The Whirlpool Galaxy in Canes Venatici (with "supernova impostor" AT2019abn)

The Whirlpool Galaxy was discovered on October 13, 1773, by Charles Messier and was designated No. 51 in his catalog. In 1845 Lord Rosse discovered its spiral structure, making it the first known "spiral nebula." Based on supernova distance estimates, it's located 23 million light years away, and is about 60,000 light years in diameter. The companion galaxy NGC 5195 may have passed through the main disk of M51 about 500 to 600 million years ago. Few of the background galaxies have redshift measurements, including the edge-on spiral (IC 4277) to the left of M51's companion and the odd bluish streak (IC 4278) below the companion.

On 22 or 25 January 2019 a new bright object was detected in the western (above center in this image) arm of M51 by the Zwicky Transient Facility at Mt. Palomar. This object first appears in my data on January 26. More information on this event, and an animation of my data from December to February, can be found on the AT2019abn page.

Exposure: Total exposure time about 13.5 hours, 226:61:62:57 x 2 mins L:R:G:B. All bin 1x1. Captured December 2018 to February 2019.
Light pollution: Bortle 7-8 (white zone, NELM about 4.5)
Seeing: FWHM of best subs about 2 arcsecs
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arsecs/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 5nm Ha/SII, 3nm OIII, 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 3.0.0.8
Guide SW: PHD2.6.3, 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 1.86

1. Calibration
BatchPreProcessing with flats, darks and bias, using Cosmetic Correction with master dark
Blink to preview and reject a few frames
Subframe Selector for luminance to confirm selections and weight by FWHM and SNR
StarAlign to register frames

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
Deconvolution with Dynamic PSF, luminance mask, and star mask for deringing support

4. Luminance Stretching
Histo Trans
Local Histogram Equalization on background, using mask, to enhance dust
TGV Denoise
Aggressive Multiscale Median Transform to remove lumpiness in background, using an inverted and blurred luminance frame as a mask to protect highlights
Additional Local Histogram Equalization on galaxy
HDR Multiscale on galaxy to enhance contrast
Mild deconvolution (parametric) on core to enhance detail

5. RGB Linear Processing
Dynamic Background Extraction
Photometric Color Calibration

6. RGB Stretching
Histo Trans
Boost saturation and blue in galaxy with Curves Trans and luminance mask

7. Color Combination
LRGB Combination of luminance and RGB images
Boost blue with Curves Trans

8. Photoshop: Background Removal
Create an artificial flat by eliminating the remaining small stars with the Dust and Scratches filter
Subtract the artificial flat from the main image, masking out the galaxy
Use Radial Blur filter to obscure comatic blue halos on bright stars
Neutralize greenish areas in dust with desaturation sponge

9. Final
Final Histogram Transformation
Save to Web as JPG