Reduction of Lateral Chromatic Aberration and Atmospheric Dispersion in Astrophotos
Since the Spring of 2000 I have done some astrophotography with a Televue TV85 and TeleVue's .8x reducer/flattener. This reducer flattener provides excellent APO results with the TV85, with stars right to the vignetted edge of the field showing fine color correction.
However, I find the resulting vignetting and relatively small image scale not to my liking for a lot of subjects, as the TV85's resulting focal length is at 480mm with the .8x unit. So recently I started shooting with a non-apo field flattener originally designed for the TeleVue Ranger/Pronto scopes. This flattener has been known for some time to also work with the TV85, and since it has a slight magnification factor of about 1.1x, it yields a longer effective focal length of 660mm @ f/7.8 with the TV85. Vignetting is also a bit less than with the .8x unit.
The big problem with this 1.1x flattener is that it's not apochromatic, being only a single meniscus lens. The resulting photos, though not perfect, are certainly acceptable but I wanted even better results from the TV85. I think I've found a great way improve the results from this scope/flattener combo, taking advantage of the capability of Auriga Imaging's RegiStar software to slightly deform images to accurately align all of the stars from separate images of the same subject into one perfectly stacked image.
Below are two crops of a small corner of the original scan of my M101 photo, This first image shows the 1.1x flattener's resulting lateral chromatic aberration affecting the stars and a small galaxy in the image.
This second image below shows the results of removing the flattener's lateral chromatic aberration by using RegiStar to do an R-G-B combine of pre-registered R-G-B channels. Definitely a big improvement!
The Method
After scanning the film and doing a basic Levels adjustment in Adobe Photoshop , save that image as a tiff file.
Then open this file in RegiStar. From the RegiStar menu select Operation-Split by channel. RegiStar then produces a grayscale image of each of the RGB channels, with the red as _R, green as _G, and the blue as _B.
I use the green channel as the reference image. To do this, in the Groups Manager, select the green channel _G, and in the Images menu in the Groups Manager, select Remove from Group.
Now go back to the images themselves. Select the red channel image, _R. Then select Operations-Register from the RegiStar menu. The registration window will show the red image as the source image, and the green image as the reference image. Click on Register . The resulting image will be called _R_reg. Next, select the blue image and perform a registration with the green image as you just previously had done with the red image. The resulting image will be _B_reg.
Now comes the fun part. Select Operations-Merge to RGB from the RegiStar menu. In the Merge control window select _R_reg as the red channel, _G as the green, and _B_reg as the blue channel. Click on OK, and stand back and watch your corrected image form as Merge 1! After saving this result, it's back to Photoshop for any other processing you have in mind.
I hope this works out as well for you as it has for me. So far my best results have been with Kodak E200. More blue sensitive films such as Fuji Provia 400F and hypered Kodak RG200 (and probably hypered PJ400 and hypered Supra 400 also) can create larger blue halos around the star images that can't be completely corrected, but are still somewhat improved.
This correction method also works very well to correct atmospheric chromatic dispersion in astrophotos, thus improving the star images in photos taken of targets that are fairly low in altitude.
This will also help correct lateral chromatic aberration in other systems such as a SCT/Lumicon GEG combo when used with the GEG's simple focal reducer lens, and in wide field camera lens photos. However it can't help any severe blue halo problems with an achromatic refractor or with any film halation problems around star images.
--John Boudreau