Calibrating Flats - Forums

Topic starter

Jul 20, 2023

Hi,

Should I calibrate my flats with darks as well as flat darks or bias frames, or is it not necessary to calibrate flats with the same darks I use to calibrate lights?

Thanks!
Jerry

Quinn Groessl

IOTD Staff

Jul 20, 2023 · Edited: Jul 20, 2023

Flat darks are basically darks for your flats. So that's all you need.

Christian Großmann

Jul 20, 2023

Hi Jerry,

as far as I understand, it is a matter of the exposure time used to create the flats. I personally use a Flat panel and the exposure times for the flats are way less than half a second for LRGB and around half a second for narrow band. So I am not concerned about flat darks, even with the heavy amp glow I see with my sensors on long exposures.

Otherwise, If you do skyflats for example, the exposure time might get quite long. Then you might think about calibrating your flats. Where the exact border is depends on the sensor, the exposure time and maybe some other stuff. If you look at the infos that many of the photographers give when they puplish their images, not many of them seem to do flat darks.

But try for yourself. If it improves your images, it's worth the effort. If you do flat darks and they won't improve your images, they won't hurt either. If you build yourself a dark library under controlled and repeatbale conditions (i.e. cooled camera and flat panel), you may reuse them for a longer period of time.

Clear Skies

Christian

Helpful

Arun H

Frequent contributor

Jul 20, 2023

There are two ways you can do this:

Method 1:

Subtract a master dark of the same exposure length and temp as your flats. Example: You use 4s flats at -10C. Calibrate them with a master dark constructed from 4s flats at -10C. With this method, you don't need to worry about bias (since your master dark already contains the same bias as your flats). It will work with any camera, but you do need to have a library of darks of the same (or close) exposure time as your flats.

Method 2:

Do not worry about darks at all and simply subtract a bias at the same temperature. Example: You have a 4s flat at -10C. Simply subtract a bias taken at -10C and don't worry about darks. This method is a bit less accurate than Method 1 since it does not account for the small amount of dark current built in the exposure time of the flat. As long as your flats are only a few seconds long, this should not make a huge difference. However, some cameras have artifacts that make taking a true bias impossible or at least very complicated, so for those you should use Method 1. Cameras with amp glow may also create issues with this method.

Whatever you do, do not calibrate flats with the same darks as your lights.

Well Written Helpful

Jonny Bravo

Jul 20, 2023

You use either… darks (called dark flats or flat darks when you take them to calibrate flats) or biases. You don't use both. Debates rage over the effectiveness of one over the other. Owning a 294MM, I always take longer flats with matching darks. I do not use bias frames.