StarTools

I have an ongoing problem with some sort of vignetting. I've been battling this issue for ~30 imaging sessions. This image is an example.
I will post 2 other sample images in this post. All three are captured with my Canon 60DA attached to the Field Flattener on my f/7 TEC 140 APO ED Refractor. They are of varying exposure lengths, all greater than 60 minutes integration. They are stacked in Nebulosity 3 with 35 flat frames stacked with Bias subtraction per imaging session, and 25 temperature matched darks. All are processed in StarTools to bring out the vignetting.

If not obvious, the artifact I'm concerned about is the irregular blue border around the image.

At first I thought the problem was some sort of optical vignetting, but it finally occurred to me that such vignetting would be more like a rectangle cut out of an annulus, IE the unvignetted portion would be round. obviously that is not the case.

My next thought was that I had missed some stacking artifacts. I reprocessed the image and verified that all of the stacking artifacts were cropped out. The artifact remained. So I tried another image and did a large crop. Here is the result;
So I took a much larger crop of the same data stack. Though diminished the vignetting is still there. By the amount of data that is cropped out it cannot be caused by stacking artifacts.
My goal is to find the cause of this problem and eliminate it. I can process it out for the most part with the crop and wipe modules, but I am concerned at the amount of data that I lose with the gradient. BTW I use the Vignetting settings in the wipe module to resolve this. So, does anyone have any ideas on what I can do to bring an end to this problem?

For anyone interested, here's a link to the stack that the M78 image came from so you can give it a go yourself.
https://drive.google.com/file/d/0BzTNnH ... sp=sharing

Most peculiar...

I can see the issue even in an initial AutoDev.

Does the issue persist if you eliminate the flats and/or bias frames from the stack? I detect a presence of some faint, large scale horizontal and vertical bands, which could indicate problems with either...

admin wrote: Does the issue persist if you eliminate the flats and/or bias frames from the stack? I detect a presence of some faint, large scale horizontal and vertical bands, which could indicate problems with either...

I don't know. I'll reprocess tomorrow as suggested and get back to you. Can't do it tonight, there's a good ice hockey game on TV.

I think I accidentally found the source of my nemesis, the dreaded Blue Fringe. It seems to be light pollution. I still don't understand how it manifests as just a fringe of gradient, but knowing the source may help me, or others with similar gradients, figure it out. My reasoning for believing the cause to be light pollution is as follows.

Tuesday night I imaged The Horsehead and Flame Nebulae, then M97 and M108. I started with the Horsehead. When I started my imaging session the target was about 48 degrees altitude almost at the meridian. I captured twelve 600 second light frames. Next I moved over to M97 and 108. When I started that capture plan the target center was at 47 degrees high to the northeast. I captured seven 600 second lights. I planned on 12 but fog was creeping in on me and I decided to call it a night early. All of the light frames were captured at 800 ISO.

The temperature was fairly stable all night so my master darks were within 2 degrees F of each other. I used many of the same dark frames for each master dark. I did not change focus or camera orientation between targets so I used the same flat frames for both targets. I use a library master bias so that was also identical between targets.

I processed these images as nearly identically as possible. Bin, autodev, crop, autodev, wipe with default settings, then dev and home in until StarTools settles on a number. I bumped up the percentage a bit on the M97 M108 about 3% just to bring out the Blue Fringe a bit. I found no trace of it in the Horsehead data. Here are the resultant images.
As all circumstances of the image processing is as near identical as possible, the only significant difference I can think of is the relative light pollution levels between the targets.

To the south I have a mountain and state park, with the nearest significantly light polluted area about 20 miles away. To the northeast I have a much lower hill with a light pollution concentration about 5 miles away. So my hypothesis is that the light pollution levels produce the Blue fringe. My habit is to start my imaging sessions as close to 40 degrees altitude and to the east so I get the darkest skies early and have more capture time before meridian flip. This might explain why this is a recurring issue with my data.

I would love to read arguments to the contrary, and any ideas on why this doesn't result in a gradient that has a higher concentration on the side of the image that is relatively nearer the ground (IE down) instead of this vignette like border that I get consistently.

Odd (and slightly intriguing). The only thing I can think of is some sort of reflection, internal or otherwise.