Link to inverted Ha and OIII images
Abell 25 (Kohoutek 1-13, PN G224.3+15.3, PK 224+15.1) was discovered by George Abell in 1955. It’s 2.8 arcminutes in diameter. The Saguaro Astronomy Club lists it as magnitude 15.4, but it’s listed as mag 18.9 in a 2005 note in the Monthly Notices of the Royal Astronomical Society, and that better matches my experience in imaging it. This 2008 article in ApJ calculated a distance of 2472 light years. The central star appears to be the very faint blue star at dead center of the nebula. It’s much brighter in OIII than Ha.
Exposure: Total exposure time 30 hours, 43:41 x 20 minutes Ha:OIII, 27:26:26 x 2 minutes R:G:B. All bin 1x1. Data collected November 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 and OIII both around 2.85 arcsecs (vs my usual 2.2 to 2.4 arcsecs)
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arcsecs/pixel (50% 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
Mild Localized Histogram Equalization to enhance contrast
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
Using a cyan color mask (created with the Color Mask script), use Curves Trans to pull up blue and pull down green in OIII areas. This shifts teal green to blue.
In Photoshop, layer RGB image on nebula image as a color layer with a star mask, so that RGB color is only used for stars, not background or nebula
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. Star Reduction
I used a modified version of Adam Block’s star reduction technique:
StarNet to create a new “Starless Image”
Extract two copies of luminance from the main image (“Halos” and “LuminanceImage,” then apply a 6-layer MLT, unchecking the residual layer, to Halos to create a rough star mask.
Binarize Halos to eliminate everything fainter than star cores
MorphTrans on Halos, using dilation to enlarge the stars
Edit Halos with CloneStamp to exclude any background galaxies
Convolution on Halos to blur star edges, lower convolution on LuminanceImage for the same reason
Pixel Math: subtract LuminanceImage from Halos so that star cores are excluded and only halos are represented
Apply Halo to main image as a mask, then run PixelMath to use Starless Image where halos otherwise would be
9. 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
