Here's an example of the use of the calibration frames which Dave demonstrated to us at the last Tutorial...
Last night I was testing a modified webcam, fitted with a 50mm lens and simply clamped to my window-sill:
I took 21x 7.5-sec exposures of a part of Perseus (including the Double Cluster, by chance), and then processed just these light frames: As well as the inaccurate focus, you can see a number of places where the sensor's "hot-pixels" have left red, green and blue diagonal trails across the combined image (since these stay fixed as the star-field drifts past).
Then I fitted the lens-cap (it was about to cloud over anyway) and took a dozen 0.3-sec bias-frames and then another dozen 7-5-sec dark-frames. From these I used IRIS to make master offset and dark frames and a cosmetic list with which to calibrate each of the lights (the flat-fields were featureless, so I didn't bother with these).
Then the stack of calibrated images shows no hot-pixel trails:
For those interested in imaging and image processing with DSLR, CCD, webcam or film.
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