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The Hole in the Trees Skybox | all galleries >> Deep Sky >> Galaxies > NGC 7331
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NGC 7331

NGC 7331, The Deerlick Group in Pegasus
Link to annotated image

NGC 7331 was discovered by William Herschel in 1784, and is the main member of a group of galaxies nicknamed the Deerlick Group . While NGC 7331 is only about 40 million light years away, the other four galaxies – sometimes known as the Fleas – are about 300 million light years away, except for NGC 7336, which has a slightly larger redshift indicating a distance of about 380 million light years. All three PGC galaxies below NGC 7331 are at the same redshift as NGC 7336. NGC 7331 A and C appear to be dwarf satellite galaxies.

In spiral galaxies the central bulge typically co-rotates with the disk but the bulge in NGC 7331 is rotating in the opposite direction to the rest of the disk. In both visible light and infrared photos of the NGC 7331, the core of the galaxy appears to be slightly off-center, with one side of the disk appearing to extend further away from the core than the opposite side.

Exposure: Total exposure time 16.3 hours, 384:35:36:34 x 2 minutes LRGB. All bin 1x1. Data collected in August and September of 2020.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border.)
Seeing: FWHM of integrated luminance around 2.2 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 Crop
Dynamic Background Extraction

3. Luminance Linear Processing
Deconvolution to Sharpen:
 Dynamic PSF to create PSF image
 Deconvolution, using a mask created in Affinity Photo to sharpen selected areas only

4. Luminance Stretching
Histo Trans x 3
Curves Trans
TGV Denoise
Aggressive Multiscale Median Transform (with an inverted luminance mask) to remove background lumpiness
Mild Localized Histogram Equalization, with a mask to limit effect to galaxy core

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

6. RGB Stretching
Histo Trans x 2
Boost color saturation with Curves
Histo Trans
Curves Trans

7. Color Combination
LRGB Combination of Luminance and RGB images

8. Background Subtraction
 a. Create an image of the background:
   1. StarNet++ to create a starless image
   2. Modify the starless image in Affinity Photo and Photoshop:
     a. Use the Healing Brush and CloneStamp tools to remove halos, leaving only the background
     b. Select the galaxy with the Magic Wand tool and expand the selection so that the whole galaxy is selected. Then delete it, and use the Smudge tool to “push in” color and patterns from the edges into the hole left by the galaxy. Then blur that area with Gaussian Blur to avoid sharp transitions.
    c. Apply a heavy Noise Reduction filter so that noise is not removed during the subtraction process.
 b. Subtract the background image from the original image (using Image>Apply Image) to remove remaining messy clumps in the background (and apply an offset so that the background is pure black)
 c. Save as TIFF and move back into PI

9. Final Adjustments
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 Galaxy Image, then apply a 6-layer MLT, unchecking the residual layer, to one to create a rough star mask.
 Binarize to eliminate everything fainter than star cores
 MorphTrans using dilation to enlarge the stars
 Edit the mask with CloneStamp to exclude any background galaxies
 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

10. Final
Final Histogram Transformation
ICC Profile Transform to sRGB
Resample at 50% scale
Save as JPG
ImageSolve
ImageAnnotation (using custom catalogs for UGC/PGC galaxies, clusters and quasars)
Edit annotated image in Affinity Photo and save as JPG


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