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 Post subject: Keeping us "in the dark"
PostPosted: Mon Jul 30, 2012 11:16 am 
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Joined: Mon Oct 12, 2009 6:47 pm
Posts: 1263
Location: Bolton
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:
File comment: 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 2465 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:
File comment: Stack of 21 images without calibration
double_nodark.jpg [ 34.68 KiB | Viewed 2465 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:
File comment: Stack of the same 21 images, but after calibration with 12x bias and 12x dark-frames
double.jpg [ 26.93 KiB | Viewed 2465 times ]

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