The Monkey Head Nebula in Orion

NGC 2174 is associated with the open star cluster NGC 2175. It is thought to be located about 6,400 light-years away from Earth. This is a tricolor narrowband image in a standard Hubble palette (SII:Ha:OIII mapped to R:G:B) with a turquoise-gold color shift.

Exposure: Total exposure time about 23.7 hours. 16:21:34 x 20 minutes SII:Ha:OIII. All bin 1x1. Captured October 2020 to January 2021.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border.)
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arcsecs/pixel (50% of original scale)

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 Type IIi LRGB, 5nm Ha and SII, 3nm OIII
Rotator: Optec Pyxis 2", connected via Andy Galasso's 0.4 driver (Optec Pyxis Rotator AG)
Focuser: Rigel Systems GCUSB nStep motor with driver version 6.0.7 on stock Celestron focuser
OAG: Orion Thin OAG
Guide cam: Lodestar (first generation). 4 second exposures
Automation SW: Sequence Generator Pro 3.1.0.558
Guide SW: PHD 2.6.9, 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 1.8.8-7:

1. Calibration
Calibration with WeightedBatchPreProcessing with flats and bias, using Cosmetic Correction with a master dark
Blink to preview and reject a few frames
Rerun WBPP for weighting and registration

2. Stack and Mure Denoise
Image Integration on each channel
Mure Denoise on each channel
Dynamic Crop

3. Linear Processing
No Dynamic Background Extraction on these images
Deconvolution on each channel, using masks made in Photoshop so that only the brightest areas were sharpened.

4. Stretching
For each channel:
Histo Trans x 2
Curves Trans
On SII and OIII: TGV Denoise to reduce fine-grained noise
On SII: Multiscale Median Transform to reduce large-scale lumpiness

5. Create Nebula Image
ChannelCombination, mapping SII:Ha:OIII to R:G:B
Star Removal with StarNet, using a mask to protect high-contrast areas*
Result = ”Nebula”
Turquoise-gold color shift: SCNR green average neutral, Curves to enhance color saturation
Deconvolution on some high detail areas, using a mask to protect the rest
32 pixel Localized Histogram Equalization, with a small mask to reduce the effect on the fine detail in the dust column at top
TGV Denoise
Curves to brighten and boost saturation
*StarNet often damages detail in these areas, so to protect it I create a mask in Photoshop: after the initial run of StarNet, I port both the original and the starless image into Photoshop, layer them, and create a mask for any damaged areas. Then I port the mask back into Pixinsight, and run StarNet again on the original image. This time the detail is protected by the mask.

6. Create Stars Image
Starting from unprocessed stacked images:
HistoTrans on each channel, keeping star sizes roughly equivalent between the channels
Combine with ChannelCombination
Run StarNet with the same mask used to create the Nebula image in step 5 above
Subtract the resulting starless image from the image with stars. The result is an image with stars, no nebulosity, and a background of zero = ”Stars”
SCNR on Green
Fix magenta halos: invert the image, run SCNR on green, and reinvert
TGV Denoise

7. Photoshop
Layer Stars as a linear dodge layer (equivalent of PixelMath add function) on top of Nebula
Use Spot Healing Brush on Nebula as necessary to remove remaining star halos
Curves and levels to increase contrast a bit

8. Final
Mild TGV Denoise
ICC Profile Transform to sRGB
Resample to 1/2 original scale
Save as JPG