Is this flat frame flat!?

[Marky1973]

Well some recent posts about flats, I've been having a fiddle with APT and my flower pot flats box as I am confused when it comes to the settings and what a flat frame should look like. Every time I think I'm in the ball park, I read something else that makes me wonder.

So, with a light source, and some white fabric to diffuse the light, I am exposing on the AV setting at the same ISO as my darks and getting an image like the one below - screen grab from APT.

I can see marks on the sensor that I know are there (and, yes, I need to clean it) - but I am wondering about the histogram - is this far enough over, or should it be exposed slightly more?

Secondly, the BLUE channel seems to be more prominent than the others, which is probably because of the LED Light source. Is that likely to be a problem? Should the channels be more evenly balanced? If so, then I need to find a better light source.

If I use the flashlight on my phone (which I only thought about using about 5 minutes ago when I nearly blinded myself with it by accident) I do get a more even spread on the LRGB channels - so perhaps that is what I should using? Flashlight on white fabric stretched over the scope.

Is this better? Should the histogram be over to the middle a little more, or does it really matter in the overall scheme of things.

I've certainly noticed an improvement on my images since using the first method but, if aligning the channels will make them even better then I need to be using that!

Thanks for humouring me!

[Alien 13]

I had always thought that the colour the flat end up makes no difference I allways used to shoot a blue sky, the important thing is to make sure that the flat is not introducing a secodary gradient because the source is uneven.

Alan

[Marky1973]

Hi Alan - I was hoping that the colour didn't matter as everyone you speak to has a slightly different way of managing them. Of course, there are also a number of different views as to how far over the histogram should be - so I might have a play next time I am out and use the mobile phone light with the cloth I have been using to try and expose between 25% and 50% and see how that goes. As you say, hopefully, as long as the field is even across the sensor it should be okay! 

[ollypenrice]

They look OK to me, but maybe a little suspiciously flat. Your kit isn't dirty, it's clean. Leave it alone! I don't know the software but maybe a screen stretch would show more contrast. Or you can check the difference in brightness across the flat. A range of maybe 2000 ADU from corner to centre is what I expect. The histo peak looks well placed.

Olly

[lensman57]

If you have used APT  in the Av mode to take the flat then the chances are that it is correct, however the way the flat is displayed has nothing to do with it being correct or not, Flats have no colour BTW as it is the Luminance that is important for correction. To judge it without looking at the end Master Light you have to stretch the Master Flat and have a good look at it. There must be vignetting present with a DSLR size sensor even with a TAK FSQ and the usual dirt traces on the sensor, the dirt on the optical surfaces shows up as doughnuts as they are further away from the sensor. I have also read somewhere that with a DSLR it is better to expose at +1 stop over in Av Mode. The easiest way of doing this is to set the exposure compensation to +1. Take two sets of flats at 0Ev and +1Ev and see which one gets you better results in the final stack.

A.G

[Marky1973]

Thanks Olly/A.G. - I'll have a stretch later and see what that shows up and look at the brightness - the attached pics aren't great I appreciate. It's just possible I read too much and end up confusing myself. I don't appear to have had much trouble taking flats to date (or at least my images don't seem to be awful when I add them) so I'm probably thinking it should be a lot harder from what I am reading!

Thanks again!

[Marky1973]

Oh, and I'll put the Fairy Liquid away for now then....