I am not smart enough to work with Flats! HELP!

[MunichAtNight]

Hello,

I read many threads here and in some forums, too about using flats. I tried to work with flats many times, but I think I am to stupid to work with flats. For me it is some how Woodo, black magic and it is doing things and causes effects I don't even have an idea what's going on.

Hopefully with the below information and samples you can help me to be a very little bit more smart! I keep my fingers crossed!

I run Shaprcap Pro SharpCapVersion=4.0.9357.0 do use a ZWO ASI294mc uncooled camera and I do my flates with Sharpcap. As telescope I have a Skywatcher Newton quattro 150P. A homebrew flatbox for which I use a LED paneel from amazon and direct on top of the newton located bewteen Newton and flatbox I use 4 pages of paper to reduce somehow light energy.

I do the flats with 600 ms, GAIN 150, Brightness 8, "Capture and substract bias frames."!

I tried it with darks substraction as well, but not with more success! Same problem.

Doing Capture

Screenshot (37).png (607.53 KiB) Viewed 1451 times

Doing bias frame

Screenshot (38).png (240.38 KiB) Viewed 1451 times

I have the feeling that the flats are "over-compensated" - Later when doing normal lights I have a circle mid zone which looks okay, but outside every thing is totally blocked. I used same brightness but of course different gain and exposure time

Screenshot (39) -stretch.jpg (259.53 KiB) Viewed 1451 times

Here you find some links:


MasterFlat_22_13_50_offset=0.745%.png
https://f2m.org/sharpcap78UZz/MasterFla ... 0.745_.png

MasterFlat_22_13_50.png.CameraSettings.txt for the above flats
https://f2m.org/sharpcap78UZz/MasterFla ... ttings.txt



Stack done with the above Flats
https://f2m.org/sharpcap78UZz/Stack_7fr ... tretch.png

Stack_7frames_112s.CameraSettings.txt fpr the above stack
https://f2m.org/sharpcap78UZz/Stack_7fr ... ttings.txt


Any ideas what I am doing wrong? What should I do different? Any help is more than welcome!

Thank you for your support! Servus - MunichAtNight - Ewald

[oopfan]

Hi,

I'm not the best person to answer but I noticed this in the Camera Settings file (the last file you attached):

Apply Flat=None

Brian

[MunichAtNight]

Hi Brian,

Apply Flat=None

you noticed right! During my test at the end I switched off the flats, because I wanted to see the difference what happens with and without flats.

In the screenshot I did during stacking there is visible that flats were activated

Screenshot (39) -stretch_V2.jpg (705.25 KiB) Viewed 1430 times

Servus - MunichAtNight - Ewald

[admin]

Hi,

good flat correction requires - at a minimum - that you let SharpCap capture and subtract a bias frame when creating the flat frame. Hopefully you did this. You should also ideally be using dark frames too, but as long as there was a bias frame SharpCap will do its best without the dark.

However, I am more worried about the fact that you are capturing at 500 gain - that's incredibly high gain for deep sky imaging. A gain of about 200 would be more suitable. I think that the very high gain is showing up in the very speckly nature of your screenshot with green 'stars' (which are most likely hot pixels).

What is even more worrying perhaps is that you have so little visible image at such a high gain - what is the telescope/filter combination you are using with your camera.

thanks,

Robin

[MunichAtNight]

Hello Robin,

What is even more worrying perhaps is that you have so little visible image at such a high gain - what is the telescope/filter combination you are using with your camera.

I thought I had prepared my documentation to ask for help in a better way and quality. I see I failed when "freeze" what happened during stacking with Sharpcap. The here shown screenshot of Sharpcap and the saved .PNG file I did after playing for about twenty minutes when I pushed finally all parameters to the max. But this isn't helpful to demonstrate my problem. Shame on me!

The telescope is a Skywatcher Quattro 150P, the filter I used is an Altair Quad Band.

that you let SharpCap capture and subtract a bias frame when creating the flat frame. Hopefully you did this.

I worked with subtract bias frames when doing the flats. Didn't you notice in my initial thread my screen-shots I did when I made the flats?

Flats are done with 600ms

Screenshot (37).png (607.53 KiB) Viewed 1397 times

Here the screenshot when sharpcap did the bias frames - exposure time 1 ms

Screenshot (38).png (240.38 KiB) Viewed 1397 times

Please can you check this screen shots? So in case it was okay I can be sure to be on the right track when repeating this again? In case I did it wrong I appreciate any hint and explanation how to do it correct.

You should also ideally be using dark frames too, but as long as there was a bias frame SharpCap will do its best without the dark.

Normally I use for darks your new "Dark function Pixel Removal Only" for removing hot pixels. This works for me like a charm. Do you think this might cause such a problem?


Nevertheless this night was in the end successfully even without using flats. I am satisfied with NGC6823 I did after finishing my efforts to work with flats this night.

NGC6823_all_lowres_Stack_750frames_6000s_WithDisplayStretch.jpg (965.43 KiB) Viewed 1397 times

It was done with Gain 300, (not 500 ) exposure time 8s but needed 6000s total stacking time to do it.

Thank you for your support - Servus MunichAtNight - Ewald

[admin]

Hi,

I think your flat captures look fine, so nothing to worry about there. Your final image also came out well, so well done

I am concerned though that perhaps the 'hot pixel only' dark option does not work nicely with flat correction. The automatic adjustment of the background when using flats and no dark frame is probably only active when dark correction is set to 'None', and it should probably also be active for 'Hot Pixel removal only'.

I will investigate to see if that could be the problem.

thanks,

[MunichAtNight]

Hi Robin,

I will investigate to see if that could be the problem.

since my last post I had indeed one time a chance for about one hours to work with flats with subtracted dark frames in combination with actual done darks. And it worked great, until clouds appeared after half an hour. I will try to investigate more, if there is an influence with "hot pixel onyl darks" and "real darks" and keep you informed here, too.

Servus MunichAtNight - Ewald

[admin]

Hi Ewald,

actually I looked into that combination and found that the code was correct - when 'hot pixel only' darks are applied then SharpCap will use the offset info encoded in the flat file name as it should. I did find an issue if the offset was negative, but that would be very unlikely for real cameras (it happens sometimes with the testing cameras).

Anyway, good to hear that you managed to get some good results in your short window of clear sky.

cheers,

Robin

[MunichAtNight]

but that would be very unlikely for real cameras (it happens sometimes with the testing cameras).

Hello Robin!
I checked it again yesterday night.

Here a screenshot with Darks "Remove Hotpixel" (screenshot with stretched Light area to 75%)

Screenshot (56)-stretched.jpg (466.81 KiB) Viewed 1221 times

Here all the same I changed only darks to "Real Darks"(screenshot with stretched Light area to 75%)

Screenshot (57)- stretched.jpg (952.35 KiB) Viewed 1221 times

I add the unmodified screenshot as well
unmodified "Remove Hotpixel"

Screenshot (56).jpg (375.78 KiB) Viewed 1221 times

unmodified " "Real Darks"

Screenshot (57) -1mb.jpg (838.71 KiB) Viewed 1221 times

In case I can test something or I can provide additional data, please feel free to advise what I should do.

Ewald

[admin]

Hi Ewald,

can you share a selection of images with me (raw frames, flat frame, dark frame) in FITS format please so that I can try to reproduce the issue as you see it. I'm afraid I do not understand quite what is going on yet

cheers,

Robin