Outters 2 and NGC 136 in Cassiopeia

Ou 2 (PN G 120.4-01.3) was discovered by chance by Nicolas Outters in November 2010 while he was taking a deep Ha image of the nearby nebula Sh2-173. Ou 2 is about 80 arcsecs in diameter, and has a very faint but intensely blue central star measured at magnitude 19.5. Ou 2 is very close to a small, faint open cluster, NGC 136. This is a bicolor narrowband image, mapping Ha:OIII:OIII to R:G:B, with the nebula layered onto the RGB star field in Photoshop.


Exposure: Total exposure time about 16 hours. 21:24 x 20 minutes Ha:OIII, 24:20:20 x 2 minutes RGB. All bin 1x1. Captured September to November 2019.
Light pollution: SQM ~18.2 (Red/white zone border, Bortle 7-8, NELM at zenith about 4.5)
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arcsecs/pixel (50% of original size)

Equipment:
Scope: C11 (standard, not Edge) with Celestron 0.63 reducer imaging at f5.7
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
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 3.0.3.169
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 1.88

1. Calibration
Weighted BatchPreProcessing with flats, darks, and bias, using Cosmetic Correction and weighting by SNR and FWHM.

2. Stack and Mure Denoise
Blink to preview frames and remove some bad images
Image Integration
Mure Noise Reduction on each channel
Channel Combination to merge RGB channels
Dynamic Crop

3. Narrowband Linear Processing
Dynamic Background Extraction
No deconvolution – image was too faint to benefit from this

4. Narrowband Stretching
Histogram Trans x 2
Curves Trans
TGV Denoise

5. RGB Linear Processing
Dynamic Background Extraction
Photometric Color Calibration

6. RGB Stretching
Histo Trans x 2
Create star mask with Advanced Star Mask script
ColorSaturation to boost blue saturation in stars, using the star mask
CurvesTrans to boost saturation on all stars
TGVDenoise
MultiscaleMedianTransform to denoise background

7. Color Combine
Merged narrowband channels with ChannelCombination, mapping Ha:OIII:OIII to R:G:B
ColorMask script to create a cyan mask, then use mask with Curves to shift cyan/teal in the nebula to blue
Slight LocalHistogramEqualization to enhance contrast in nebula

8. Merge Narrowband and RGB Stars
Create starless nebula image with StarNet
In Photoshop, layer nebula onto RGB stars image with a mask
Boost contrast in nebula using curves and the same mask
Repair blue halos using Photoshop’s Radial Blur tool on a color layer

9. Final
Resample to 50% scale
ICC Profile to sRGB
Save as 100% quality JPG