Abell 13 in Orion

Inverted Ha and OIII Images

Abell 13 (PN G204.0-08.5) is a highly evolved, faint (magnitude estimates range from 16 to 20, though the latter is certainly too faint), 2.5 arc minute planetary nebula located between 3100 and 3900 light years away. Although the nebula appears roughly spherical, a 1999 study concluded that Abell 13 is actually a bipolar PN viewed nearly pole-on, with the bright ring representing the high-density equatorial waist of the bipolar nebula. The nebula appears to be interacting with the local interstellar medium.

Luminance for this nebulosity in this image is pure Ha since, as the linked inverted image shows, the OIII signal, with two hours of data, is barely detectable. Careful stretching of the OIII reveals little more than a ghostly smudge. For color, Ha was mapped to pure red. The stars are RGB.

Exposure: Total exposure time about 20 hours. 54 (!) x 20 minutes Ha, 20:20:20 x 2 minutes RGB. All bin 1x1. Captured November 2020.
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: 0.8 arcsecs/pixel (75% original size)

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 IIe 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 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
Dynamic Crop

3. Narrowband Linear Processing
Slight deconvolution on the bright ring of the nebula in Ha
STF applied and inverted images created

4. Narrowband Stretching
Histo Trans x 2
Curves Trans
TGV Denoise
Denoise with Multiscale Median Transform, using a blurred, inverted luminance mask, to remove blotchiness in background areas

5. RGB Linear Processing
Photometric Color Calibration, using Average Spiral Galaxy white reference

6. RGB Stretching
Histo Trans
Curves
Boost color saturation with additional Curves
TGVDenoise

7. Colorization
Remove stars from Ha image with Starnet++
Colorize starless Ha with PixelMath to red
Mild curve to brighten
Mild Localized Histogram Equalization to enhance nebula structure

8. Photoshop
Layer starless, colorized Ha nebula image as a lightness layer (equivalent to a PixelMath “max” instruction in PI) onto RGB starfield image
Since this eliminated a few embedded stars visible in the Ha image, I applied the monochrome, stretched Ha image as a luminance layer, using a mask so that only those few stars were added to the final image
Save as TIFF and move back into PI

9. Star Reduction
I followed Adam Block’s star reduction technique to reduce the brightness of the stars embedded in the nebula:
StarNet to create “Starless Image”
Extract two copies of luminance from main image, then apply MLT to one to create a rough star mask
Binarize to select only the stars
MorphTrans to enlarge stars
Convolution to blur star edges
Pixel Math: subtract luminance image from blurred star mask so that cores are excluded from mask, and on ly halos are represented in the mask = “Halo Mask”
Apply Halo Mask to main image, then run PixelMath to use Starless Image where halos otherwise would be
I applied this technique twice, applying MLT to different layers and targeting different sizes for MorphTrans to separately target small and large stars

10. Final
Final Histogram Transformation
ICC Profile Transform to sRGB
Resample to 75% of scale
Save as JPG