Best way to get flats for larger scopes

[Spookyer (Brett)]

Hi all, I have used my lightbox for my refractor but now with my RC-12 I have to work out how best to do flats. I know some people do them at twilight, what is the best way of doing this? Are there other methods I should be looking at?


cheers
Brett

[rustigsmed (Russell)]

hi brett,

yes i have the same problem as you. i take twilight flats, which isn't too handy in the middle of winter... i need to retake some actually as i've changed a part in the imaging train, and was thinking off attempting t-shirt flats (think bedsheet) in the direction of a closeby flood light... well i'm hoping it will work.
twilight i find just after sunset usually to the south before any stars come up in the flats. avoid cloudy areas or too low to the horizon.

Russ

[Atmos (Colin)]

Never had any experience whatsoever with these things but I think they could work quite well for you.

http://www.gerdneumann.net/english/a...-overview.html

[RickS (Rick)]

I have one of the Gerd Neumann panels and it works pretty well but I prefer sky flats. I find they calibrate a little better and I don't need to do them in the dark. Doing sky flats manually would be a bit of effort. I use ACP and it automatically tracks the anti-solar point and adjusts exposure to keep the ADU levels within set limits. I think some of the other automation tools offer this too. I know that CCDAP does, at least.

Cheers,
Rick.

[DavidTrap (David)]

CCD Commander does as well Rick.

DT

[glend (Glen)]

I use a white t-shirt over the front of my newt and it works just fine and costs nothing. Shoot them againt a flat backlight, the sky is fine away from the sun late in the day.

[multiweb (Marc)]

A3 EL panel. Flats on demand. Anywhere, anytime.

[Paul Haese]

I use both sky flats and I have a flat box that Peter made for the tubed RC12. I simply adjusted the box to fit the truss. The flats it produces are very good, but I need to get a solution that will work in an automated environment. I am thinking of putting a large white circle in the observatory and shining a bright light from the pier onto the white circle at an angle. That will produce good flats too.

[RB (Andrew)]

Originally Posted by multiweb

A3 EL panel. Flats on demand. Anywhere, anytime.

+1

[gregbradley]

Originally Posted by Paul Haese

I use both sky flats and I have a flat box that Peter made for the tubed RC12. I simply adjusted the box to fit the truss. The flats it produces are very good, but I need to get a solution that will work in an automated environment. I am thinking of putting a large white circle in the observatory and shining a bright light from the pier onto the white circle at an angle. That will produce good flats too.

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.

[SpaceNoob (Chris)]

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.

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.

[mjfitzhenry (Matthew)]

Cover with white shirt then take advantage of the full moon to take your flats