NGC 3372 - Carina Nebula
First light with the Antlia EDGE 4.5nm SHO filter-set 

For Christmas, I received my first SHO filter-set. I had purchased an ASI2600MM mono camera last year, and have been testing it out with LRGB images, but the SHO filters were the real point of going mono, so I was excited to receive them. However, the moment I unboxed them, clouds set in, and it stayed that way for more than two months.

Finally, last week, the clouds parted, and I managed to capture some data for my first SHO image. I chose the spectacular Carina Nebula to test the filters out, and I managed to capture two nights of data. Seeing on the first night was reasonable, but seeing on the second night was excellent. Imaging was performed with my heavily-modified GSO 6" f/4 Newtonian.

After capturing roughly 40 frames each of SHO, and some token RGB frames for star colours, stacking was performed with WBPP. I was pleased to see a sharp stack (with FWHM ranging from 1.8 - 2px across the three channels), giving me crisp data to work with.

The individual stacked lights were then run through BlurXTerminator, linear fit was applied, they were stretched identically with GHS, light noise reduction was applied to the shadow areas, and some very gentle sharpening applied to better bring out edge definition. As I wanted to experiment with different SHO combination techniques, I kept the individual channels separate for as long as possible.

There are two revisions of the image:

Natural SHO
https://www.astrobin.com/bbmyhp/0/

I have selected this as the main image, as it is a more natural presentation of the data. This was combined with the NB Colour Map script developed by Mike Cranfield and Adam Block, and uses a quite natural palette (Ha purplish-red, SII orange, OIII teal). The relative strengths of the channels were selected to best show the contrast between the emission lines, while still having a natural appearance. Thanks to Rich for suggesting this tool, I really enjoyed using it.

RGB SHO
https://www.astrobin.com/bbmyhp/M/

For this combination, I decided to use the full RGB colour space, leave in the green, avoid SCNR, and do everything I could to show separation between emission lines in vivid colour. I applied SII to red, Ha to green and OIII to blue with minimal blending. Then I colour-balanced the image in Affinity, while carefully observing the result to ensure it maintained colour separation. In doing this, I found  the obvious: the more I suppressed the green channel, the less clearly separated the Ha signal was, so I left plenty of green in there.

While the result may be a bit garish for some, the approach does at least do a better job at highlighting the relative intensity of the various emission lines, with areas with especially dominant Ha highlighted with a greenish tint. In particular, I like the watercolour-like effect in the bottom-right of the image.