Clarification needed for capturing Flat frames

[Ehud]

Hi Robin,
The User Manual say:
"Capture and subtract dark flat frames – If this option is selected, SharpCap will capture dark flat frames (at the same exposure length as the flat frames) after completing the capture of flat frames. This option can give the best flat correction results (particularly with long exposure flat frames of 1s or more). When using this option, you must either cover the telescope when prompted to allow dark flat frames to be captured or specify a filter to use that blocks light to the camera. "

However, I am using 5 seconds exposure' and when testing through Darks capture histogram, even with Gain at 0 , the image is overexposed.

Please advise,
Ehud.

[Ehud]

Hi Robin,
also, I am not sure how this goes with with the direction to capture Darks using sane gain and exposure as using for the picture stacking itself. I am using a gain of 300 (for Galaxies EAA from Bortel 9).
Thanks.

[admin]

Hi,

light frames and dark frames need to be captured with the same gain, exposure and offset values to work together properly. Flat frames and dark flats need the same settings as each other, but do not need to be matched to the light/dark frames. That means you can use shorter exposures and lower gains for the flats and dark flats than for the light/dark frames.

If the image is looking over-exposed when capturing dark frames, the first thing I would check is to see if you have left the display stretch turned on , which could be making an image that is actually quite dark look light. The other thing to check is the offset (black level/brightness) control - sometimes if that is turned up high it can push a frame towards being light even when no light is hitting the sensor.

cheers,

Robin

[Ehud]

Hi Robin,
I think I got it:
Dark-Flats at the Flats capture process are not instead of the Darks that I capture separately.
Am I correct?
While capturing the Flats & Dark Flats, Darks subtraction should be enabled or
not?

Thanks,
Ehud.

[admin]

Hi,

yes, that's right - 'dark flats' are the dark frames matched to the flats - they are used to create a flat with the dark level background removed and not used in place of normal darks.

Best to turn dark correction off when capturing flats and allow the 'dark flats' option to do its bit.

cheers,

Robin

[Ehud]

Robin,
Thanks for the clarifications.

[Ehud]

Hi Robin,
I was playing with various options. I can't make up my mind if when capturing flats, should I set the Black level (I set it to 40 whne using SVBony 605cc) for the value I use for EAA, or should it be set to 0.
Please advise.
Thanks,
Ehud.

[admin]

Hi,

as long as you use either bias or dark flat frames when capturing flats, *and* you use dark frames when capturing lights, you don't have to worry about the black level being the same between flats and lights. It does have to be the same between darks and lights, and separately the same between flats and bias/dark flat.

If you are taking flats with a low gain, you could probably get away with a lower black level without black clipping the bias frames, but the overall effect on final image quality will be next to nothing, so best to just leave it at one setting and never change it... The biggest risk is that you change it then forget to change it back and end up with mismatched darks and lights or similar.

cheers,

Robin

[Ehud]

Thanks Robin!

[TheAstrophotographer]

Robin,

I am trying to get my flat for my ZWO ASI533MC Pro and using a Optolong L-Quad Filter and am getting the histogram provided below other than the white peak which is not shown; therefore, is this OK?

2025-09-28 21.42.12.jpeg (88.49 KiB) Viewed 1225 times

Also, when I look at the capture flat frame, I do not see histogram status OK but it indicates that there is saturation for some of the pixels; therefore, is this OK?

2025-09-28 21.45.46.jpeg (163.15 KiB) Viewed 1225 times

Thanks,

Edmund