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Help with double-colored stars due to low-angle Lights

12 Oct, 2016 23:56
I am stacking my M16 data from last month - since the object has been setting, I would image it from about 9-11pm, earlier in the later half of the month. I would typically stop once it reached 25 degrees or so, but even so my stacks have red/blue stars due to being relatively near the horizon.

I use Star Tools for processing, but have access to Lightroom and Photoshop CS2 as well.

So far my solutions have revolved around stripping down bad data. Most of my Lights are good, just diffracted. But even removing the latter half of all my data for each night, so I could stack only the earliest / highest-in-the-sky images still shows stars with dual color.

Does Star Tools have any fixes for this? Perhaps some type of tool which radially blurs/averages out color for a star to counter this effect?
13 Oct, 2016 17:45
I'm very interested in this as wel!  Jon Talbot and I spoke at length about this being an unsolved processing problem in Astro photography. Typically a star mask and a reduction curve in whatever color is fringing is what I use now. It works but not quickly or as well as I'd like.
13 Oct, 2016 18:15
For this dispersion problems, you should just have to deconvolve and realign/register per channel.
There are multiple"sources" for this, including the atmospheric dispersion, the lateral and longitudinal chromatic aberration of the optics (including from the filters). Thus it will depend on what registration model your software is implementing : if it is only a rigid model (typically, rotation and translation) it might not be sufficient to correct the fringing. The per-channel deconvolution is mandatory if the fringing is centered, with for instance a large red or blue halo around starts (depending on your focus position).
For the software, I am not sure, but I guess PI would have this kind of tools.
13 Oct, 2016 18:46
The deconvolution per channel is something that should definitely help and I have done this in extreme cases before. It is a really good point and something that is time intensive but quite robust. Ill have to look into and experiment with this more.

Since i I tend to shoot pretty high in the sky and have very well corrected system, I've always attributed the issue somewhat to sky gradients from very nasty lp that I shoot in. I wonder if there is some validity in this or not.
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