DEIMOS Bias Level Adjustment Procedure

Background

Over time, the bias levels of the CCDs in the DEIMOS science mosaic will drift from the nominal value of 1000 DN/px. The required adjustments can be completed easily and quickly through the procedure described below, which will produce new values for the VIDOFFn keywords controlling the bias level.

Procedure

Adjust VIDOFF Keywords

Log in to an HQ machine as user dmoseng.
Launch DEIMOS control software.
Launch an IDL session in a HQ dmoseng window using the command
```
	start.idl62
```
and in the IDL window execute the commands
```
	do_deimos_check_vidoff, /dir
	do_deimos_check_vidoff, /spec
```
This procedure will:
- configure DEIMOS in the specified mode,
- acquire bias images,
- analyze them for offsets in bias level,
- make appropriate adjustments to the CCD keywords to equalize the bias

Save Values

When new bias levels are determined, they should be entered into the Default DEIMOS Configuration settings file so that these values get re-loaded by the observer each afternoon. Otherwise, they will disappear when the CCD crate is rebooted. To do so:

Open an xterm window and log into deimosserver as user kics:
```
	ssh kics@deimosserver xterm
```

In the xterm window, type:

	/home/deimos/src/scripts/inst/update_vidoff

and verify that the script completes successfully:

	[update_vidoff] Successful completion!

Examples

To adjust the bias levels:

IDL> do_deimos_check_vidoff, /dir 
setting MOSMODE = Direct (wait)
setting ampmode = DUAL:A+B (wait)
setting ttime = 1 (wait)
------------------------------------------------------------------------
Interation:        1
------------------------------------------------------------------------
Fri Oct 17 20:37:28 HST 2008
  DARK Exposure 1/1 Image d1018_0026.fits
  Exposing...reading out...transferring to polo...done.
  Exposure sequence completed.
Video_Input_1 i=  1 amp=1A vidoff= 1 bias=1001.0 old=2535 dv=   0 new=2535
Video_Input_2 i=  2 amp=1B vidoff= 2 bias= 996.0 old=2438 dv=  -1 new=2437
Video_Input_3 i=  3 amp=2A vidoff= 3 bias=1000.0 old=2503 dv=   0 new=2503
Video_Input_4 i=  4 amp=2B vidoff= 4 bias= 981.0 old=2513 dv=  -3 new=2510
Video_Input_5 i=  5 amp=3A vidoff= 5 bias=1001.0 old=2555 dv=   0 new=2555
Video_Input_6 i=  6 amp=3B vidoff= 6 bias=1001.0 old=2544 dv=   0 new=2544
Video_Input_7 i=  7 amp=4A vidoff= 7 bias=1002.0 old=2518 dv=   0 new=2518
Video_Input_8 i=  8 amp=4B vidoff= 8 bias= 987.0 old=2590 dv=  -2 new=2588

Image: /s/sdata1003/dmoseng/2008oct18/d1018_0026.fits
Changes:        3

------------------------------------------------------------------------
Interation:        2
------------------------------------------------------------------------
Fri Oct 17 20:38:36 HST 2008
  DARK Exposure 1/1 Image d1018_0027.fits
  Exposing...reading out...transferring to polo...done.
  Exposure sequence completed.
Video_Input_1 i=  1 amp=1A vidoff= 1 bias=1001.0 old=2535 dv=   0 new=2535
Video_Input_2 i=  2 amp=1B vidoff= 2 bias=1002.0 old=2437 dv=   0 new=2437
Video_Input_3 i=  3 amp=2A vidoff= 3 bias=1000.0 old=2503 dv=   0 new=2503
Video_Input_4 i=  4 amp=2B vidoff= 4 bias=1000.0 old=2510 dv=   0 new=2510
Video_Input_5 i=  5 amp=3A vidoff= 5 bias=1001.0 old=2555 dv=   0 new=2555
Video_Input_6 i=  6 amp=3B vidoff= 6 bias=1001.0 old=2544 dv=   0 new=2544
Video_Input_7 i=  7 amp=4A vidoff= 7 bias=1002.0 old=2518 dv=   0 new=2518
Video_Input_8 i=  8 amp=4B vidoff= 8 bias=1000.0 old=2588 dv=   0 new=2588

Image: /s/sdata1003/dmoseng/2008oct18/d1018_0027.fits
Changes:        0

Adjustments completed in       2 iterations.

To update the stow positions file:

[123] kics@polo% /home/deimos/src/scripts/inst/update_vidoff
[update_vidoff] Verified /local/kroot/data/deimos/stowPositions.tcl exists and is writeable
[update_vidoff] Verified existence of dir /local/kroot/data/deimos
[update_vidoff] Verified /local/kroot/data/deimos/stowPositions.tcl and /tmp/update_vidoff.5003 both have 114 lines
[update_vidoff] Created backup file /local/kroot/data/deimos/stowPositions.tcl.2008Oct17_4
[update_vidoff] Replaced /local/kroot/data/deimos/stowPositions.tcl with /tmp/update_vidoff.5003
[update_vidoff] Successful completion!