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Exercise #4: CCD Calibration (JKalen)

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Exercise #4: CCD Calibration (JKalen)

Exercise 4: CCD Calibration

8” SCT-wedge mounted
DSLR: Canon 40D
Analysis software: AIP4WIN (2.4.0)

I transport my equipment to another site for observing and try to arrive and set-up during twilight to acclimate the telescope and DSLR. Images are acquired in RAW format at ISO 1600 without automatic dark compensation.

During twilight, I plan to take several flats for the evening’s session. Since taking this course, I now plan to diffuse the light at the telescope using either a T-shirt or a thick white plastic cover to improve the overall uniformity and increase the exposure which can reduce any effect due to the shutter. Since the DSLR is not cooled, my plan is to take darks with the telescope covered at several time-points during the evening and analyze the variations (mean and standard deviation) of the dark images. Subtracting the bias from the dark and comparing the mean and standard deviation for an evening and over several months should allow me to determine the frequency to acquire dark correction images.

To test image quality and calibration procedure, I plan to standardize the star-image exposure to 20 seconds; bright stars. Taking darks for the same exposure (20 sec) reduces the number of dark correction images to be acquired in an evening and allow me to fully test the quality and variation (mean and standard deviation) of the dark correction images. Since the dark and star-image are similarly exposed (20 seconds), bias frames are not required. Once I fully understand darks, I plan to test various methods for improved flats. I am interested in testing the “headlight” approach since I am not always able to arrive at the observing site during twilight.

Image files are opened, split into the 3 color components due to the Bayer filter, and the green component which is similar to the V-filter is saved in the FITS format. FITS file format data are saved as 32-bit floating point. Presently I process the files according to the Citizen Sky DSLR tutorial.

After I understand DSLR calibration for differential photometry, I am interested in acquiring a CCD camera and appropriate filters.

I welcome any helpful criticism and wishing everyone clear skies!

HQA's picture

your plan looks fine.  Darks really have a low signal level, so taking a few through the night and then expecting to get a decent mean and standard deviation may be difficult.  You should take a thermometer with you and write down the ambient temperature as the night progresses.  It may take a couple of nights of experimentation to see how stable things are and how often you need to take darks with such a non-temperature-controlled system.  In fact, I'd suggest picking a cloudy night or two and just take darks, rather than using up precious clear sky time.  The main thing is to experiment and plot everything against everything else and see what correlations that you find.

Good luck!  I'm looking forward to lots of excellent DSLR photometry in the years to come.


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