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Just figured out interesting technique for artificial flats


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This technique requires light pollution :D, so if you have dark skies - bad luck, you'll need flat panel.

Another requirement is to use darks and that you dither. You can probably get away without darks. I actually developed this technique few minutes ago when I was asked to help out with processing of rather poor data.

About 8 x 10min subs were taken in moonlight with DSLR and lens. Due to wide field there is a lot of LP and gradients. Vignetting is about 50% in the corners. I did not have any other calibration files but I managed to remove vignetting. I did not manage to remove dust shadow as that is too fine feature.

Here is what you should do to produce artificial flats:

- load your subs and do sigma clip stacking with aggressive values without alignment of subs. I used 5 iteration rounds with sigma 2 for this case.

- take resulting image and do feature removal - like background extraction or low pass filter or similar

Idea is that we need to remove stars and any strong signal from the image. If subs are properly dithered then stars will be in different place in each sub. If we omit alignment of subs that gives us opportunity to use sigma clip stacking to actually remove the stars. Vignetting and other features that are due to flats will not move since they are sensor related and not sky related.

Here are example results:

image.png.c23572d588fd62c8446b9350e11c6a3b.png

Single green channel sub (debayered frame and then binned x2 to improve SNR) - this is wide field shot of Heart & Soul nebulae region.

image.png.8f69cbb6c4440db3573386db741e73af.png

This is "master flat" derived from flats from R, G and B channels (this is OSC camera so vignetting will be the same - each flat was normalized to 0-1 and then average taken)

image.png.693bb8b47829bf61e8d35aaa01ea10db.png

Single frame after calibration ...

Here is what sigma reject looks like on 8 subs:

image.png.502784783384e78dbc4ee6e06e3c0d7e.png

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