Andromeda Galaxy (M31) The Saga Continues: Siril + APP.
UPDATE: I want to thank Mabula Haverkamp, author of Astro Pixel Processor (APP). Beginning with version 1.075 APP successfully processes this image. The original story was written using version 1.074. As you read you can see that I had to fallback to using Siril which is no longer the case. The image shown above is part of the original story which used Siril for star alignment and stacking. While it was successful the alignment was not perfect. If interested you can see the high-quality APP image here: https://u235.herokuapp.com/#lrgb-exposure and scroll two-thirds of the way down.
Original story:
The night of this shot I was blessed with excellent seeing conditions, a rarity in this part of the country. The atmosphere was calm. Stars were stable. On most nights however the atmosphere is quite turbulent, causing starlight to rapidly twist and turn.
Sounds idyllic, right? Well, yes and no. On the one hand I can capture some very fine detail which normally would be lost to poor seeing conditions. On the other hand however the small size of the stars on the camera’s sensor can lead to a condition called “under-sampling”.
Ideally you want the average brightness star to cover a 3×3 area of pixels. With average to poor seeing conditions this is no problem but with excellent seeing conditions the star may only cover a 2×2 area.
After capturing the image at the telescope I began processing with Astro Pixel Processor (APP). Almost immediately it complained that it could not find enough stars in the color frames! I was shocked but not surprised.
When I purchased this CCD camera I knew that it had a tendency to under-sample (see notes at the end of the article.) This condition is exacerbated when capturing color frames using bin2 mode. Bin2 can dramatically reduce the exposure time of color frames but there is a downside. It cuts image resolution in half because it reduces each 2×2 matrix of pixels to one pixel.
This isn’t as terrible as it sounds. I think it was Trevor Jones at AstroBackyard who made the analogy of a child and his coloring book. The child provides the crayons and the publisher provides the detail in the form of the outline. If the child draws a little outside the outline the picture still looks good. If he gets sloppy it gets worse but is still acceptable especially if seen from a distance. This is the analogy that Trevor made with binning: bin2 is like the child drawing a little outside the outline, and bin3 is sloppy but acceptable. Remember that the outline is the job of the luminance filter running at bin1 and since the sensor sees three times more photons than with a color filter the exposure time is short.
So Astro Pixel Processor did not like my bin2 color frames. I checked the log file. Initially it said it found 100 stars but ultimately decided that only 4 of them qualified as real stars. Apparently the software looks at the star’s profile. Since there were so few pixels it failed.
I contacted my friend David Richards in the UK. He suggested I try Siril. I dedicated several hours and took a crash course from the online tutorial guide. Soon thereafter I had a reasonable looking final image. Thanks to Dave he helped me with some of the finer points of using Siril and now I have this wonderful image that you see here.
The subtitle of this blog post is “The Saga Continues”. It never ends but with regards to this image there is much more to say. I’ll leave that to a later post.
Technical Details:
William Optics 71mm f/5.9
Atik 314E CCD
Optolong LRGB filters
Luminance: 38x120s bin1
Red: 18x90s bin2
Green: 10x90s bin2
Blue: 17x90s bin2
Bias: 100 each for bin1 and bin2
Darks: 50 each for 120s bin1 and 90s bin2
Flats: 50 each filter
Total Integration Time: 2.4 hours
Siril for calibration, stacking, and color balance.
APP for histogram stretch and sharpening.
Note: Before purchasing an astronomy camera, no matter if it is CCD or CMOS, you should make sure that you match the camera to your telescope. Astronomy Tools has an excellent resource called the CCD Suitability Calculator. Scroll to the bottom of the page. There are two boxes I want you to fill in. Focal Length: 418. CCD Pixel Size: 4.65. Notice the warning: “this combination leads to slight under-sampling.” Now change CCD Binning from 1×1 to 2×2. Notice it goes off into the red. Ideally for 1×1 binning you want the indicator to be at the lower end of the green region and for 2×2 binning at the higher end of green. For my telescope that would be a camera with a pixel size of about 2 microns.