The Medusa Nebula in Gemini

Bicolor Ha/OIII image. Ha mapped to red, OIII to green and blue, using 50/Ha as luminance (alpha channel in Pixel Math) and RGB for stars.



Exposure: Total exposure time about 36 hours, 45 x 20mins Ha, 58 x 20 mins OIII, 13/15/15 x 2 mins RGB. All bin 1x1. Captured over 17 nights in January and February 2017 and January to March 2018.
Light pollution: Bortle 8 (white zone, NELM about 4.5)
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arcsecs/pixel (50% reduction)

Equipment:
Scope: C11 (standard, not Edge) with Celestron 0.63 reducer
Mount: Paramount MX+, connected via ASCOM Telescope Driver 6.1 for TheSkyX, with MKS 5000 driver 6.0.0.0
Camera: SXVR-H694, connected via SX ASCOM driver 6.2.1.17140 (SX 1.2.2 also installed)
Filter wheel: Atik EFW2 with 7x1.25 carousel and Artemis 2.4.3.0 driver
Filters: Astrodon 5nm Ha/SII, 3nm OIII, Type IIi LRGB
Rotator: Optec Pyxis 2", connected via Andy Galasso's 0.4 driver (Optec Pyxis Rotator AG)
Focuser: Rigel Systems GCUSB_nStep with driver version 6.0.7
OAG: Orion Thin OAG
Guide cam: Lodestar (first generation). 4 second exposures
Automation SW: Sequence Generator Pro 3.0.0.4 beta
Guide SW: PHD2.6.3, connected to guide cam via native SXV driver
ASCOM: ASCOM 6.3.0.2831
Platesolving: PlateSolve 2, failover to local Astrometry.net 0.19 server
Collimation: Metaguide 3, using ASI120MM connected via ZWO Direct Show driver 3.0.0.2

Processing Workflow by Workspace in PixInsight 1.85

1. Calibration
BatchPreProcessing with flats, darks and bias, using Cosmetic Correction with master dark

2. Stack and Mure Denoise
Blink to preview and reject a few frames
Subframe Selector to confirm selections and weight by FWHM and SNR
Image Integration
Mure Denoise

3. Luminance Linear Processing
Dynamic Crop
Dynamic Background Extraction
Deconvolution with Dynamic PSF

4. Luminance Stretching
Histo Trans
Masked Stretch
Curves Trans
Noise Reduction with TGV Denoise and Multiscale Median (following David Ault’s Astro Imaging Channel tutorial).
Local Histogram Equalization to enhance contrast inside nebula
Deconvolution
Final TGV Denoise

5. RGB Linear Processing
LRGB Combination to merge RGB channels
Dynamic Crop (using the same icon as with luminance)
Dynamic Background Extraction
Photometric Color Calibration

6. RGB Stretching
Split luminance from RGB
Masked stretch on chrominance
Curves to boost saturation on chrominance
Histo Trans and Curves on luminance
LRGB Combination to merge luminance and chrominance
TGV Denoise

7. Color
Merged narrowband channels in PixelMath: mapped Ha to R, OIII to green, 85% OIII and 15% Ha to blue, and used 50/50 Ha/OIII as alpha channel

8. Combine
Star removal, following David Ault’s technique
With PixelMath max command, add starless nebula to RGB star field

9. Final
In Photoshop, clean up blue comatic halos on color layer with Radial Blur and registration error by creating and subtracting artificial flat. Save as TIFF
Downsize 50% and save as JPG