Quote:
Originally Posted by gregbradley
Perhaps a side by side comparison of an image with and without flats would be good.
Greg.
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even better Greg, attached is an unprocessed flat from my system scaled to 8 bits. The variation across the frame is about 20% and it varies very smoothly, so for galaxy imaging, I just rely on the standard PI gradient removal process to take this out as well - which it does most effectively. As you can see, dust bunnies are not a problem at f4. Flat calibration may just possibly be useful if I ever decide to image something that fills the frame, but for now, there is simply no point in using it for galaxies and PNs.
In the absence of good weather, did some darks to test out the effect of dark calibration on a few combination methods. Using a small featureless central region of some dithered red subs of the Helix, I tried a variety of combination methods with the following results. Note SNR here is that that of the signal+sky in the chosen central region - 2 sigma outlier rejection was applied to the PI stacks and default normalisation was applied:
Median, no calibration - SNR=29.7 (PI)
0.1% Hot pixel reject - SNR=30.5 (Nebulosity)
Median with dark cal - SNR=31.3 (PI)
Average no calibration - SNR=33.6 (PI)
Average with dark cal - SNR=35.7 (PI)
The SNR difference between median and average is not quite the theoretical ~20%, but it is still reasonable for the experimental uncertainties. Simple hot pixel mapping works OK.
The choice is between the simplicity and high overall efficiency of the uncalibrated approaches vs the higher SNR of the calibrated approaches. Average stacking with no calibration and with outlier rejection seems to me to be the compromise that generally best suits my imaging and processing opportunities, although I have used calibration when trying to extract the last skerrick of info from a low brightness target (the difference was right on the edge of being perceptible).
I have not yet done a similar test on narrowband imaging.