Histogram and vignette issues

[speckofdust]

Hello all, Could someone please look at the problems I am having with image capture and tell me what I am doing wrong? As you can see from the attached images (labelled "histo......." ), my histogram always has a little bit of unwanted extension in front of the foot of the main curve. These images have been captured with the same setup ( HEQ5 Pro mount, SW Evostar 80ED Pro, ZWO Electronic filter wheel with Baader LRGB filters, ASI1600MM cooled with no field flattener or reducer) and then stacked with Deep sky stacker using dark, offset and flat frames. There have been no manual changes to the position of the camera or the mount during image capture session and usually I would get  all LRGB images in the same session. But the frames are not aligned in the same way when creating individual LRGB stacked final image which leads to that little extension in the histogram curve. I end up cropping the individual final LRGB images to get rid of that little extension and then have to manually align the LRGB images in photoshop.

Also I have noticed quite a significant amount of what looks like vignetting on the right hand side of the images. It is not circular like you would expect to see with vignetting. So I am not sure if this is something to do with anything on the surface of the filters. I have looked at the filters and they look clean to me. Also, the pattern of the abnormal vignetting seems to be the same in the R, G and B images and probably in the L images as well, but it is not seen clearly in L. I have stretched the images to extreme to make the "vignetting" problem obvious. Any help would be really appreciated

[ollypenrice]

In order to get an accurate and meaningful view of the histogram in Levels you need to edge crop the image. If this change in size is going to cause you problems when you want to combine another layer using the same or matching data then, rather than edge crop it, you can exclude the edges by creating a large marquee selection and looking at the histo within it. Continue to process within that selection.  If you still have a gap on the left before the start of the data line just move the black point in to meet it. This will give you the full brightness range for continuing. You don't want any edge artifacts appearing in the histo.

Some CMOS cameras have a reputation for amp glow on the right hand side of the chip if I'm not mistaken. I haven't used a CMOS but a bit of internetting on that term might give you some answers. Why don't flats correct it? I'd guess that their short exposures don't produce as much glow but that's a guess.

With CCD you can use bias (offset) as a universal 'dark for flats' but with CMOS you cannot. You should take darks for your flats (AKA flat darks) on the same settings as your flats. It's just possible that doing so might solve your problem. I'm not sure what purpose your bias frames are serving at the moment. You might be better off without them.

Olly

[speckofdust]

Thanks Olly, your advice on adjusting the histo using the marquee selection really worked well. I was doing edge cropping previously and as I could not find a way to crop all the 3 RGB images in the same way, it created problems with matching the data when overlayed.

I took dark flats as per your advice and restacked the images, but unfortunately that did not do anything about the vignetting/ amp glow. I will certainly look it up to find a solution as it is very difficult to get a good contrast between the target object and the dark sky around it.

Thanks