Newbie question regarding flats

[PetePeterson]

Been running a new ASI2600MC with SharpCap for 3 months now and just this week upgraded to Pro. The Histogram color control and the FWHM focus routine are super, and I did manage to calibrate the camera as well. Have been doing astro photography and scientific imaging for 15 years and know how to use darks and flats. But I find the file system in SharpCap to be a puzzlement, and am having a problem figuring out SharpCap flats.

Here's what was supposed to be a flat taken last night:

It'd be really handy to know the file exposure time, temperature, gain used, etc in a file title as I've no idea as to what's located where and what it is. Am I correct in guessing that the software automatically finds the proper dark and flat file depending upon the camera setup?

Believe darks are functioning. But the image is of a 1 minute DSO exposure using a flat taken earlier in the evening (the bright glow in the center is a monitor screen reflection). For this exposure darks were turned off. Will be taking another set of flats ASAP but it'll be a week before the sky is forecast to clear once again. Any thoughts as to what I've done wrong here?

Feedback much appreciated.

[admin]

Hi,

that's certainly looks more like noise than a flat frame

Below are a couple of screenshots that indicate what I'd expect you to see while first capturing a flat frame and then see after the flat has been captured and it when it is then being applied to each frame (assuming you haven't changed the illumination yet).

It's worth checking the histogram when you're capturing the flat to make sure that all the peaks are well separated from the left and right hand sides. Also check your camera settings are sensible – for instance if you have been using the stretch in the mini histogram, turn that back to defaults to avoid the stretching of the image distorting your view of what's really going on. It would probably be worthwhile including the control area in any future screenshots if you're still having problems.

Cheers, Robin

captureflat.PNG (779.55 KiB) Viewed 942 times

withflat.PNG (769.35 KiB) Viewed 942 times

[PetePeterson]

Thank you Robin. Thank you very much. Success.

First, the camera was wrapped to factor out possible light leaks. Then the flat field exposures were done for 3 seconds rather than 1/2 second as somewhere I'd seen comments that super short flat exposures don't work Following your directions closely, and apparently successfully, here's a screen shot doing a twilight flat with T shirt:

Concentrating so hard that in this instance I'd neglected to check the bias frame box.

And here's the resulting flat frame:


This is the end result:

Meade 14" @ f/8, ASI2600MC-P 19 X 2 min, -15°C, gain at 200, darks & flats, 2 sec PHD2 guiding

Still trying to understand your lecture regarding optimization of exposure, but this short series of 2 minute exposures under my urban sky is significantly better than a longer series of 5 minute exposures. For a total exposure of only 38 minutes it looks pretty good to me.

[PetePeterson]

A few weeks ago I’d centered the exposure histogram and the flats were useless. Someone pointed out that at the time that due to using a focal reducer I had major vignetting in the corners and such a severe variation may be throwing off the flat field calculations. The focal reducer is now off.

An ADU level histogram of 50% didn’t work with the focal reducer setup. Subsequently flats were run at 30% and 70% well depth. The low ADU flats produced black doughnuts and the high ADU flats produced white doughnuts! Can you see where this is going?

On a very dark gray morning at 07:45 so I opened the shutter, fired up the camera, and modified the T shirt frame, adding an additional 2 layers so the camera’s not oversaturated. A 1 sec exposure 100 frame stack of 50% ADU was run for yet another trial. And eureka! It worked.

Conclusion: Centering the histogram is the way to go but the resultant flats probably won't be usable if there's severe vignetting.

[admin]

Hi,

All sounds sensible – flat correction is a great tool, but it can't work magic. Once the variation in flat field brightness across the frame starts becoming extreme (probably anything much more than a factor of two) then you have to be very likely to get to work properly.

Cheers, Robin