Messier 94, the “Croc’s Eye,” in Canes Venatici

M 94 (NGC 4736) is a spiral galaxy located about 16 million light years away. It was discovered by Pierre Méchain in 1781 and catalogued by Charles Messier two days later. M94 has an inner ring with a diameter of 5,400 light-years and a very faint outer ring with a diameter of about 45,000 light-years. The inner ring is the site of strong star formation fueled by gas driven into the ring by the inner oval-shaped bar-like structure.

Many images of this galaxy show the outer ring as a very bright ring. From my Bortle 7.5 skies, that's not really possible, so I concentrated on bringing out detail in the bright core and the faint spiral arms.


Exposure: Total exposure time about 15.3 hours, 345:37:39:38 x 2 minutes L:R:G:B. All bin 1x1. Data collected from December 2021 to February 2022.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border.)
Seeing: FWHM of integrated luminance 2.4 arcsecs
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 0.9 arcsecs/pixel (67% of full scale).

Equipment:
Scope: C11 (standard, not Edge) with Celestron 0.63 reducer
Mount: Paramount MX+, connected via ASCOM Telescope Driver 6.2 for TheSkyX, with MKS 5000 driver 6.0.0.0
Camera: SXVR-H694, connected via SX ASCOM driver 6.2.1.18212 (SX Windows Drivers 15.26.50.450 [i.e., version 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 2.6.11, connected to guide cam via native SXV driver
ASCOM: ASCOM 6.6 SP1
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, Photoshop CS2

Processing Workflow by Workspace in PixInsight 1.8.9:

1. Processing¬
Calibration with WeightedBatchPreProcessing with flats and bias, using Cosmetic Correction with a master dark
Blink to preview and manually reject a few frames
Weighting, registration and integration with WBPP
RGB Combination for RGB frames
Dynamic Background Extraction on luminance/narrowband and RGB images
ImageSolve RGB, then run Spectrophotometric Color Calibration, using Average Spiral Galaxy white reference
BlurXterminator using Correct Only on luminance/narrowband and RGB
BlurXterminator on luminance/narrowband and RGB
NoiseXterminator on luminance/narrowband and RGB
DynamicCrop on RGB and luminance/narrowband

2. Luminance
I made three different stretches, one for the bright core, one for the spiral arms, and one for the outer ring, using various combinations of Histogram Transformation, Curves and Masked Stretch
Mild Localized Histogram Equalization to enhance contrast in core
I then layered the three images together in Photoshop, using masks, to create a single blended luminance image
3. RGB Stretching
Create a saturation mask: apply ScreenTransferFunction to Histo Trans, then apply to the stretched luminance image. Blur the mask slightly with Convolution.
Histo Trans x 2
Curves Trans to boost saturation, using the saturation mask to prevent spurious background colors from being boosted
Curves Trans to brighten

4. Color Blending
LRGB Combine, boosting saturation slightly
(using Ha as lum for HaRGB images)]

5. Background Subtraction (Artificial Flat)
Broadband images captured with my (non-Edge) C11 usually had circular artifacts left over from flat calibration, because it’s basically impossible to create accurate flats when the mirror shifted while capturing light frames. To remove these artifacts, I create and subtract an artificial flat, which is simply an image of the messy background, with all stars and imaging targets removed.
a. Create an image of the background by removing stars with StarXterminator
b. Clean this image up in Photoshop, removing the target(s) and any leftover stars
c. Blur this background image slightly (otherwise in the next step you’ll remove all the noise, creating an unnatural-looking noiseless image)
d. Subtract the background image from the main image (adding a pedestal, to avoiding having a pure black background), using a mask to prevent the galaxy from being subtracted

6. Final
Final Histogram Transformation
ICC Profile Transform to sRGB
Rescale
Save final image, annotated image, and inverted narrowband images (if any) as JPG