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anybody want to tell me how to take flats?


adamphillips

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something ive put off for too long. its been a while since ive read anything on taking flats.

I just remember when I researched it before, the info had some grey areas about how long the exposures should be, I think you want the same ISO.

should I build a light box to put over the telescope? what do you need for a light box?

any help would be greatly appreciated.

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I can't remember how to do it with a DSLR, but I'm sure someone will come along soon to help!

Some things to remember though. Don't move anything in the imaging train from your lights, everything needs to be the same to replicate the potential dust bunnies and natural optical vignettes etc.

You can take sky flats, with a T shirt over the front of the lens to get the right brightness. I aim for 20-22k ADU but I don't know if your capture programme will give you the same readout or just gives you a histogram? I take mine against my white obs wall the morning after - Works well. No need to a light panel and a whole lot less fiddly to do I found. I do have a light panel, but definitely prefer the white wall method!

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right now im on the edge 11 HD with a canon 5d mark ii. later on ill buy the dovetail to be able to mount the old nexstar 8se OTA. and Ill get the atik 383l later but for now its just the DSLR.

another question, when I first read about mono cameras it seemed like people were saying you don't even need calibration frames, but I see people taking darks and flats with their ccd cameras.

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I popped in Asda and brought a pillow case, this goes over the end of the scope, i used a large jubilee clip to hold it in place so there are no creases, then set the DSLR to Av point the scope at the sky so the sun cannot shine on the pillow case, run off around 20 subs....well thats how i have done mine......

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thank you for that. that might be enough info to try out for myself.

in AV mode would you just keep the same manual focus point and the camera would determine its own exposure time?

I heard something about that before, letting the camera decide the exposure time. and I heard people say flats are so easy it just takes a couple minutes to fun through them. there is no reason not to take them.

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Flats should definitely be taken at the same ISO as your lights but temperature and exposure can, and probably will, be different. Ideally you want your histogram hump to be near the mid-point. This ensures that the vignetted data will be brought away from the noisy left side of the histogram.


It's ideal to take flats with your camera in the same exact position, with the same exact filters/adapters, as when you took your lights as vignetting can vary due to position and focus. Also, if you have a problem with dust motes then it becomes essential to take your flats before removing your camera from the scope. Even a subtle shift can cause a dust mote to move and then you've got a situation where the dust donuts in your lights are not only _not_ corrected, but new dust donuts are introduced due to the newly relocated dust mote.


I always try to take flats in the field. This is done by draping a clean t-shirt over the scope aperture, securing it with velcro, smoothing out the wrinkles, and pointing it at an evenly illuminated light source (e.g., the daytime sky or an exterior wall of my house.) Since I've got a live histogram on my DSLR live view, I just switch the camera into manual mode and adjust exposure until the hump is at the midpoint and begin taking the flats. Even though taking flat-darks won't next you a huge reward, it's almost effortless to do at this point and they certainly don't hurt. While you can use AV mode to set exposure, be aware that even a subtle shift in sky brightness will cause you to lose that frame due to exposure change. This can be especially frustrating if you're using the morning or evening sky as a light source.


Since my camera is almost always in the same position on my scope, I do maintain a "flat library" which I generated with a dust free sensor while inside. These work surprisingly well and are very useful for those times I don't feel like taking flats in the field (e.g., due to mosquitoes, weather conditions, dead batteries, etc.)

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Some thoughts on DSLR flats in my routine.

I normally fire off about 20 using APT software.

I do this at same iso but on av DSLR setting.

I like to take them at similar temp as my main run.

I do this by holding a white image on my laptop or kindle in front of my scope.

Make sure not to change any thing between the run and the flat.

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