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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David Wood avatar
David Wood avatar

Thanks for the advice leonardo

As you probably guessed I’m relatively new to this hobby learning off a mish mash of you tube videos

i assume its normal for the flats on inspection of the individual subs appear to have a vignetting to them

Tony Gondola avatar
Tony Gondola avatar

David Wood · Apr 13, 2026, 12:16 PM

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

Offset yes but there is no need to match gain or temp. for flat frames. That is only required for darks. Has long as you get the histogram correct as outlined above you should be fine.

Concise
David Wood avatar
David Wood avatar

Thats interesting to know Tony thanks for that

Leonardo Ruiz avatar
Leonardo Ruiz avatar

David Wood · Apr 13, 2026, 02:34 PM

Thanks for the advice leonardo

As you probably guessed I’m relatively new to this hobby learning off a mish mash of you tube videos

i assume its normal for the flats on inspection of the individual subs appear to have a vignetting to them

Yes, that is perfectly normal. A flat frame is supposed to show the vignetting and any dust shadows. In fact, if you can see those clearly in the flat, that is usually a good sign.

The purpose of the flat is for the stacking software to subtract those defects from your light frames, so after calibration the final image should have a much more even background, with the vignetting and dust spots greatly reduced or gone completely.

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David Wood avatar
David Wood 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.

Well Written Helpful Insightful Respectful Engaging Supportive
David Wood avatar
David Wood 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

Well Written Helpful Respectful Engaging Supportive
David Wood avatar
David Wood 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.

Light Frame.jpg

Flat Frame.jpg

Stacked result.jpg

Thanks for your time Leonardo attached are the unstretched images saved from Siril

What I would like to mention (don ‘t know if this has any bearing on the issue) I have just started to use my svbony uv/ir cut filter after a long while and noticed the stacked image has a very pronounced blue hue which i had not come across before

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