Link to annotated image
NGC 6463 is the large elliptical galaxy at the center of this field, and it’s tempting to conclude that most of the other galaxies are gravitationally bound to it. SIMBAD does indeed report a galaxy group at this location ([TTK2018] 6186, with 75-100 members, which may be the same as [SPD2011] 49492, with 23 members). That group has a redshift of 0.04162, very close to the redshifts reported for NGC 6463, NGC 6456 and PGC galaxies 2702587, 2701716, 2701592, and 60721. This would put the group about 570 million light years away. But some of the other galaxies in this field of view have redshifts that put them closer to 650 million light years away, perhaps indicating a second node to the cluster. That node includes NGC 6477 and PGCs 2701885, 2701701 and 2701676, and probably includes the odd double-ring galaxy NGC 6470 (redshift reported as 0.00488, but I’m guessing that’s a typo and should be 0.0488, putting it at the correct range). A third group, including NGC 6472 and PGCs 60779 and 2701283 may be closer, at 450 million light years. The edge-on spiral NGC 6471 has a redshift placing it about 370 million light years away.
Exposure: Total exposure time 27.3 hours, 567:88:81:82 x 2 minutes LRGB. All bin 1x1. Data collected from March to July 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 10.5, with MKS 5000 driver 6.0.0.0
Camera: SXVR-H694, connected via SX ASCOM driver 6.2.1.18182 (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 4.2
Guide SW: PHD 2.6.7, connected to guide cam via native SXV driver
ASCOM: ASCOM 6.3.0.2831
Platesolving: ASTAP, 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 1.8.9:
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 in both luminance and RGB
Dynamic Background Extraction
3. Luminance Linear Processing
Deconvolution to Sharpen:
Dynamic PSF to create PSF image
StarNet2 to create a star mask from the original image
Deconvolution, using the star mask for local deringing support
In Photoshop, layer the sharpened image on top of the unsharpened image, and using a mask paint in only the areas you want sharpened. Then save the mask and port it back into PI.
Use the mask with PixelMath to combine the sharpened and unsharpened images. You have to set PixelMath to replace the target image, otherwise the mask will be ignored in the PM operation. The resulting image will be sharpened only in the places selected in the mask. Save the image as Lum Linear.
4. Luminance Stretching
Histo Trans x 2
Curves Trans
TGV Denoise
Aggressive Multiscale Median Transform denoise (with an inverted luminance mask) to remove background lumpiness
5. RGB Linear Processing
Platesolve RGB image with the SolveImage script
Spectrophotometric Color Calibration, using Average Spiral Galaxy white reference
6. RGB Stretching
Histo Trans
Boost color saturation with Curves
Histo Trans x 2
Curves Trans
7. Color Combination
LRGB Combination of Luminance and RGB images
8. Background Subtraction
I usually have to do this because my flats aren’t perfect. I image with a moving mirror (pre-Edge) SCT, and because the mirror shifts through the course of the imaging session I find it impossible to make a flat that can correct all of the lights perfectly. I shoot flats with a flat panel, with the scope pointed up.
Create an artificial flat:
a. Create an image of the background:
1. StarNet2 to create a starless image
2. Modify the starless image in Photoshop as necessary:
a. Use the Healing Brush and CloneStamp tools to remove galaxies and any remaining star halos, leaving only the background
b. Apply Noise > Dust & Scratches with a blur of about 20px. This prevents the artificial flat from removing all noise 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, applying an offset so that the background isn’t completely black. I sometimes do the subtraction in PI instead, using a formula like “(original + background_level) – artificial_flat”. Adding in the background level of the image functions similarly to using an offset in Photoshop.
9. Star Reduction
I use a modified version of Adam Block’s star reduction technique, first creating a star halo mask:
StarNet2 on the original => “Starless Image”
Extract the luminance image from the original
Extract another luminance from the original, then apply a 7-layer MLT, unchecking the residual layer, to create a rough star mask
Binarize to select only the stars in the star mask
MorphTrans using erosion to eliminate the smallest stars from the star mask (unless I also want to shrink those stars)
MorphTrans using dilation to enlarge the remaining stars in the star mask
Edit the star mask with CloneStamp (or in Photoshop) to exclude any background galaxies
Convolution to blur star edges in the star mask
Pixel Math: subtract the luminance image from the 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 original image, then run PixelMath to use the Starless Image where star 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)
