Curing Blue-Purple Star Halos

As I mentioned in a recent post, I am tinkering with using an Orion ST-80 refractor telescope for EAA and astrophotography, mainly asteroids right now. This telescope is quite inexpensive, with new ones selling for $110. Unfortunately, to keep the price low, these telescopes don’t use the expensive glass and multiple lenses necessary to produce images free of chromatic aberration and field curvature defects.

As is typical of images from simple achromat refractors, the star images in my ST-80 showed the large blue-purple halos around stars caused by chromatic aberration. And, images showed heavily distorted stars outside the center 30% of the field of view caused by field curvature.

To avoid sinking big bucks into an 80 mm APO refractor with field flattener and focal reducer, I opted to adapt my existing ST-80 to bring these two optical issues under control. To fix the ugly blue-purple halos, I added a Baader Fringe Killer filter in the optical path between the telescope and camera. The filter worked much better than I expected, but still left a bit of a residual blue-purple fringe around the brighter stars. I don’t find this small residual fringing too bothersome, and am inclined to just leave it alone most of the time. However, I just found a tool that will eliminate these blue-purple halos completely in post processing.

I use the freeware GIMP for post processing and just happened across a plugin for it, that is not specifically intended for astrophotography, but has one tool that is perfect for eliminating purple halos around stars. The plugin is called G’MIC-Qt, and the tool is a repair filter called ”Unpurple.“

Below are two live-stacked images of the Perseus Double Cluster taken with my ST-80 and ZWO ASI482MC one-shot-color camera in which the “raw” image with blue-fringed stars alternates with the G’MIC “unpurple” filtered image. The first image is the full 1.6° x 0.9° field. The second image is a cropped and enlarged view of the center section of the field of view. The “unpurple” filter has a number of adjustable parameters, but for the image here I just used the default settings. The basic image consists of a stack of 90 ten-second images.

I’m pretty impressed with this tool. If you are thinking about using an old ST-80, or other inexpensive refractor, for astrophotography, you will need to solve the blue-purple fringing halo problem. I suspect PixInsight or Photoshop, or both, have something similar to G’MIC’s “unpurple” filter, but, I’m always looking for ways for doing this hobby on a budget. In this case, you can’t beat the price. Both GIMP and the G’MIC plugin are freeware.

G’MIC-Qt can be downloaded here. It is very easy to install into GIMP, and it is very easy to use. A short two-minute YouTube video showing how easy this tool is to use for star images is here.

Full Field View


Cropped and Enlarged View


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Great tips, Russ! Thank you!

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After that, you need to knock down the green a bit.


You’re right. The green definitely needs taming. This is basically the live-stacked image that dumped from the camera onto the computer. No post processing other than the “unpurple” filter. If you press your ST-80 into service, this GIMP plugin could help.

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