") the use of colors to mark pixels outside the mapping range. When enabled pixels below the range are shown in blue and those above the range are shown in green. Note this does not affect the marking of saturated pixels which are always shown in red.

map -Defines the way the 16-bit CCD data is to be mapped to the 8-bit display. The default parameter values ( algorithm = "mode", low = 0.2, high = 2.0) work well for normal star fields. Try low = 0.2 and high = 1.3 to bring out nebulosity.


algorithm = "mode" (mode|stdev|minmax|constant|show)

The mapping algorithm to use. Options are:

mode -Map range is specified number of standard deviations below mode and above mean.

stdev -Like mode, except bottom of range is specified number of standard deviations below mean.

minmax - Map range has specified percentages of pixels above picture minimum below picture maximum. (e.g. 0.5 and 1 would have 0.5% of the pixels below the map range and 1% above the map range.)

constant -Set map range to specified values.

show - Display current picture with specified values, but do not change the map parameters for subsequent picture displays.

low = 0.2

Low value for mapping algorithm. This is expressed as a number of standard deviations for the mode and stdev algorithms, the percentage of pixels below the bound for minmax and the absolute level in ADU's for constant and show .

high = 2

High value for mapping algorithm. This is expressed as a number of standard deviations for the mode and stdev algorithms, the percentage of pixels above the bound for minmax and the absolute level in ADU's for constant and show .

5.2 Controlling when display and remapping take place

The 16-to-8-bit mapping is performed based on image statistics accumulated as the picture is read out. However, it is necessary to wait until enough of the picture data is available, so that these statistics are meaningful, before using them for the mapping. Until then the mapping from the previous exposure is used. This works well when sequential exposures are of the same type and comparable duration. Inthe future the system will be made smarter so that the preliminary mapping used is based on information from the previous exposure of the same type scaled according to exposure time.

The remap task (also temporarily in the contributed package) controls how soon remapping is performed:

remap -Set remapping options for real time display.


delaydisplay = no

Do not start displaying until percent of the picture has been read. This prevents starting to display pictures with the mapping from the previous picture, but delays any feedback on the current picture.

percent = 43

The percentage of the picture which must be read out before the mapping for the first display pass is changed from that used for the previous picture.

redraw = yes

Perform final high resolution display pass.

With the default value of percent repainting of the screen with the new look-up table will finish bef ore CCD read-out completes. A smaller value will result in earlier re-mapping, but an increased risk that incomplete sampling of the field will result in a poor choice of look-up table. In some cases such as when an isolated bright star or a compact star cluster are placed near the center of a quad readout detector, the correct mapping cannot be known until the end of the readout. When the new look-up table is poorly chosen, the display may be repainted three times; once at the start using the look-up table from the previous exposure, then again based on statistics accumulated from the top and bottom edges of the image, and finally when the readout is complete based on statistics from the entire picture. This final remapping is done at the same time that the image is displayed at full spatial resolution. Some people find this repeated repainting of the screen confusing. If you decide you do not want any remapping, set percent = 100. and delaydisplay = yes. Display of the picture will then not start until the read-out is complete, at which time you will see a fast, medium spatial-resolution display, followed by a second slower pass at full resolution, but there will be no change in the mapping.

6.1 Getting the Telescope in Focus ........

As a starting point the telescope focus can be set to the "nominal" value for the filters you are using, which can be obtained from observer support. They will also advise you on an appropriate focus step to use when taking focus frames and the direction in which the focus should be set. If necessary this rough focus can be refined by using the movie command (see section 3.3) to continuously take short exposures, displaying them on the real time display, while you slowly adjust the focus.

Once rough focus has been established, the accurate focus can be determined by taking a seriesof snapshots of a star, each at a different focus value. To minimize the overheads in this process all the exposures are taken on a single CCD frame successive images being displaced from one another. Such focus frames are obtained by simply running the observe command, specifying "focus" as the exposure type. As for any other exposures the controlling parameters will be found in the obspars pset:


nfexpo=7 Number of focus exposures

(refis= middle)Reference is first, middle or last exposure?

freferen=5000focus value

fdelta=30 Focus increment

(focmode=auto)Focus mode

(shtype=detector)Shift type

(fra_off=0.)Focus offset in RA

(fdec_of=30.)Focus offset in Declination

(nrvrows=30)Number of rows to reverse shift

Before taking your first focus frame you should review the values of the hidden parameters (those shown in brackets above); all the remaining parameters will be prompted for when you run observe and so need not be set.

The parameter refis selects whether you will be prompted for the focus value corresponding to the first, middle, or last exposure in the focus sequence. Experiment has shown that at 4:00am many astronomers (well at least one) are incapable of performing the complex algebra required to determine the starting focus value needed to bracket a given central focus.

At the 4.0-m, where the telescope focus is under computer control, you should set focmode=auto

to have the program automatically step through the focus sequence. At the 1.5-m and 0.9-m telescopes where the focus is set manualy by pushing buttons on the handpaddle you should use focmode=manual . Finaly at the Schmidt telescope where the focus is adjusted by running upstairs to the platform and turning a knob, set focmode=delay . For Schmidt observers there are two additional

parameters; initial_delay specifies the time it takes you to travel from the computer terminal in the pleasently warm control room to the arctic cold platform and make the initial focus adjustment; delay specifies the time taken to make subsequent focus steps. Be warned that the default values were selected by a postdoc, familiar with the telescope, and who competes in triathlons.

