I would be interested to hear some suggestions on how I can improve the quality of my stacked LRGB images. Note that in portion of the LRGB image below you can see that there is a red halo around all of the stars. This is giving my images a distinctive reddish hue. I am pretty sure that this halo is there because in most of my images, the stars in the Luminance and Red sub-frames are always larger than the corresponding stars in the Green and Blue sub frames. I always use the same exposure time for my sub frames. In the example below, the FWHM for the large star at the top of the image is: L - 3.818, R - 2.882, G - 1.934, B - 2.205. The saturation level for the same star is: L - 65535 (completely saturated), R - 48743, G - 65535, B - 27718. I can not rule out focus or seeing as the reason for the difference in the size of the stars, and I am continually trying to improve my focusing techniques, but should I be using different exposure times for the different filters ?
What kind of telescope are you using? Some of the larger Schmidt-Cassegrains have spherochromatism because of their single-element corrector. I used to have a Meade 16" SCT (the classic optical design) and it was pretty bad from this perspective. I'd have to refocus between red, green, and blue and really couldn't use luminance at all. It looks like the color image planes aren't perfectly aligned with each other. Make sure you're using Auto Star Matching or Astrometric alignment mode. (If you are guiding you may still need to realign... also I always recommend dithering in any case as it helps remove the effects of hot pixels.) Saturation can cause issues for stacking cleanly. Usually this only affects the very brightest stars in the image. In some cases you might want to use shorter exposures and stack more frames, but it really depends on the target.
Most optical systems have compromises to try to bring the range of wavelengths to focus at the same spot, but they're not perfect. So this is why we have Filter Offsets - values that can be added/subtracted to the focuser position to allow for differences in individual filters. Ideally, you'd work these out, OR refocus for each color.
Thanks Doug & Colin. To your questions & comments: The imaging scope is an Astro-Physics 180 EDT with a Quad TCC telecompressor / flattener (1166.4 mm effective focal length). The scope was recently sent back in to Astro-Physics to get some upgrades and to have the lens checked. Roland Christen checked the lens out himself and said it was in good condition. The imaging camera is a SBIG STXL16200 with the FW8G-STXL filter wheel and the 8 filter set that came with it. The mount is an Astro-Physics 1100 GTO (non absolute encoder version). The guide scope is a Tele-Vue Pronto (480 mm focal length) piggybacked on top of the 180 EDT. The guide camera is a SBIG ST2000-XM (main imaging chip). Guiding and imaging is via Maxim DL Pro (version 6.25). Stacking is also via MaxIm DL using Astrometric alignment and median combine. There were no anomalies or errors encountered during the alignment process. The final image is a stack of 24 X 600 second sub frames (six through each filter; L, R, G, B). I have been using a Bahtinov mask for focusing, and I have been checking and refocusing whenever I switch to a different filter. I do suspect that the seeing may be a factor. I have read that this is an issue with longer focal lengths, and that if the seeing is bad it can be difficult to get good focus. The L and R images were taken on the same night, and then the G and B images were taken the next night.
You can get remarkably different seeing conditions from night to night. That might be a major factor here. Telecompressors tend to amplify optical aberrations of all kinds. That could also be a factor. You might want to try doing a set of image on the same night and see if you get consistent FWHM across all the filter bands.
