Link to inverted images
Abell 31 (Sh2-290, PK 219+31) is one of the largest planetary nebulae in the sky, with an angular size of about 17 x 15.5 arc minutes. Most web pages list the range at 2000 light years, apparently based on parallax measurements made by the Hubble Space Telescope. This would make the nebula about ten light years in diameter. The central star appears to be the very small, deep blue star near center.
Exposure: Total exposure time 20.7 hours, 28:28 x 20 minutes Ha:OIII and 44:44:44 x 2 minutes LRGB. All bin 1x1. Data collected in January and February of 2022.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border.)
Seeing: FWHM of integrated Ha 2.4 arcsec, OIII 2.2 arcsecs
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.5 arcsecs/pixel (40% 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
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 3.1.0.457
Guide SW: PHD 2.6.7, connected to guide cam via native SXV driver
ASCOM: ASCOM 6.3.0.2831
Platesolving: ASTAP, failover to local Astrometry.net 0.19 server
Collimation: Metaguide 3, using ASI120MM connected via ZWO Direct Show driver 3.0.0.2
Processing Software: Pixinisight, Photoshop CS2
Processing Workflow by Workspace in PixInsight 1.8.9:
1. Calibration
Calibration with WeightedBatchPreProcessing with flats and bias, using Cosmetic Correction with a master dark
Blink to preview and reject a few frames
Weighting and registration with WBPP
2. Stack and Mure Denoise
Image Integration on each channel
Mure Denoise on each channel
RGB Combination for RGB frames
Dynamic Crop
Dynamic Background Extraction
3. Narrowband Linear Processing
Deconvolution to Sharpen:
Dynamic PSF to create PSF image
Deconvolution, using an inverted luminance mask
4. Narrowband Stretching
Histo Trans x 2
Curves Trans
TGV Denoise
Aggressive Multiscale Median Transform (with an inverted luminance mask) to remove background lumpiness
5. RGB Linear Processing
Photometric Color Calibration, using Average Spiral Galaxy white reference
6. RGB Stretching
Histo Trans x 2
Boost color saturation with Curves
Histo Trans
Curves Trans
StarNet v2 to create a star mask => Stars Image
7. Color Combination
Color for Nebula:
PixelMath to blend stretched narrowband images, using Ludo’s Dynamic Narrowband formula
Curves to brighten and saturate
Color shift: extract green channel and use it as a mask with Curves Trans to pull up blue and pull down green. This shifts teal green to blue.
Mild Localized Histogram Equalization to enhance contrast
StarNet v2 to remove stars => Nebula Image
8. Merge Nebula and Stars
PixelMath to add Stars Image to Nebula Image using the formula ((1-$T)*(Stars Image))+$T
Curves to brighten and increase saturation
9. Final
Final Curves Transformation
ICC Profile Transform to sRGB
Resample at 40% scale
Save as JPG
