Advice on capturing effective flat frames in SharpCap

AstroSharp Ltd SharpCap
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David Wood avatar
David Wood avatar

Some advice on taking flats in sharpcap needed

My flats are not always very effective

I use sharpcap’s “Capture Flat Frame” feature and set my gain,offset and temp to match my light frames.

As I use the ASI294MC pro I set my exposure time to ~3secs then adjust the illumination on my flat panel

to bring the white line on the histogram to about 50%, with a mean value of about 28000 to 30000

the blue green and red lines spread out at different degrees with varying mean values but Sharpcap persists in asking me to get more pixels above a certain percentage so that by the time it declares histogram ok the white line can be up to about 80% along the histogram with one or more of the other colours tipping off over the right edge of the histogram

Do I go by Sharpcaps assessment or continue to take my flats with the white line at a mean value around 30000

Any advice appreciated

Leonardo Ruiz avatar
Leonardo Ruiz avatar

Hi David,
With a one-shot colour camera like the ASI294MC Pro, it is completely normal for the red, green and blue histograms to be separated. The sensor uses an RGGB Bayer matrix, so the green channel usually has a higher response than red or blue.

For flats, do not try to make all three colour histograms line up. The important thing is that the overall (white) histogram is around the middle, typically 40–60%, which for the ASI294MC Pro is roughly 25,000–35,000 ADU. Your 28,000–30,000 value sounds fine.

I would not follow SharpCap if it keeps asking for more brightness until the white histogram is near 80%, especially if one of the colour channels is clipping on the right edge. A clipped channel will make the flat much less effective.

So I would keep using your flats around 50%, with no colour channel touching the right edge, even if SharpCap still says “more pixels needed”.

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Leonardo Ruiz avatar
Leonardo Ruiz avatar

At the moment it is difficult to know whether there is actually a problem with your flats, or whether you are just concerned because the RGB histograms do not match.

The real test is not what SharpCap says or how the histograms look, but whether the flats actually correct your images properly. Try calibrating and stacking a few light frames with these flats and see the result. If the vignetting and dust shadows disappear, then the flats are working, even if the colour channels look different.

Please upload:

  • One uncalibrated light frame

  • One flat frame

  • The calibrated or stacked result

That will make it much easier to see whether the issue is underexposure, overexposure, clipping of one colour channel, or whether the flats are actually fine and SharpCap is simply being overprotective. Right now we are all trying to diagnose the patient by looking at the thermometer still in the box.

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