NGC 4236 in Draco

NGC 4236 is a barred spiral (class SBdm) galaxy located about 14.5 million light years away in the constellation Draco. The galaxy is about 100,000 light years across, or about the same size as the Milky Way. It's one of the three largest members of the M81 Group (the others being NGC 2403 and of course M81 itself), which is one of the nearest galaxy groups to our own local group. Despite the fairly large number of background galaxies, I found no redshift measurements or distance estimates for any of them. I also found no cataloged galaxy clusters – including the fairly obvious one just below the right tip of NGC 4236 – or quasars, so I didn’t bother to prepare an annotated image.


Exposure: Total exposure time 16 hours, 390:34:25:32 x 2 minutes LRGB. All bin 1x1. Data collected in March and April 2021.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border.)
Seeing: FWHM of integrated luminance around 2.8 arcsecs
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.8 arcsecs/pixel (33% 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
Deconvolution:
Dynamic PSF to create PSF image
Deconvolution, using a mask created in Photoshop so that only a few detailed spirals were sharpened

4. Luminance Stretching
Histo Trans x 5
Curves Trans
TGV Denoise
Aggressive Multiscale Median Transform, using an inverted luminance mask, to remove lumpiness in background

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
Curves Trans
TGV Denoise
Aggressive Multiscale Median Transform, using an inverted luminance mask, to remove lumpiness in background

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

8. Background Subtraction
 a. Create an image of the background:
   1. StarNet++ to create a starless image
   2. Modify the starless image in Photoshop:
     a. Use the Healing Brush and CloneStamp tools to remove halos, leaving only the background
     b. Select the galaxy with the Magic Wand tool and expand the selection so that the whole galaxy is selected. Then delete it, and use the Smudge tool to “push in” color and patterns from the edges into the hole left by the galaxy. Then blur that area with Gaussian Blur to avoid sharp transitions.
    c. Apply a heavy Noise Reduction filter so that noise is not removed during the subtraction process.
 b. Subtract the background image from the Galaxy image (using Image>Apply Image) to remove remaining messy clumps in the background (and apply an offset so that the background is pure black)
 c. Save as TIFF and move back into PI


9. Final Adjustments
Histo Trans to darken background
Slight MultiscaleLinearTransform on galaxy for sharpening
SCNR Max Neutral and Average Neutral on green
Curves Trans to slightly boost blue in galaxy

10. Star Reduction
I used a modified version of Adam Block’s star reduction technique:
StarNet to create a new “Starless Image”
Extract two copies of luminance from the Galaxy Image, then apply a 7-layer MLT, unchecking the residual layer, to one to create a rough star mask.
Binarize to select only the stars
MorphTrans using erosion to eliminate the smallest stars
MorphTrans using dilation to enlarge the remaining stars
Edit the mask with CloneStamp to exclude any background galaxies
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 Galaxy image, then run PixelMath to use Starless Image where halos otherwise would be

10. Final
Final Histogram Transformation
ICC Profile Transform to sRGB
Resample to 50% of scale
Save as JPG
ImageSolve
ImageAnnotation (using custom catalogs for UGC/LEDA galaxies, clusters and quasars)