To be practical, CCD tests must be simple enough to be carried out without fancy test equipment. You can do basic testing without even taking your CCD off the telescope! This consists of shooting two bias frames, two flat-field frames, and one dark frame. That's it. The analysis takes about five minutes. Details are given in Section 8.2.1.
Advanced CCD testing is more complex. It requires building a low-level light source (L3S), and with it shooting nine bias frames, nine low-level flat-field frames, 32 flat frames taken with differing integration times, and three long-integration dark frames. The test sequence can be done any place that can be darkened, including indoors on a cloudy night or in your observatory. Shooting a set of advanced test images takes about three hours, and analyzing the results takes about an hour. You will find a complete description of advanced testing in Section 8.3.
The purpose of making initial test images is to get a quick measurement of the conversion factor, readout noise, and dark current of your CCD camera. This method gives you accurate results, and it takes only a few minutes. Furthermore, you can test the camera while it is attached to your telescope, so you can easily fit it into your schedule during a night's observing.
To make the flat-fields, you will need a light box like the L3S described later in this chapter, the evening sky, or your usual method for making flats. Make the bias frames and the dark frame immediately before or after the flats so there's no time for the CCD or electronics to drift or change.
Turn on the camera and allow it to reach thermal equilibrium. Using the readout mode that you use to make celestial images, shoot the following:
Two Flat Frames. Make two flat frames using the same integration time. Adjust the integration time to produce average pixel values one-third to halfway to the maximum output of the camera. For 12-bit cameras, the flat should have an average value between 1,500 and 2,000 ADUs; and for a 16-bit ones, the flat should average around or 20,000 to 32,000 ADUs. Save the flat-frame files as BCT-FF1 and BCT-FF2.
Two Bias Frames. With the dark slide closed or with telescope capped, set the integration time of your camera to the shortest available, and then make two bias frames. Save them as BCT-BF1 and BCT-BF2.
One Dark Frame. Close the dark slide or cap the telescope, and make a dark frame using a 60-second integration time. Save the dark frame as BCT-DF.
8.2.2 Basic Test Analysis
These images contain enough information to determine the conversion factor, the readout noise, and the dark current.
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Although we usually tend to think of the digital camera as the best thing since sliced bread, there are both pros and cons with its use. Nothing is available on the market that does not have both a good and a bad side, but the key is to weigh the good against the bad in order to come up with the best of both worlds.