Mosaic Overview: At least Read This!

General Characteristics:

Arrays : 8 2048x4096 SITe CCDs; thinned , science grade
Image size : 8192 x 8192 @ 16 bits, plus header, overscan:~135 Mbytes
Pixels size : 15-um (0.27"/pixel at the 4-m)
Read-noise : ~6 -8e-, very low fixed pattern noise
DQE : 86% peak at 6000Å (average for 8 CCDs; see also Figure 3.1.2)
Dark-current : ~ <2 e-/pixel/hr; but charge injection introduces a ramp of about 10-12 electrons; needs to be taken out with a summed zero exposure.
Read-out time : 2.5 minutes in 8 channel mode; expected ~100sec in 16-channel mode
CCD Gaps : ~0.7 mm (~50 pixels) in rows; ~0.5 mm (~35 pixels) in columns
Cosmetics : Good to excellent: typically 2 bad columns per CCD, but many 2-8 pixel areas of 5% variation (flatfield completely) and some very large areas of 10% variations that all flatfield to <0.5%
Filters : 5.75"x5.75"; almost parfocal, 15 filters now available (UBVRI; see Sec 2.6)
Saturation : Typically, linear to 0.1% to 70,000 e-
Gain : ~2 e-/ADU



4-Meter Parameters:


Count Rates : At UBVRI=20th mag: U: 35; B:330; V: 340; R: 410; I: 225 e-/sec
FOV : 36'x36', XIMTOOL Orientation: North- right, East-up
Scale : 0.27"/pixel at center; decreases quadratically by 6.5% out to corners
Image quality : PSF reasonably constant across the FOV, but ~6% larger in linear scale at the corners. Focussing on a star approx halfway to the corner of the array gives a slightly more even image quality over the whole field, however the focal plane is not tilted wrt to the CCDs, and focus at center is only around 30 microns less than the outer parts of the array. to corner.
Artifacts : No obvious filter ghosts; electronic crosstalk between pairs of CCDS at ~0.1% and can be removed accurately when reducing the data
Typical focus : 14500 at ~15C ; change with temperature: -135 units per degree C
ADC: : Atmospheric Dispersion Correction: Use ADC in "ENABLED" mode via the operators's TCS console. Provides continuous trackinge except for zenith distances less than 8 degrees, where it returns to its nominal position


Data Acquisition

Acquisition commands are given on computer named ctioa0

Analysis commands are given on computer named ctio4m


All the Commands That Are Likely To Be Needed

Setup Commands (ctioa0)

enable enable or disable filters/motors/some TCS functions
motor init initialize the filters, including the name assignments
motor stat list of status of motors, including filter assignments and position
setdet change detector parameters (careful!)
instrpars change instrument parameters (careful!)
telpars change telescope parameters (careful!)
obspars change observing parameters

Observing Commands (ctioa0)

observe take one or more exposures prompting for the exposure type
doobs a script which takes flats/objects for a list of filters
mosdither takes N (typically 3-5) dithered images in a single filter to fill the gaps in array. Dither patterns are stored (by default) in /u4m2/mosaic/dithers
more take more exposures just like the last one
test take a test exposure. The output image, test, is overwritten each time
object take one or more object exposures
zero take one or more zero (bias) exposures
dark take one or more dark exposures
dflat take one or more dome flats
pflat take one or more projector flats
sflat take one or more sky flats
focus take a focus frame
snap take a binned exposure for quick readout
settemp pokes serrulier truss temperature into instrpars


Exposure Control Commands


pause pause exposure (e.g. clouds) [do not ABORT or STOP from within pause!]
resume resume a paused exposure [then ABORT or STOP if necessary!]
tchange increase or decrease the exposure time
stop stop an exposure (and sequence of exposures) reading out the detector
abort abort an exposure (or sequence) discarding the data
pictitle change the title of the picture


Quick-look and Taping Commands (ctio4m)

mscdisplay display an entire mosaic frame
mscexamine general tool for examining images
mscwfits write mosaic frames to tape in multi-extension FITS format


Caution :

Tape your data as you go>; DLT-7000 (~250 images/tape), DDS3(~80 images/tape), and Exabyte drives (~35 images/tape) are available. Write time: ~55 seconds/image to DLT, ~90seconds/image to DDS3,~3 minutes/image to Exabyte. You are expected to be off the computer by noon of your last day!

You can buy DLT, DDS3 and Exabyte tapes on Tololo, but bringing your own is cheaper.


Calibration data:

Take dome flats and/or twilight flats. Dome flats may show slopes across the Mosaic with amplitude of a few percent, twilights are better (1%) although we are not sure if an "artifact" in the south-west corner has gone away. The long read time makes taking twilight flats a real challenge, don't expect to get more than 6 or 7 at a time. For really flat images you'll need to flatten using dark-sky frames combined and filtered to remove stars. You may be able to use your object frames or fields (preferably with not too many bright stars) taken for the purpose. Remember bright stars bleed east-west, and you should jog at least 30 arcsec and preferably more.

Take dark exposures of similar length to your science exposures, note that the general dark level is zero and hot pixels are usually non-linear, so this is really only for the truly paranoic.

Take zeroes (i.e., biases) too -- Be sure that the dome is dark for darks and zeroes! The instrument is reasonably light tight but we do operate with the cage doors removed, so bright lights are not a good idea.

It's a good idea to take a careful look at your calibration frames individually, before combining. And remember that some of the CCDs have very considerable slopes and variations as you go up the overscan, so fitting the overscan with a constant or line may not be appropriate.

CAUTION: The Mosaic 2 camera allows 2x2 to 4x8 binning although not fully tested for science frames reductions.


This page was last updated on Feb 22, 2000 by ARW
Please send comments or questions to awalkerATnoao.edu AND csmithATnoao.edu. THANKS!!.

Updated on June 4, 2021, 10:08 am