NGC 2841 in Ursa Major

NGC 2841 was discovered by William Herschel in 1788. A 2001 Hubble Space Telescope survey of the galaxy's Cepheid variables determined its distance to be about 46 million light-years. This is the prototype for the flocculent (“fleecy”) spiral galaxy, where the spiral arms are patchy and discontinuous. Its disk can be traced out to a radius of around 228,000 light years, making it a bit larger than our Milky Way. Of note in the annotated image are a small galaxy cluster at about 4 billion light years and a very distant quasar with a redshift of almost 4, placing it about 12 billion light years away.


Exposure: Total exposure time 21 hours, 517:33:35:45 x 2 minutes LRGB. All bin 1x1. Data collected November 2020 to February 2021.
Light pollution: Bortle 7-8 (white zone, NELM about 4.5)
Seeing: FWHM of integrated luminance around 2.4 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 IIe 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 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

3. Luminance Linear Processing
Dynamic Background Extraction
Sharpening with Multiscale Linear Transform (No deconvolution on this image)

4. Luminance Stretching
Histo Trans x 2
Masked Stretch
Localized Histogram Equalization
TGV Denoise
Curves Trans

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

6. RGB Stretching
Histo Trans
Boost color saturation with Curves Trans
Additional Curves Trans to brighten

7. Color Combination
LRGB Combination of luminance and RGB images to create a “Galaxy” image

8. Star Reduction
I followed Adam Block’s star reduction technique:
StarNet to create “Starless Image”
Extract two copies of luminance from main image, then apply MLT to one to create a rough star mask
Binarize to select only the stars
MorphTrans to enlarge stars
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

9. Photoshop
Use CloneStamp to remove halos from Starless Image, then subtract it from main image to remove remaining messy clumps in the background
Save as TIFF and move back into PI

10. Final
Final Histogram Transformation
ICC Profile Transform to sRGB
Resample to 50% of scale
Slight Histo Trans to darken
Save as JPG
ImageSolve
ImageAnnotation (using custom catalogs for galaxy clusters and quasars)
Save annotated image