SharpCap Forums

Hi there,

I noticed an issue with using Sharpcap for creating master flat using sky flat approach. The issue is simple -- the individual flat frames and bias frames (and even the master bias generated) are pretty good but the calibrated master flat over corrects.

I tested a couple different ways/ First I generated a master bias in SC Pro at pre-sunrise conditions. Next I collected 50 flat frames at 100 gain and 300 ms exposures for the zwo 2600 camera using SC's flat tool.

1. stacked the frames in Sharpcap's process, to generate a master flat
2. used the WBPP process in PI to stack a master flat

I compared both with a synthetically generated flat that I worked on iteratively to calibrate about 5 different stacks of star fields and faint nebulae.

Master flat generated in Sharpcap consistently over brightened the edges.

Master flat generated in PI was perfectly on point, within a couple ADU's for the edge in the raw stack.

So something in the stacking process works differently in SC Pro vs WBPP in PI. It could be the default settings especially for variance handling but I am not sure.

Windows 11 25H2 Pro
ZWO 2600 MC Pro
SC 4.14 from April 20, 2026

Hi,

are you using SharpCap's master flat in other software? If so then that is the problem - SharpCap pre-subtracts the bias from the master flat before saving, which I don't think the other software will be expecting, so then the subsequent processing using a bias level again would be inappropriate. I'd usually recommend running the entire processing pipeline in one application (ie everything done in SharpCap, or everything done in PI or AstroAst or DSS, etc).

Another common cause of over/under subtraction is using a dark frame that is not properly matched to the light frames when using the flat correction (this applied either for flat correction inside SharpCap or outside). If there is no dark frame, SharpCap will 'try its best' using the bias level stored with the flat frame data, but that only works out if the camera bias level has not changed since flat capture and the thermal noise is practically zero.

cheers,

Robin