Background

The new LRIS red-side detector electronics exhibit a change in noise characteristics when the temperature of the controller changes. In order to minimize fixed pattern noise, it is necessary to periodically check the temperature of the detector electronics and to reset the clock delay parameter (keyword LSERCLKD). The appropriate value of this keyword depends not only on temperature (keyword UTBTEMP), but also on the CCD readout speed (keyword CCDSPEED) and CCD binning mode (keyword BINNING). This document describes the procedure for determining the optimal clock delay value for a given set of these parameters, plus the procedure for updating the LSERCLKD keyword when conditions change.

Clock Delay Parameters

The LRIS keyword library includes 4 keywords which control the value of LSERCLKD in various operating modes, as shown below:
Keyword CCD Speed Binning
USER1 Normal 1,1
USER2 Normal 2,2
USER3 Fast 1,1
USER4 Fast 2,2
When the observer changes binning or CCD speeds by using the appropriate script, the software will identify the appropriate USERn keyword to use and sets the value of LSERCLKD to that value.

Setting USERn

As part of the afternoon setup procedure, the observer or support astronomer must reset the values of the USERn keywords appropriately for the current UTBTEMP value. This is accomplished by executing the following command frmo the lriseng account on lrisserver:
	set_clock_delay
This script will read the current value of UTBTEMP and will look up the optimal values of USERn for that temperature.

Calibrating LSERCLKD

The framework above relies on the existence of a table listing the optimal clock delay for the various observing modes and CCD controller temperature. Following this procedure to derive new values for the current temperature:
  1. Acquire images. Log into lrisserver as user lriseng. Execute the clock_delay_tests script to acquire images while varying LSERCLKD. Note that the default is to acquire data in all 4 modes, which requries about 3 hours to complete. You can use the -fast or -normal flags to restrict the data acquisition to one mode and cut the time roughly in half.
  2. Analyze images. From an xterm window as user lris on an HQ machine, execute start.idl62 and do the following:
    	cdata
    	.compile /kroot/idl/keck/lris/readmhdufits
    	.compile ~lris/widgets/src/clockdelay/clockdelay
    	clockdelay
    Select a set of images to analyze (images used to determine USER1 are numbered starting at 0100, etc.). The script will generate a PostScript plot in the directory /home/lris/data/clockdelay.
  3. Analyze plot. Inspect the variation of noise level as a function of LSERCLKD as shown in the plot and select a value which appears optimal.
  4. Update data table. Edit the data table /home/manuka/lriseng/data/clockdelay/clockdelay.dat and insert a new line with the ccd speed, binning, utility board temperature, derived optimal clock delay, and corresponding postscript plot name.