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.
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- 2015_01_20 Horshead Neb V1 ST Vig.jpg (495.97 KiB) Viewed 5030 times
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- 2015_01_20 M97 M108 Neb V1 ST Vig.jpg (493.44 KiB) Viewed 5030 times
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.
