Mosaic Overview: At least Read This!
|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|
|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|
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|
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.
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