North America Nebula: An Alternative Processing Approach

This post explores an alternative approach to processing OSC dual-narrowband data, aimed at achieving a distinctly different simulated color palette for the iconic North America Nebula. As a relatively new astrophotographer, this marks my third processing attempt, and I'm always eager for feedback and suggestions!

Equipment Used:

• Scope: Apertura 90mm Triplet w/1x Flattener
• Mount: SkyWatcher EQ6-R
• Guidescope: Svbony 60mm
• Main Camera: ZWO ASI2600MC
• Guide Camera: ZWO ASI120MM
• Focuser: ZEO EAF

Acquisition Details:

• Exposure: 34 x 180 seconds (1.67 hours total exposure)
• Temperature: -10°C
• Gain: 100
• Acquisition Software: NINA
• Guiding Software: PHD2

Processing Workflow:

The journey to this final image involved a series of meticulous steps:

1. Siril OSC Preprocessing: The initial steps involved standard preprocessing routines in Siril to clean and calibrate the OSC data.
2. GraXpert: Noise reduction and artifact removal were performed using GraXpert.
3. Channel Separation in Siril: The processed image was then taken back to Siril to separate the Red, Green, and Blue channels.
4. Channel Assignment for Narrowband Simulation:
   • The Red channel was designated as Hydrogen-alpha (Ha).
   • A new image simulating the Oxygen-III (Oiii) channel was created using PixelMath with the formula: Oiii = 0.5*G + 0.5*B.
5. Star Removal: Starnet was used to remove stars from both the simulated Ha and Oiii images, creating starless versions.
6. GHS Applied: Gradient Harmonic Separation (GHS) was applied to each starless image to further refine the background.
7. Deconvolution and Star Desaturation: Deconvolution was used to sharpen details, and star desaturation was applied to the star masks to reduce color fringing.
8. Combining Ha and Oiii: The simulated Ha and Oiii images were combined using PixelMath with the Paulyman formula for a specific false-color palette:
   • R = Ha
   • G = ((Oiii*Ha)^~(Oiii*Ha))*Ha + ~((Oiii*Ha)^~(Oiii*Ha))*Oiii
   • B = Oiii (For a deeper dive into this formula, check out this Cloudynights discussion: )
9. Stars Re-added: The previously removed stars were carefully integrated back into the combined image.
10. Final Adjustments: Curves and histogram adjustments were performed to fine-tune the contrast and overall appearance of the final image.

This method allowed for a unique interpretation of the nebula's hydrogen and oxygen emission lines, resulting in a color palette that differs from more traditional narrowband combinations. I'm incredibly grateful to resources like this subreddit and Cloudynights for sharing such valuable techniques!