NGC 2403 is an outlying member of the M81 Group and is approximately 8 million light-years distant. It bears a similarity to M33, being about 50,000 light years in diameter and containing numerous star-forming H II regions, many of which show in the image as hot pink glows. Just above and left of the core is the star forming region NGC 2404 which, at 940 light-years in diameter, is one of the largest known H II regions. This region is strikingly similar to NGC 604 in M33, both in size and location in the galaxy.

Exposure: Total exposure time about 18 hours, 362:54:55:56 x 2 LRGB. All bin 1x1. Data collected in January and 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 2.3 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 6.2 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 First on luminance and RGB
NoiseXterminator on luminance and RGB

2. Luminance
Histo Trans x 3
Curves Trans
Second curves trans, using a mask to bring out the arms without blowing out the core
Mild Localized Histogram Equalization to enhance contrast between arms

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

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 shifts 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 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 from the main image (adding an offset, to avoiding having a pure black background), using a mask to protect the galaxy from being subtracted out

6. Final
Final Histogram Transformation
ICC Profile Transform to sRGB
Rescale to 50%
Save as JPG