Link to inverted images
Abell 16 (PN G153.7+22.8) lies is a fairly empty part of Lynx. Distance estimates range from 2400 light years to 6200 light years. The magnitude is estimated at 15.9 (Cartes du Ciel, per the Saguaro Astronomy Club catalog). The nebula is about 135 arcseconds wide. The progenitor of the nebula is the faint blue star near center. Per a 2013 MNRAS paper the central star is about magnitude 18.7 and it has an M3V companion. This image uses RGB for the star field, a 50/50 blend of Ha and OIII for luminance for the nebula, and H:O:O mapped to R:G:B for color in the nebula. The OIII signal is only about 20% stronger than the Ha signal here, so a 50/50 blend for luminance seemed about right.
Exposure: Total exposure time 24 hours, 38:28 x 20 minutes Ha:OIII, 20:18:22 x 2 minutes R:G:B. All bin 1x1. Data collected from January to March of 2021.
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 images both around 2.35 arcsecs
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arcsecs.pixel (50% reduction)
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 Crop
Dynamic Background Extraction
No deconvolution on these images
3. Narrowband Stretching
On each image: Histo Trans x 3
Curves Trans
TGV Denoise
4. RGB Linear Processing
Photometric Color Calibration, using Average Spiral Galaxy white reference
5. RGB Stretching
Histo Trans
Boost color saturation with Curves
Histo Trans
Curves Trans
Background Subtraction
a. Create an image of the background:
1. StarNet++ to create a starless image
2. Apply Convolution to the starless image so that noise is not removed during the subtraction process
b. Subtract the background image from the original image in PixelMath to remove remaining gradients and any messy clumps in the background, applying an offset so that the background is not pure black: ($T+0.06)-BackgroundImage
6. Color Combination
Nebula color: map Ha:OIII:OIII to R:G:B with PixelMath
Nebula luminance: use 0.5*Ha:0.5*OIII with PixelMath
Mild Local Histogram Equalization on the nebula luminance image
Merged nebula luminance and nebula color with LRGB Combination
Range mask on nebula image to highlight only the nebula
Apply the mask to the RGB starfield, and use PixelMath to add the nebula image. The mask prevents the nebula background (about 0.06) from being added to the RGB starfield background (about 0.07). A background of 0.06 also was subtracted from the nebula image in the PixelMath merge operation: $T+(Nebula-0.06).
7.
Final
ICC Profile Transform to sRGB
Save as JPG
