My second attempt at narrow-band SHO imaging was the Elephant’s Trunk Nebula, IC1396. I’m pretty pleased with the way this one turned out. This go round I shot 10-minute subs instead of 5-minute subs. I was able to shoot a total of 22.5 hours of data (7.5 hours for each of my Optolong narrowband filters–hydrogen-alpha (7 nm), sulfur-II (6.5 nm), and oxygen-III (6.5nm)–through my mono camera, the ZWO ASI533MM Pro. I shot through my Sky-Watcher Esprit 100 f/5.5 refractor on its iOptron GEM45 mount.
Once again I processed using PixInsight. I think I’ve settled on a recipe for turning the mono images from each filter into a single color image.
- Preprocess with the Weighted Batch Preprocessing (WBPP) script
- Crop slightly
- Use the LinearFit process to equalize the brightness for each filter
- Run NoiseXTerminator to reduce noise, as needed
- Combine the linear images using the ChannelCombination process (for SHO palette, SII=red, Ha=green, and OIII=blue)
- Run StarXTerminator to produce the linear starless image and the star-only image
- Use HistogramTransformation on the starless image, stretching the color channels separately to obtain the desired image colors Generally, Ha will dominate the combined image, causing it to be mostly green. Stretching the color channels individually allows me to reduce the green and get the blues and golds that I usually prefer.
- Use CurvesTransformation on the starless image to make more fine adjustments to color and saturation.
- Follow the same process with HistogramTransformation and CurvesTransformation to stretch the star-only image and adjust the star colors (which are often quite magenta to begin with). Processing the star-only image separately also gives you the opportunity to do star reduction by simply not stretching them so much.
- Use PixelMath to combine the starless and star-only images.
Playing with the image in CurvesTransformation is always a bit of a trial-and-error proposition, as I’m not always sure exactly what I want the final image to look like. So I play around to see what happens and generally settle on something that I find appealing.