Quote:
Originally Posted by gregbradley
Paul,
I usually shoot dusk flats but sometimes I shoot them during the day. My wife made me a white cover for the end of my scopes with elastic so it fits over the end. I then simply point the scope towards a wall and there is enough ambient light to do a nice flat. They work very well, just as good as dusk flats and more convenient.
So your idea probably would work if your observatory is fairly dark with the roof closed. Mine is painted black on the inside.
A white cloth over the end of the scope with elastic like I just mentioned and dusk or dawn flats are hard to beat. Point towards the sky opposite the sunset for a null area. You have to be a little fast as its amazing how quickly the light levels drop. You see it in the reported brightness levels of the flats. So I usually increase the exposure length a little with each new filter to compensate. I go for about 20-30,000 ADU. Usually closer to 30,000. I subtract a bias from my flat when using it in processing but not when creating a master flat. Simply average them to create a master flat. There was a thread a while ago about flats and how many to take to not introduce noise into your image but as a guide you want to take as many as you can in that time. So ideally your camera, cools off fast, has fast downloads and you are organised with the naming of files, setting up the next filter etc so there is no delay between sets of flats for each filter. I do flats for each filter usually. The scope should be in focus and cooled to the same temperature as the lights.
If you move the camera (rotate) the flats should still be fine as its more the dust donuts that stay oriented if you rotate the camera rather than the scope. Just don't rotate the camera with regards to the filters.
Greg.
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I use a flatman XL and get my flats manually at the exact orientation I collect the subs. With the rotator I dial in whatever angles I captured the previous night. Focus isn't really a concern, but I will sometimes select a mid-point from beginning to end of night depending on how much my focus shifted due to temp changes. I rotate 180deg on meridian flip when operating an AO, so I can keep the bright guide star. For flats I take 20xflat for each angle / filter.
Be cautious having such high ADU for your flats.... I've found 17k-20k was optimum for the 8300. Any higher, say 25k to 30k, over correction was visible, for me at least. I've seen quite a few of your images on PBase, with a calibrated monitor I can see your flats over correcting.