Lick Infrared Camera User's Guide

Testing the Array

Dark and Noise Tests

Set all wheels to their dark positions by selecting the `Dark' option in the motor control program. From the data-taking system, set the parameters for a zero-second exposure with three erases. If Lirctop is not running, start it. From Lirctop's main menu, select `Tests', from the submenu, select `Dark and Noise Levels'.

The script begins by taking, displaying, and plotting a one-second dark. If Vista's own image and control windows do not already exist, a few moments will be required for their creation. The dark-frame should appear in the image display window as a more or less uniform square composed of four quadrants, resembling Figure 10. Note the positions of the few blemishes.

Figure 10. Typical dark frame

A black plot window will appear with a row plot which should look something like Figure 11. Values should vary about 100 DN peak to peak. The level should be in the neighborhood of 2,000 DN, representing the bias level. (Remember that if you've enabled baseline subtraction, the level should be much lower--about 100 DN--but the peak to peak variation should be the same.) If your image is markedly different, you may have a light leak or a baseline problem. Make sure that all wheels are in their `Dark' positions. Have a technician check your baseline setting.

Figure 11. Row plot of typical dark frame

The procedure will have paused at this point to allow you to examine the image and plot. To resume, type `continue' (or just `c', as it is often aliased) in the Vista window. A second dark frame will be taken and subtracted from the first. The remainder is then displayed and plotted. The plot should resemble Figure 12, with levels of about 30 DN peak to peak. This number, divided by six and multiplied by 12e- DN-1, will yield the approximate read-noise level in terms of e- pixel-1.

Figure 12. Row plot of dark minus dark

Stability

The constancy of the systems response can be measured by examining flux levels in repeated exposures, or the stability of the baseline can be checked by looking at a series of darks. Such tests measure the stability of the array and its associated electronics, including exposure timing.

If it's not already running, open a Vista window and start Lirctop. From the main menu, select `Tests', from the submenu select `Stability'. This procedure automates the stability test by taking the requested number of exposures, calculating the mean DN within one or more user-defined boxes, and finally plotting mean DN against exposure number.

The response test is only as accurate as the illumination is constant. A good, stable thermal source, over periods of time during which the dome temperature does not change significantly, is the backside of the TUB diagonal mirror or the inside of the primary mirror cover.

Figure 13. Plot of baseline stability The mean of one hundred, 1-second dark exposures, plotted against exposure number. The y-axis is the mean DN measured in a 56x56 pixel box, centered on the array.

Sky Test

This check must be performed after dark. From the motor control program, select the K' filter and the intermediate field of view. Take an exposure of the evening sky, long enough to bring the average pixel to about 15,000 DN, and make a rowplot. It should resemble Figure 14.

The appropriate exposure time will depend strongly on air temperature and other local conditions, varying by as much as a factor of three or four between summer and winter. The 45-second exposure shown in Figure 14 was made with the intermediate field of view, at 39° Farenheit, under a clear, moonless sky. The same exposure level is reached in about half the time at 55°. See the `Times to Saturation' table.

Figure 14. Row plot of the K' sky This 45-second exposure was made with the intermediate field of view, at 39° F, under a clear, moonless sky. Local conditions, especially temperature, can cause K' sky brightness to vary several fold.