The Northern Trifid in Perseus

The Northern Trifid is an emission and reflection nebula complex reminiscent of the much brighter Trifid Nebula (M20) located in Sagittarius. If found no distance estimates for this object, and very little other information except a note in Wikipedia that it’s part of the California Molecular Cloud, which presumably includes the nearby California Nebula. This is a tricolor narrowband image, but the Northern Trifid has almost no OIII signal, so blue is very weak. Blue color in images of this region generally comes from the reflection nebulosity, but my light polluted skies won’t allow me to capture much of that.

Exposure: Total exposure time about 32.3 hours. 30:34:27 x 20 minutes SII:Ha:OIII, 20:20:20 x 2 minutes RGB. All bin 1x1. Captured October-December 2019.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border.)
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arcsecs/pixel (50% original size)

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 (the integrated SII showed an airplane track, so I used
Adam Block’s large scale pixel rejection trick to modify that subframe, and then restacked it, and the track disappeared)
Mure Denoise on each channel
RGB Combination for RGB frames
Dynamic Background Extraction
Dynamic Crop

3. Narrowband Linear Processing
Deconvolution on Ha image only

4. Narrowband Stretching
Histo Trans
Masked Stretch
Curves Trans, using a mask to protect the core from stretching
Local Histogram Equalization (radius 32) on core to bring out details
Less aggressive LHE (radius 256) on outer regions
TGV Denoise
Denoise with Multiscale Median Transform, using a blurred, inverted luminance mask, to remove blotchiness in background areas

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

6. RGB Stretching
Histo Trans
Curves
Boost color saturation with additional Curves
TGVDenoise
Result = “RGB Starfield Image”

7. Create Separate Narrowband Images
Narrowband luminance: PixelMath: max(OIII, Ha, SII)
Remove stars with StarNet++, then subtract starless image from original to create “Narrowband Stars Image”
Mild Localized Histogram Equalization, with a Range Mask, to enhance nebula structure
Narrowband color:
PixelMath, mapping SII:Ha:OIII to R:G:B
Color Mask to select cyan regions, then stretch with Histo Trans and blur with Convolution
Shift cyan to blue with CurvesTrans, using the modified color mask
LRGB to merge narrowband luminance and color = “Starless Narrowband Image”

8. Photoshop
Repair leftover star halos in Starless Narrowband Image with spot healing brush

9. Combination of Images
Sum Narrowband Stars and Starless Narrowband in PixelMath
Using the Narrowband Stars image as a mask, use LRGB combination to colorize the stars with the RGB Starfield image

10. Star Reduction
I followed Adam Block’s star reduction technique to reduce the brightness of the stars embedded in the nebula:
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
I applied this technique twice, applying MLT to different layers and targeting different sizes for MorphTrans to separately target small and large stars

10. Final
Final Histogram Transformation
ICC Profile Transform to sRGB
Resample to 50% of scale
Save as JPG