There are two methods of producing the displacement between successive images in focus frames. These are selected with the shtype parameter:

* shtype = detector % the displacement is produced by shifting charge on the CCD detector. This is the preferred (and default) method. The amplitude of the shift is set with the parameter nrvrows although the default value will usually be appropriate.

* shtype = telescope % a traditional focus frame is taken in which the telescope is offset a short distance between each exposure. The size of the offset is determined by the parameters fra_offset and fdec_offset which specify the step size in RA and Dec, respectively. Even if you use thismethod, you can still place your chosen focus star near the center of the detector; the telescope will automatically be offset by the correct amount before starting the focus sequence and will be returned to the original position at the end.

Whichever shifting method you use the program inserts a double space after the first exposure for easy identification. Also note that if shtype=detector and you are using quad readout, stars in the upper and lower halves of the chip will shift in opposite directions ! (the motion is always from the center towards the closer edge).

The following example illustrates the sequence of events when taking a focus frame:

cf> observe

Exposure type (|zero|dark|object|dflat|sflat|focus) (focus): focus

Exposure time (0.:) (10.):

Number of focus exposures (7):

Middle exposure (number 4) of sequence to have focus value (4900): 5000

Focus increment (30):

Filter in wheel one (b): v

Filter in wheel two (clear):

Title of picture (B focus frame): V focus frame

As usual you will be prompted for the parameters needed to complete the exposure. These include, in addition to the parameters used for normal exposures, the number of focus exposures, the starting focus value, and the size of the focus increment.

At the 4.0-m this is all that is required. Once you enter the picture title, the program will automatically obtain the focus frame. That is it will set the focus to each of the specified values in turn, take an exposure and then either shift the charge or move the telescope.

At the 1.5-m and 0.9-m telescopes you still have a little work to do. Once you enter the picture title the program will prompt you with the focus value for the first exposure. You must manually set the focus to the requested value using the buttons on the hand paddle (be careful to move in the correct direction). The telescope will also offset to the starting position if shtype=telescope . Once you have set the focus, and the telescope has finished moving, hit (the focus value for each exposure is recorded in the image header, so you may wish to enter the actual focus value if you overshoot by a significant amount). The first focus exposure will be taken, the charge will be shifted (or the telescope will be offset), and you will be prompted with the next focus value. Again, you must set the focus (and wait for the telescope to finish moving) before hitting . The sequence of events will look something like:

Enter focus value


abort - abort sequence

last - stop sequence after next exposure

nexposures - change number of exposures

telescope focus (5000):

Set focus and wait for telescope to stop moving.

Then enter new focus value

telescope focus (5030):


telescope focus (5180):

At the Schmidt telescope you have a lot more work to do! After you enter the exposure title you will see the following:

Program will wait 60s before taking the 1st exposure

and 15s between subsequent exposures

The initial focus value should be

telescope focus (5000):

There will be no more prompts!!!

Start running!

..... focus = 5000<-- you shouldn't be there to see this message!

..... focus = 5030

Be sure you are ready to run upstairs before you hit the on the initial focus prompt. Once you do so the program will wait initial_delay seconds before opening the shutter fro the first focus exposure. When the exposure is over the shutter will close with a clearly audible clunk. This is your prompt to step the focus to the next position in the sequence. The program will wait delay seconds to allow you to do this before taking the next exposure and so on.

Note that while the controller is waiting between successive focus exposures the status screen will read "MIDDLE_OF_FOCUS". Occasionally something will go wrong part way through a focus sequence and you will receive a message "command completed with error", followed by the IRAF prompt. The status screen continues to show the message "MIDDLE_OF_FOCUS" instead of returning to the normal "CONTINUOUSLY_ERASING" and the controller will refuse to start any new exposures. You can clear this condition by typing:

cf> send observe fabort

You will then be able to start the focus sequence again from the beginning , although any accumulated focus data will be lost. You will also need to recenter the focus star on the detector if shtype = telescope .

The completed focus frame can be examined using the standard IRAF task imexamine in order to determine the optimal focus.

6.2 ..... And Keeping it That Way

In general the optimal focus differs somewhat from filter to filter, especially for the narrow band interference filters. The focus also changes during the night, since it appears to be a function of temperature and possibly telescope position. If your program calls for several filters, or even if it doesn't, you can waste an awful lot of time taking focus frames. We have tried to alleviate thisaggravating situation by designing a couple of features into the software.

