SharpCap - Flat Field Illumination Puzzle!

[Norris Adams]

I have recently been trying to improve my EAA images with the subtraction of Flat frames in SharpCap.

I have made use of a light box (with three different light settings) which has enabled me on occasion to create an averaged Flat with the right kind of histogram profile.

So far so good.

What is puzzling me is that when trialling optimal Flat Field Illumination set-ups (using light box & stretched white T-shirt etc) I sometimes seem to get very different profiles from the histogram even though I don''t think I have changed anything or I have simply recopied earlier successful settings. 

I have attached a couple of examples using nominally the same physical set up (see also below) and SharpCap settings.

What am I missing? Why such big differences when nothing seems to have been altered?

Secondly, should I be seeing the histogram itself vary in real-time, when for example I alter the light levels in the box or increase the exposure time? At present it only changes when I do a full reset - usually by clicking 'Live Stack' twice and then changing the relevant parameters (e.g. using a different Light Level settings or increasing exposure time to seek out the best profile)

I wonder sometimes if my laptop simply lacks processing power to do this in real time - or is there something else I should be thinking about?

Finally, I should add that I also take some darks prior to acquiring flats, but I can't see how this should make any difference. Nor I think should reattaching the camera in a slightly different angular position.

Any help or guidance gratefully received!

Thanks,

Norris

Skywatcher P1501 / ZWO ASI183MC 

 

[Elp]

For flats you don't alter the light level at all, once you've determined a good light setting and diffusion method, and the right exposure length you take a few images, they should generally all look the same. Stack them to make a master flat. A lot of software can now auto expose so they calculate automatically the correct length for each flat. I use perspex sheets to dim my tracer panel to make it look like it's almost off when at the lowest setting.

Darks should be the same length as the lights with the telescope/camera completely capped off so you're imaging complete darkness. These are needed for the 183 due to the amp glow. Again take a few with the same setup, then stack to get your master dark.

As you're using an uncooled camera try to take all of the above at a close similar temperature to when you're taking the lights, I normally leave it until the end of the session (for long exposure DSO imaging) but for EAA you likely have to take them at the start of the session, stack then apply to the EAA session.

Edited Saturday at 19:31 by Elp

[Norris Adams]

Thanks for you advice Elp.

The workflow you describe is pretty much exactly the pattern I have tried to follow. I only move between the three light settings to help me settle on the best histogram shape. But I sometimes find that when I try to replicate the best profile a little later, I sometimes get a completely different outcome (which seems ridiculous I know)!

I even wondered if I'd left the end cap on after taking the darks!!

And you are right of course that for EAA it is best to take Darks and Flats at the start of the session. This is what I normally do. I decided  to do these trials during the day only because I wanted to get the process right and to try and find out why I'm not getting a consistent outcome.

Still scratching my head here!!?!

By the way, am I right in saying that I cannot build a Flats 'library' (unlike Darks) because the camera's position will vary (even if only by a few degrees) with each imaging session?

Norris

 

 

[Elp]

Generally if you can guarantee the image train will not change you can build a flats library as many DSO imagers reuse flats, but the slightest rotational or focus change (or filter change if applicable) will render them next to useless.

Regarding testing during the day, you may have a light leak, or more significantly, dont underestimate the power of light during the day, I recently tested some thicker perspex when I received it during the day, even though the camera was directly on the material pointing down at the light panel indoors, the flats looked odd to me, each different slightly and with odd grain/patterns, I tried the same test again at night indoors with minimal outside light near the test area and the flats were as expected and consistent flat to flat.