Link to inverted images
NGC 246 is roughly 1,600 light-years away. The nebula's central star is the 12th magnitude white dwarf HIP 3678. This image maps Ha to red and OIII to green and blue. For a different processing, which uses an Ha-OIII composite for luminance and RGB for color, see here. Smoke from forest fires in the western United States was a major problem while collecting this data, and I had to throw out about one-third of what I had collected.
Exposure: Total exposure time 20.7 hours, 27:28 x 20 minutes Ha:OIII, 22:24:24 x 2 minutes R:G:B. All bin 1x1. Data collected in August and September of 2021.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border.)
Seeing: FWHM of integrated luminance around 2.6 arcsecs
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arcsecs/pixel (50% of full 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 Crop
Dynamic Background Extraction
For the narrowband images, I then integrated the Ha and OIII integrated images into a single Ha-OIII image. The combined image had slightly better SNR than either of the separate images. To do this, I had to create copies of each image, because ImageIntegration in PI requires at least three frames to run. I set ImInt to use noise evaluation as the weighting factor.
3. Narrowband Linear Processing
Deconvolution:
Dynamic PSF to create PSF image
Deconvolution, using a mask created in Photoshop to sharpen selected areas only
4. Narrowband Stretching
Histo Trans x 2
Curves Trans
TGV Denoise
5. RGB Linear Processing
Photometric Color Calibration, using Average Spiral Galaxy white reference
6. RGB Stretching
Histo Trans x 2
Curves Trans
TGVDenoise
7. Color Combination
LRGB Combination of Ha-OIII as luminance and RGB as color = Nebula Image
Curves Trans to boost color saturation on Nebula Image
Mild LocalizedHistogramEqualization to enhance contrast in nebula
8. Background Subtraction
a. Create an image of the background:
1. StarNet++ to create a starless image. I cleaned this up a bit in Photoshop, using the spot healing brush to remove the nebula and a few leftover star halos.
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+0.10)-BackgroundImage
9.
Final
Resample to 50% of original
ICC Profile Transform to sRGB
Save as JPG
