Link to inverted Ha and OIII images
Abell 7 (PK 215-30.1, PN G 215.5-30.8) is quite large (12.7 arcminutes) but the low surface brightness (integrated magnitude 13.2) and far southern location (declination -15.6) makes it a challenging target for my northern hemisphere suburban backyard. It’s estimated to be 1800 light years away, making the diameter about 8 light years. The mag 15.4 central white dwarf star is designated as WD 0500-156, and may be part of a binary pair based upon a Hubble image showing a red dwarf companion 0.91″ away. The galaxy is 14th magnitude MCG-03-13-058 (PGC 16611), located at about 160 million light years.
Exposure: Total exposure time about 28 hours, 46:31 x 20 minutes Ha:OIII and 45:56:53 x 2 minutes R:G:B. All bin 1x1. Data collected September 2021 to January 2022.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border.)
Seeing: FWHM of integrated Ha 2.9 arcsec, OIII 3.4 arcsecs (vs my usual 2.2 to 2.4 arcsecs)
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.8 arcsecs/pixel (33% 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: 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 Software: Pixinisight, Affinity Photo, Photoshop CS2
Processing Workflow by Workspace in PixInsight 1.8.8:
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 Background Extraction
3. Narrowband Stretching
Histo Trans x 2
Curves Trans
TGVDenoise
Aggressive Multiscale Median Transform (with an inverted luminance mask, so that the effect was not applied to the nebula) to smooth out background.
4. RGB Linear Processing
Photometric Color Calibration, with average spiral galaxy as white reference
5. RGB Stretching
Histo Trans
Curves for color saturation, using a luminance mask so that color in background wasn’t also boosted
Histo Trans
Curves Trans to brighten
6. Color Combination
Color for Nebula:
PixelMath to blend stretched Ha and OIII images (R:G:B = Ha:OIII:OIII * 0.85 + Ha * 0.15)
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++ to remove stars
In Photoshop, layer RGB stars image onto nebula image as a lighten layer (equivalent to Pixelmath max command), using healing brush repairs on nebula layer to remove some leftover star halos.
7. Background Subtraction
a. Create an image of the background:
1. StarNet++ to create a starless image. I used the spot healing brush in Photoshop to remove the nebula and a couple leftover star halos.
2. In PS, apply a dust & scratches filter at very low strength (5 pixels) to smooth image so that noise is not removed during the subtraction process
b. Subtract the background image from the original image in Photoshop to remove remaining gradients and any messy clumps in the background, applying an offset so that the background is not pure black
8. Final
Final Curves, with a mask, to boost color saturation in nebula and stars
ICC Profile Transform to sRGB
Resample at 50% scale
Save as JPG
