Gradient with and without calibration frames under full moon

[cdanil]

When I stack DSO raw frames obtained under a nearly full moon, the gradient and amp glow are quite more noticeable if I use dark and flat frames vs. stacking without them. This is both with and without linear gradient removal. Any logical explanation?

[admin]

Hi,

the only thing that comes to mind is that if you have corrected for dark and flat then you might tend to apply a stronger stretch to the image, which would make the remaining imperfections in the image seem brighter. Not sure about that though. Perhaps a couple of sample images might help show exactly what you are seeing...

cheers,

Robin

[cdanil]

https://1drv.ms/f/c/5aa77654af78c39d/Et ... g?e=Je35Yv


Here is a link to download the files. Yes, a stronger stretch might explain it. Why would it be stronger with calibration applied? I just blindly alternate clicking Auto Stretch and Auto Color Balance until the appearance of the image stabilizes somewhere I like...

[admin]

Hi,

thanks for sharing - I ran through the stacking both with and without the dark and flat frames to see what was happening, and it soon became apparent that the dark frame is over-correcting (particularly for the starburst amp glow). That starburst glow shows up as a bright area in the normal images, but once the dark frame is applied, you see a dark copy of the starburst in that same position.

I had a quick look at the capture settings files for the individual frames and the dark frame, and it looks like it comes down to the darks being captured at a much higher gain (300) compared to the light frames (125). ZWO cameras measure gain in 0.1dB increments, so a difference of 175 means 17.5dB or a factor of about 7.5 times brighter.

Your best bet would be to capture a new master dark frame at settings to match the light frames (30s exposure, gain 125, WBR=52, WBB=95, Brightness=8), then re-process and see how things turn out.

cheers,

Robin

[cdanil]

Thank you Robin. Rookie mistake. However, what’s also going on is that the flats are overcompensating for vignetting, which seems to be a fairly common problem. You tackled it in a recent thread and it’s been discussed elsewhere. I’m still reviewing the exchanges as it’s not clear at all to me how to proceed. I think I’m doing everything by the book. I just reshot my darks and flats and the problem seems to have gone away, although I don’t see what changed. I use a ZWO ASI294MC Pro cooled to -10C. My recent lights are 30s with 300 gain. I reshot my darks with the same setting. The flats were shot against a cloudy sky covering the telescope with white fabric, using Auto Adjust Gain/Exposure that yielded an exposure of 1 sec and a gain of 269 (thus following your recommendation to avoid the 120-200 range for this camera); ticked “Create Monochrome Flat Frame” and “Capture and subtract dark flat frames”. Brightness set at 8 for lights, darks and flats. My images are pretty nice (to me at least!) without flats or darks, so this more a case of “curious minds want to know”. Previous flats were shot with an exposure of 2.5s and a gain of 250, so similar, but the light source was a LED light tracing panel, rather than daylight. Maybe that’s significant?

[admin]

Hi,

I think the best thing to do is to get the dark subtraction sorted out and then re-test to see how that affects the flat correction. Incorrect dark subtraction will definitely make flat correction go wrong (the dark is supposed to take out the part of the signal that didn't come from photons entering the telescope, leaving a signal that is just due to photons entering the telescope which can be flat corrected. If the dark is wrong then the data you get after subtracting the dark is wrong, so the flat correction gets applied to the wrong input data).

With a little luck you'll find that fixing the dark correction will also help the flat correction work better. If not then we can look into the flats again with confidence that we have the right data to hand.

cheers,

Robin