QHY 268 C SharpCap flat frames problem?

[Tom1980]

Hi,

I can´t create correct flat frames with my cam QHY 268C in Sharp Cap. Results in stacking are stil bad and the corners of image are overbrighted. Histogram status "green OK" is impossible without sensor analyse at first. Than it´s OK but histogram curve looks just differently and flat frame is still unusable.
I´ve tested brighter and darker versions of flats, with/without dark frames, with/without "background removal", but without good results. There must be probably a mistake in SharpCap or in QHY 268 C or do I have a flats still too bright? My configuration is 10´Newton (4,7) + GPU Maxwell comacorrector 0,95x. I am using Doerr LT 3838 UltraSlim LED panel.
Does anybody have an idea where´s could be a problem?

Thanks much!
Tom

[admin]

Hi,

to get good flat correction for cases where the captured images get stretched a lot (i.e. deep sky and EAA) then you need to use one of the options to correct for the camera's black level offset (the pixel value of a perfectly dark pixel not being zero).

The easiest way is to tick the box to capture and subtract bias frames when creating the dark. Capturing and subtracting flat darks is slightly better if you are taking flats at a relatively long exposure (more than 1s or so).

You should then make sure that the camera 'Offset' control is unchanged between capturing the flats and capturing the light frames. This allows SharpCap to calculate the portion of the image pixel value caused by light (as opposed to the black level offset) and only apply correction to that.

You could also use dark frames to help here, but you still need to tick one of the 'capture bias' or 'capture dark flat' options when capturing your flats. Flats without these options will probably work OK for lunar/solar/planetary, but not for deep sky/EAA.

cheers,

Robin

[Tom1980]

Hi Robin,

OK, thanks for reply and I´ll test it soon. But I can´t normally achieve "green status OK" in flat creating. So Shoud I to ignore it? I can´t manage to have curve between 50-60% of brightness, cause the curve is more stretched. It s a pity, that this is not mentioned in SharpCap manual.

Tom

[admin]

Hi,

green OK status can be hard to get in some situations - particularly when there is a big difference between the brightness of the colour channels. If that's the case then just aim to keep clear of both ends of the histogram graph by roughly equal amounts. Setting the option to create monochrome flats on a colour camera helps reduce the impact of this too.

cheers,

Robin