Applied flats look horrendous

[JerryG]

Hello,

I am trying to apply flat field images to my live EAA outreach sessions. However, my flats over-correct and give these horrendous looking gradient rings around the DSO. I've tried following the manual on how to capture and apply flats (and dark flats and darks), and I've also tried reading other flat aquistion problems on the forum, but I can't seem to get a proper flat-field. I am using no offset, and capturing darks, flats, and flat darks using the tool tab in sharpcap. Histogram stretching really makes it look bad. Attached are the Master Flat, flat capture settings, and Master Dark settings. I used the dark flat option so the dark flats are same exposure/gain as the flats. The log says darkflats but they are actually flats, I must've typed in wrong. Any help would be appreciated

https://drive.google.com/drive/folders/ ... sp=sharing

[Menno555]

Hello and welcome

People make their calibration files in different ways.
For example: my thumb of rule is that for (darks)flats a I always use the same Gain, White Balance and Offset values as my captures. I only change the exposure time to get the right flats. Depending on the kind of filters I use, those times are anywhere between 3 and 15 seconds.
Others do change those values and have also good flats, but for me it works best this way.

And how do your make your flats? Do you use the white t-shirt methode, flat panel, or otherwise?

Menno

[admin]

Hi Jerry,

from the screenshots of the camera settings files, I can see you are using RGB24 mode for both the flat and the dark. This is going to be at least part of the problem, possibly the main cause.

For deep sky imaging, you should use RAW16 mode for all images (darks, flats, dark flats, light frames, etc). In RAW16 mode, you get 65536 different brightness levels out of the camera, whereas in RGB24 (or RAW8) you only get 256. The coarser levels in RGB24/RAW8 may be stopping dark subtraction from working properly, meaning that the flat frame correction is then being applied to the wrong part of the signal (ie being applied to the actual image *and* some of the dark background included).

I would suggest re-taking all images in RAW16 mode, and also make sure that the same camera settings are used when capturing the dark frames as when capturing the light frames (don't change exposure, gain, brightness, white balance, etc, etc between darks and light frames).

The thing with the DarkFlat in the settings for the flat frame is a minor bug - when you capture a flat and ask SharpCap to subtract dark flats (the right thing to do for deep sky), By the time that file gets written, SharpCap is already setting up to do the dark flat capturing, so the frame type has been changed. That can be ignored...

cheers,

Robin

[JerryG]

Thank you, Robin. Changing the capture format to RAW 16 did help quite a lot. Much appreciated