Over Correction

[Stub Mandrel]

I've got an issue with over-correcting flats in Sharpcap. I'm fed up of relying on Gradient exterminator!

 

I'm also posting this on the SHarpcap forum.

 

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 - see below!

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?

 

Thanks!

 

[AstronomyUkraine]

4 hours ago, Stub Mandrel said:

I've got an issue with over-correcting flats in Sharpcap. I'm fed up of relying on Gradient exterminator!

 

I'm also posting this on the SHarpcap forum.

 

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 - see below!

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?

 

Thanks!

 

I have the ASI 1600 MM. I use a cheap light panel to take my flats, but I use the flats wizard in SGP. My histogram is always around the 30% mark from the left for my flats, and I've never had any problems with calibration. I never use bias frames with my 1600, just flats and dark flats taken at room temperature, and darks taken at the same temp as the lights. I always use unity gain for my NB filters. I set the ADU in SGP to 20,000 in the flats calibration wizard and my exposure lengths for my flats is around 25s for SII, 10s for Ha and 4s for OIII. Not sure if you can adjust the ADU in Sharpcap, but a lot would also depend on your brand of filter for length of exposure, mine are 7nm filters.

[Stub Mandrel]

After some advice I got yesterday, last night I took new flats and let DSS stack them rather than doing it in Sharpcap.

 

The difference is striking (not much to see in this image, due to haze):

 

[Greg M]

I have found much better results with my flats at between 30-40% ADU rather than 50% or higher.

Also, FWIW, according to Adam Block, don't use BIAS frames with most CMOS cameras, use Dark Flats instead.

[AstronomyUkraine]

14 minutes ago, Greg M said:

I have found much better results with my flats at between 30-40% ADU rather than 50% or higher.

Also, FWIW, according to Adam Block, don't use BIAS frames with most CMOS cameras, use Dark Flats instead.

I set my ADU at 60% and have never had any problems. Will try lowering it to see what kind of results I get.

[TerryMcK]

I also set my ADU around 30% for flats too. I have a different sensor, having the ASI183MM and MC, but don’t have any issue at that setting.

I only shoot flats with an illuminated flats panel and take darks at the same temp as the lights for calibration. No bias frames at all.