Flats - longer or shorter exposures better?

[BrendanC]

I can't decide whether it's better to have flats that are taken with a dimmer light source and a longer exposure, or a brighter source with a shorter exposure.

Is there any theoretical reason why one should be better than the other?

[BrendanC]

Actually I think I just found the answer - looks like longer is better for a DSLR with a mechanical shutter.

So, dimmer with longer exposures it is!

If I could delete the original post, I would...

[Budgie1]

In my case I use a blank MS Word document on the desktop screen as the light source and when I started taking flats I used one layer of a white t-shirt over the end of the scope. With the DSLR set to AV mode, the exposures were too short and it was causing a shadow at the top of the flat because the mirror was still lifting. 

By adding a second layer of t-shirt to the end of the scope slowed the exposure enough to get a clean flat image.

So it's not so much a dimmer light source and longer exposure, but getting it just right for your setup.

Edited November 18, 2020 by Budgie1
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