This is an experiment in 4-color processing using two 5nm dual-narrowband filters: Antlia ALP-T Ha/[OIII] and Antlia ALP-T [SII]/Hb. I wanted to see if you really could put the [SII] and Hb signals to use beyond the more traditional bicolor HOO processing. This is also my first workflow using StarXTerminator (or any star removal) to separately process the nebulosity. I'm writing this out to share that 4-color processing with OSC and dual narrowband filters is indeed very possible in AstroPixelProcessor and Photoshop.
Over several nights, I gathered 11 hours (132 x 5 min) on each panel using the ALP-T Ha/[OIII] filter, and an additional 8 hours (96 x 5 min) on each panel with the newer ALP-T [SII]/Hb filter. Total integration across all filters and two panels is 38 hours (456 x 5 min).
Then in AstroPixelProcessor:
- For each panel, extract and integrate Ha, [OIII], [SII], and Hb separately, and remove light pollution.
- Stitch separate Ha, [OIII], [SII], and Hb mosaics.
- Register and normalize the mosaics
- Combine RGB. Here's my formula in AstroPixelProcessor, relying upon the better signal in Ha and [OIII] and using [SII] and Hb for deeper colors:
- [SII]: x2.5, (R,G,B) = (50,5,1). L = 0
- Ha: x1.25, (R,G,B) = (50,15,1). L = 20
- [OIII]: x2.5, (R,G,B) = (1,25,75). L = 0
- Hb: x2.5, (R,G,B) = (1,5,25). L = 0 .
- Save with 20%, 3 sigma stretch, saturation at 0.30
In Photoshop:
- Use StarXterminator to remove stars
- Process stars:
- Use difference from original to get a star image
- Use levels to adjust the black and white points for each color
- Use a nonlinear curve that maxes out at about 75% brightness, with a concave down shape
- Process nebulosity:
- Use NoiseXTerminator (60% denoise, 60% sharp)
- Adjust black levels for R, G, B separately.
- Use nonlinear curves to differentially boost R and B.
- Boost vibrance
- Slightly increase contrast
- Adjust colors in Hue/Saturation to further separate colors
- Shift red -5 to -10
- Shift yellow -5 or -10
- Leave Green
- Shift cyan -5
- Shift blue +5 or +10
- Leave Magenta
- Combine:
- Copy stars as a new layer
- Set star layer to linear dodge (add)
- Adjust curves and levels as needed.
![Lion Nebula Mosaic: 4-color [SII]-Ha-[OIII]-Hb with OSC, Paul Macklin](https://cdn.astrobin.com/static/astrobin/images/placeholder-gallery.811cd2122597.png)
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