SharpCap Forums

Hi, my apologies if there is a definitive answer somewhere else, but I haven't found it!

I have been using Sharpcap and stacking in DSS for a couple for years but have always struggled with what appears to be a degree of residual over-correction of the flats.

I use an ASI1600MM-pro and the problem is common to all my scopes/setups.

Last night I accidentally had 'automatically apply flats' checked and the preview images clearly had the 'crater like' image of a dust bunny (quickly removed!) which emphasised how bad the problem is - the resulting artefact was more noticeable than the dust bunny and its complex gradients would have been harder to correct.

When I stack in DSS I always use temperature matched lights, DSS master dark, Sharpcap master flat and a DSS master Dark Flat, with no bias frames.

Typically, I get over corrected vignetting (brighter corners) and any gradient is reverse, although the effects aren't as severe as in the original image.

I am always careful to get the histogram peak near 60% and use unity gain (139 for the ASI1600).

All the 'cures' I have seen involve making sure you use darks and bias/flats.

Are there any suggestions?

[img]https://content.invisioncic.com/r307508 ... 16.png[img]

Thanks

Neil

Hi,

experience suggests the following

1) ensure settings are the same between dark and light frames and also between flat and dark flat.
2) ensure all of the above are taken with the same version of SharpCap (there have been cases in the past where an SDK update from a camera vendor has changed the meaning of the values of certain controls, that has meant that - for instance - gain 100 now is not the same as gain 100 before the SDK update).
3) test flat correction - the way I do this is to make a flat (and dark flat) and then apply it to another set of flat/dark flat data taken with a different exposure length and a different peak brightness level (maybe 80% histogram if your flats are usually 60%). If your flats are working fine then applying the 60% flat to the 80% flat/dark should give an image of uniform brightness, even when stretched.
4) Remember that if you stretch enough you will get problems no matter how good your flat correction is. Your correction can be accurate to within 0.1%, but it's still possible to stretch far enough that the 0.1% error becomes visible.

hope this helps,

Robin

Hi Neil

To add to what Robin said.
Did you already try to make separate flats in SharpCap and then make the masterflat in DSS?
I had similar problems like you described but once I let DSS make my master calibration files, it was all fine.

Menno

Thanks both,

Tonight I will just capture some single sub flats and see how I get on with DSS making the master.

It's the easiest thing to change.

If that doesn't work, I will regenerate my dark flats as they are the only control frames more than a few months old.

Thanks for the advice folks, it looks like letting DSS do the stacking of the flats is the answer.

https://content.invisioncic.com/r307508 ... 76f309.png

Good to hear Neil!
Have fun

Menno

I have the same camera and ran into the same issue. Like a reverse vignette and dust mote worse than raw lights.

I had Preprocessing done with flats and then applied darks in DSS I took with Sharpcap.

Looks like just make own separate library and don't Preprocessing for night time deep sky. This method works for Ha mono solar imaging for rings very well though.