Flats are overcompensating for vignetting

[turfpit]

Craig

Don't worry about fat or thin histograms - these can vary depending on the camera or FITS software.

As Brian suggests, the flats are ok for a large sensor. Are you applying only flats to your light frames? If that is the case then you need to also capture some darks and apply darks + flats to your light frames.

Given your modern camera you probably need to be applying darks, flats and dark flats. I notice in your ASTAP image that you are using Sharpcap 3.2. An upgrade to SC 4.0 will allow you to capture dark flats along with the flats.

Below is a Sharpcap 4.0 flat capture with an Altair 183C (sensor 5440x3648). Note the option to capture and subtract dark flat frames. Also the master flat displayed in FITS Liberator - note the mean ADU of 29,000.

Dave

SC4-flat-capture.JPG (151.69 KiB) Viewed 608 times

This was the Master Flat generated

master-flat-generated.JPG (79.28 KiB) Viewed 607 times

[CraigG]

Thanks Brian and Dave

More good info! As I mentioned earlier I only take 2 or 3 dark frames for most of my imaging sessions averaging a total of about 1 1/2 hours. I used only 2 for this session as appeared that was sufficient for the ASI2600mc. I'll plan to add more in my next session and see how they mix with the lights and flats. Good to know about the dark flat capture feature in 4.0. I had some initial issues with 4.0 so I've stuck with 3 but I'll revisit.

I'll post a Liberator shot of my latest flat when I get back to my PC to see if you still think that it's good pending more calibration tweeks.

Thanks guys!
Craig

[CraigG]

So here's one of my single flats from early this morning on the patio.

Craig

[turfpit]

Craig

That flat looks good with a mean ADU of ~25,000 and histogram just left of centre. What was the exposure of the flats?

Time to have a proper imaging session with some lights and a set of calibration frames. Deep Sky Stacker allows you to load darks, flats and dark flats. If you use SharpCap 4 then Capture > Flat Frames will guide you through the capture of flats/dark flats.

Dave

[CraigG]

Thanks Dave,

200ms exposure for that one. Well, on to the next round of imaging! SC 4 seems to be playing well with my rig now.

Thanks so much for all of your input!

Best wishes,
Craig

[turfpit]

Craig

Good luck with it. Have a read of Robin's post above re the types of calibration frames you need to capture.

Dave