Strange color around edges of images

[ionia23]

Hello,

Included here is a stacked image of NGC 7129. I took a screenshot of the image stretched from Autodev, default settings. No additional processing at this point other than cropping out stacking artifacts from the edges.

There's this very strange greenish color that runs around the edges of the image. I've seen this in quite a few of my shots. While I've had a few ways to counter it, I've never been able to get a source. Regardless of external ambient lights, this appears in anything. I'm not sure if this is amp glow, some buried hot pixels, or whatnot.

The images were captured on an Orion Starshoot G4 Color OSC. I'd appreciate any thoughts as to what the source of this greenish glow might be and/or settings that may make its removal easier in Wipe.

Green Side Glow.jpg (272.33 KiB) Viewed 2181 times

[Mike in Rancho]

I am currently working on something like that myself. Sometimes I get what I think is sensor heat glow, mostly during the summer though, and that was with my refractor too. Most of it is along a couple edges, and with an uncooled DSLR that is known to have problems making effective dark frames, well, one can try to get lucky. Sometimes they work. Other than that, changing the gradient edge options in Wipe is sometimes a huge help (though I think there needs to be a fourth option).

Using my Newt lately though, I've had issues that look much more like yours, though not exact. And of course the temps are cool for winter.

I believe this is due to light leaks, and am currently trying to engineer some solutions. I can see from the star spikes that you are using a reflector of some type?

With my Orion 6" f/4, I also realized that I was getting very similar patterns - most all of it in the green channel - night after night, regardless of what direction the scope was pointing, where the LP domes were, or how the camera was rotated for framing. So I put the cap on the scope and started taking some test darks, inside the house with a little bit of ambient light. I gave the darks some big stretches in Gimp to see what they looked like.

Holy moly light leaks galore. So I started testing it out further. The dust cap leaks but that was easy to cover. The backside of the scope leaks quite a bit, so I built a sheath that slips on to cover that up. Will be easy to slide on after the scope cools down and I am ready to image. One of the biggest problems I found though was the edge of the drawtube where it goes into the housing. That's the hardest to get covered up, since one needs the focuser to move!

I haven't been able to test things out yet, but hopefully will soon. My theory is that stray light, from the neighborhood, the moon, whatever, has been leaking into the back end of the scope and the edge of the drawtube, and as things move throughout the night, that light leak just gets smeared about a bit and ends up creating these strange patterns.

Honestly at first I thought it was the red channel, as I would also have this big red streak across my images. But turning off the channels I realized that instead it was "absence of green," that streak just being a part where the light leak hadn't smeared over.

Anyway, I believe these leaks are more than enough to wreck the images when we try to stretch out the target. You might try taking some test darks to see how sealed up your scope is. I mostly just used 30 seconds at ISO400, which was sufficient to really reveal when the scope was leaking and when it was leak-tight.

[Mike in Rancho]

A follow-up, Ionia,

My external stray light blocking has only produced minor improvements.

Still needs to be done, however, I think. I do need to work out how to engineer my focuser shield. Right now it is a bit unwieldy, so I think I will look for some rubber innertube or the like to fashion something better.

Anyway, I went back to the drawing board and started working on my flats. My next theory is that I might be getting stray light bouncing all over the place inside this Newt, and probably eventually up the drawtube. This thing actually might need some baffles?

For my test flats, I moved back from my light panel about three feet instead of close to it, and made a diffusing shield of several pieces of white paper to fit around the end. Then, totally in a dark room and with the scope and camera otherwise fully covered by dark towels, I took sets of flats. They needed to be about 6 seconds long despite the panel being almost at it's brightest setting.

Using these flats actually made a noticeable improvement with the oddball gradients and edge colors, compared to the exact same lights using my original flats, and the same settings in Wipe.

My next step will be to improve my diffuser, and look around inside the scope for possible reflective spots. This could be coming in during my lights as well.

Sigh. Goofy Newt.