Link to annotated image
NGC 6286 and NGC 6285, discovered by the American astronomer Lewis A. Swift in the 1880s, form the interacting pair Arp 293. The galaxies are separated by about 130,000 light years, with a faint bridge spanning the gap. NGC 6285 is an elliptical (S0) galaxy, and NGC 6286 is a Sb-type galaxy seen almost edge-on with a dust lane inclined to the galaxy’s disk.
Arp 293 is located approximately 248 million light-years from Earth in the constellation of Draco. The other two large galaxies in the image, UGC 10641 and UGC 10646, are at the same distance, so this appears to be a small group. In the background, the galaxy at upper right is at 1.1 billion light years, and the one at lower left is at 1.4 billion. No redshift was located for the other galaxy.
Exposure: Total exposure time 24.5 hours, 530:66:69:70 x 2 minutes LRGB. All bin 1x1. Data collected in April and May of 2022.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border.)
Seeing: FWHM of integrated luminance around 2.7 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 1.8.9:
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
Copy image and stretch to create luminance mask
StarNet2 to create a star mask from the luminance mask image
Deconvolution, using the star mask for local deringing support and the luminance mask to sharpen bright areas only
4. Luminance Stretching
Histo Trans x 2
TGV Denoise
Histo Trans x 2
Aggressive Multiscale Median Transform denoise (with an inverted luminance mask) to remove background lumpiness
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
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 halos, leaving only the background
b. Apply Noise > Dust & Scratches. This prevents the 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)
