I forgot to take flats...

[Baskevo]

Hi guys,

I'm on my way back from my camping trip and got about an hour per LRGB filter. However, we had to leave early and I disconnected everything before I could take flats.

There aren't any dust motes (from I can tell before stacking), so can I just reconnect my camera and take flats? Or does anyone have a good method for artificial flats?

I don't want to lose this data!

thanks guys

[KathyNS]

You have a couple of options.

You can re-shoot the flats. They won't be as good as flats shot at the time of the lights, and if there are dust spots, they could make them worse: you'd have some dark dust spots and some light dust spots. But worth a try if the spots you have aren't noticeable. Make sure that you assemble the gear exactly the way it was during the imaging session, paying particular attention to the orientation of the camera, filters, etc. Make sure, also, that you focus well on a target at infinity. If they work, great.

You could also try "synthetic flats". In your favourite processing software, make a duplicate layer of the stacked image. On the duplicate version, you basically blur all details out of existence. The goal is to create an image that is background only. This requires some care if the target occupies a large part of the image. (My first attempt, on M33, turned the process into a Romulan cloaking device: the galaxy disappeared! Don't do that!) If the target is large, you may want to remove it from the duplicate with a clone-stamp or similar tool. Leave the background intact. I blur the duplicate using first a point-spread function (in GIMP, I use Enhance / Noise / Spread), followed by a Gaussian Blur. For both functions, I specify a very large pixel distance: 200 pixels in most cases. Finally, set the mode of the duplicate to subtract from the original image. The resulting image will have the blurred background subtracted from it, which should remove any vignetting. You will have to tweak the result a bit, since the overall brightness will be reduced.

[XCalRocketMan]

One other thing. Dust motes and stuff are sometimes not noticeable at all on a single sub. But once you stack and stretch, bingo.

[Baskevo]

Thanks for the advice guys. I will give new flats a try, and then I'll probably have to do the synthetic flats lol Thanks for the advice guys! wish me luck