Globular Cluster in Serpens Caput

Messier 5, located 25,000 light years away in the western half (Caput) of Serpens, is one of the oldest and largest globular clusters in the Milky Way. Its stars formed nearly 13 billion years ago. The cluster contains over 100,000 stars packed into a region only 165 light years in diameter.



Exposure: Total exposure time about 5.7 hours, 84:25:33:28 x 2 minutes LRGB. All bin 1x1. About 3x this much data was actually captured, but I used only the sharpest images. Data collected January to April 2020.
Light pollution: Bortle 7-8 (white zone, NELM about 4.5)
Seeing: Average FWHM of subs around 2.6 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 6.1 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 IIi LRG, Chroma B (the Chroma blue has a higher short wavelength cutoff than Astrodons, resulting in slightly less coma in blue stars)
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
BatchPreProcessing with flats 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 PSF on luminance frame, using a luminance mask created with the Screen Transfer function and HistoTrans

4. Luminance Stretching
Histo Trans x 3
Curves Trans
(No further noise reduction required)
Slight Local Histogram Equalization, using a range mask, to enhance contrast in the core

5. RGB Linear Processing
Dynamic Background Extraction
Photometric Color Calibration (because PCC couldn’t platesolve the RGB image, I had to use the ManualImageSolver script, using the luminance frame as the reference. I never had to do that before!)

6. RGB Stretching
Histo Trans x 2
Curves to boost saturation
Histo Trans x 2 (again)
Curves for contrast and more color boosting
Desaturate background with Curves, using an inverted Range Mask

7. Color Combination
LRGB Combination

8. Star Halo Reduction
Most small stars had bloated blue halos, so to reduce those:
Color Mask script to create blue mask, which picked up the blue halos pretty well
Curves to stretch the mask
SCNR/Blue set to maximum neutral to desaturate the blue halos

9. Final Repairs in Photoshop
Repair comatic halos: On medium-sized stars, Filter>Radial Blur, then sharpen slightly with Unsharp Mask.

An artificial flat, created in Photoshop, was used to flatten the background.

Save as TIFF and move back into PI

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