Calibrating the CCD-39 biases and darks

The CCD-39 has four readout quadrants, each with its own readout electronics. Due to the differences between the four quadrants, there are four bias parameters that need to be calibrated and set individually.

Periodically, you will wake up one morning to a WFS intensity display that looks like this, even when the CCD is illuminated:


Checking the camera

The first thing to do is to check the connectors, as shown in the picture below (click for enlarged version). Once we had a problem with the CCD and found that the top of the connectors were hanging off.

If the connectors are fine, then the camera should be power cycled. Instructions on how to power cycle the camera will be included in the Hardware Information webpage. After power cycling the camera, be sure to reboot the WFC.

Setting the biases

There are four quadrants, each with its own bias. The keyword for the camera bias is wsgain.

For every program, wssmprg, and camera gain, wssmgn, there is a bias setting consisting of four bias values. The settings are saved in the file /kroot/rel/ao/qfix/data/biases.dat. There are 28 rows, each corresponding to a combination of program (0-7) and gain (0-3) setting. The six columns represent the program, gain, and four bias values.

To see what the CCD biases for a given configuration look like do the following. First, ensure that there is no light on the WFS (i.e., light turned off, FSS set to block).

Example 1: Program 2 (2x2 binning), Gain 0.
IDL> setupwfc,2,0,0
IDL> loadbkgnd, 'zeros40x40.dat'

Example 2: Program 7 (1x1 binning), Gain 3.
IDL> setupwfc,7,0,3
IDL> loadbkgnd, 'zeros.dat'

This configures the WFC correctly and then loads the corresponding background file containing all zeros. You should now be able to see four quadrants with differing intensities. The idea is to have all the intensities be close to each other and have about 500 counts per pixel. This ensures that the pixel value will never go negative while not using up too much of the dynamic range of the pixels (0-16383). Note that this implies a subaperture intensity of about 8000 for 1x1 binning and 2000 for 2x2 bining.

The ordering of the quadrants in the wsbias keyword differs for Keck I relative to Keck II and is shown below:
2 3
0 1
3 2
1 0
Keck I Keck II

To change the bias, all that is needed is to modify the wsbias keyword. For example
modify -s ao wsbias=340,450,432,398

There is no need to set wcoper to 0 in order to change the bias. Once the desired bias for a given program and gain combination has been found, the biases.dat file should be changed. Make sure you save a backup of the file before making changes.

Example of bad bias settings Example of good bias settings

Creating dark frames

Once you are happy with all the bias terms, it is time to make dark frames for all the programs and gain combinations. The darks are very important, as they are used as the background at night and during calibrations if no other background is taken.

Ensure that there is no light on the WFS. Then run the IDL script to make darks:
IDL> makedarks

The darks are stored in /kroot/rel/ao/qfix/data/ControlParms/BackGround/*.dat.


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