Keeping us "in the dark"

For those interested in imaging and image processing with DSLR, CCD, webcam or film.
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Keeping us "in the dark"

#1 Post by rwilkinson » Mon Jul 30, 2012 11:16 am

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:
Modified webcam fitted with 50mm lens, and fixed to the window-sill. Unguided, but I could still manage exposures of 7.5 sec without significant trailing.
callumcam.jpg (161.78 KiB) Viewed 2760 times
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:
Stack of 21 images without calibration
double_nodark.jpg (34.68 KiB) Viewed 2760 times
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:
Stack of the same 21 images, but after calibration with 12x bias and 12x dark-frames
double.jpg (26.93 KiB) Viewed 2760 times

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