Firstly, it is possible to store a table of focus corrections to be applied to the focus value when using each filter. Since the difference in focus between any two filters is constant, it is then only necessary to determine the filter-to-filter offsets for the set of filters you are using once at the start of the run. Indeed if you are using one of the more commonly used filter sets the corrections may be known already; ask your friendly observer support person. To use this facility first focus the telescope for one filter, which you will use as reference. Enter the focus value for this filter as obspars.basefocus . If you are unlucky and the offsets are not known for your other filters you must also determine the focus for each of these in turn, and note down the difference in best focus value between each filter and the reference (in the sense filter - reference). These values can then be stored in the appropriate filter wheel pset wheel1 or wheel2 as shown in the following example,

cf> wheel1


Image Reduction and Analysis Facility

PACKAGE= cfccd

TASK= wheel1

(id1=u)Short identifier for filter in position 1

(name1=U Hamilton #1)Full name of filter in position 1

(focus1=-40)Focus value for filter 1

(id2=b)Short identifier for filter in position 2

(name2=B Harris set #3)Full name of filter in position 2

(focus2=10)Focus value for filter 2

(id3=v)Short identifier for filter in position 3

(name3=V Harris set #3)Full name of filter in position 3

(focus3=0)Focus value for filter 3

(id4=r)Short identifier for filter in position 3

(name4=R Harris set #3)Full name of filter in position 3

(focus4=20)Focus value for filter 3

In this example the V filter has been chosen as the reference. Best focus for the U filter is 40 units lower, and that for B and R 10 and 20 units higher respectively. Once set up you can adjust for focus changes during the night, or from night to night, by redetermining the focus for the reference filter (or any other filter) and changing the value of obspars.basefocus accordingly.

Secondly, the focus value may be corrected for changes in the telescope temperature. To do this you must set the value of obspars.reftemp to the telescope temperature at the time the reference focus value obspars.basefocus was determined. You must also supply the coefficients of the temperature-focus relationship which is taken to be a power-series polynomial of the form

%F = a1 × %T + a2 × %T2 + .......

The coefficients are specified by the parameter obspars.tfrcoefs a white space delimited string having the form "n a1 a2 ..... an", where n is the number of coefficients. For instance obspars.tfrcoefs = "1 10.0" would specify a simple linear dependance with the focus increasing by ten units for every one degree increase in temperature. Entering a null string ("", the initial value) for this parameter disables calculation of the temperature correction. While from time-to-time it has been demonstrated that such a simple temperature correction works, the values of temperature coefficients are not currently known for any of the telescopes. It is hoped that making this feature available will encourage visitors and staff to determine the coefficients. These should be recorded on tablets of stone, &/or sent to S. Heathcote or A. Walker, if possible together with the empirical data from which they were derived. Once this has been set up you can compensate for the temperature dependance of the focus by setting the parameter obspars.temperature to the current telescope temperature and adjusting it periodically as this changes during the night.

Finally, to make use of these elegant mechanisms, you must set obspars.setfocus =yes. Subsequently, for each object or flatfield exposure the following will be displayed:

Filter in wheel one (u): u

Filter in wheel two (clear):

Focus calculation:

base focus = 5000

correction for filters = -40

correction for temperature = -180(Delta T = -18.0 )

final focus = 4780

At the 4.0-m the focus will be automatically set to the calculated value. At the smaller telescopes you will be prompted to enter the focus value, with the result of the automatic calculation being given as the default; you should then set the focus to this value using the handpaddle buttons (or by running upstairs at the Schmidt) hitting when you are ready.

Note that if obspars.basefocus =INDEF (the initial value), no corrections to the focus (neither for temperature nor filters) will be calculated, but you will still be prompted for a focus value; this may be useful if you just want to record the current focus value in the image header. The prompt can be disabled altogether by setting obspars.setfocus =no.

7. Data Reduction

Appendix A:Software Summary - The Only Page(s) You Really Need to read.

Loading packages etc.

arcon-Load main Arcon package

cfccd-Load instrument specific package for Cassegrain focus CCD direct

pfccd-Load instrument specific package for Prime focus CCD direct

nfccd-Load instrument specific package for Newtonian focus CCD direct

connect-Make connection to detector controller (automatically done by cfccd etc)

disconnect-Break connection to detector controller (automatically done on bye from cfccd)

Data taking commands

observe-Take one or more exposures prompting for type

dark-Take one or more dark exposures

dflat-Take one or more dome flat exposures

object-Take one or more object exposures

sflat-Take one or more sky flat exposures

zero-Take one or more zero exposures

focus-Take a focus frame

more-Take more exposures of the previous type

preview-Take an exposure preview frame. Data is shown on real time display but not saved to disk.

movie-Continuously take preview exposures until stopped with abort

Exposure Control Commands

abort-Stop exposure and do not readout detector

stop-Stop exposure and readout detector

tchange-Change exposure time

pause-Pause current exposure

resume-resume paused exposure

Parameter Sets

obspars-Observing parameters

detpars-Detector parameters

telpars-Telescope parameters

instrpars-Instrument parameters

wheel1-filter info parameters for wheel1

wheel2-filter info parameters for wheel1

Instrument Control

instrument-Generic instrument control bases on instrpars pset

setdewar-Set dewar tilt and rotation. pfccd only

settv-Set tv filter and focus. pfccd only

setscantable-Set scan table position. pfccd only

tvfocus-Set tv focus. pfccd only

Updated on April 10, 2024, 11:03 am