Globular Cluster in Ophiuchus
Exposure: Total exposure time about 5.5 hours, 80 x 2 mins luminance, 24:23:37 x 2 mins per channel RGB. All bin 1x1. Luminance captured June 2016, RGB captured May 2018.
Light pollution: Bortle 7-8 (white zone, NELM about 4.5)
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arcsecs/pixel (50% reduction)
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 5nm Ha/SII, 3nm OIII, Type IIi LRGB
Rotator: Optec Pyxis 2", connected via Andy Galasso's 0.4 driver (Optec Pyxis Rotator AG)
Focuser: Rigel Systems GCUSB_nStep with driver version 6.0.7
OAG: Orion Thin OAG
Guide cam: Lodestar (first generation). 4 second exposures
Automation SW: Sequence Generator Pro 3.0.0.4 beta
Guide SW: PHD2.6.3, 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 1.85
1. Calibration
BatchPreProcessing with flats, darks and bias, using Cosmetic Correction with master dark
Skipped Local Normalization, as it was leaving “fog” between faint stars
2. Stack and Mure Denoise
Blink to preview and reject a few frames
Subframe Selector to confirm selections and weight by FWHM and SNR
Image Integration
Mure Denoise. This had little effect on lum image, but good results on RGB.
LRGB Combination to merge RGB channels
Dynamic Crop
3. Luminance Linear Processing
Automatic Background Extraction. Executed first for subtraction, then for division.
Dynamic Background Extraction
4. Luminance Stretching
Masked Stretch
Exponential Trans
Curves Trans
Noise Reduction with TGV Denoise and Multiscale Median (following David Ault’s Astro Imaging Channel tutorial).
5. RGB Linear Processing
Dynamic Background Extraction
Photometric Color Calibration
6. RGB Stretching
Split luminance from RGB
Masked stretch on chrominance
Histo Trans and Curves on luminance
LRGB Combination to merge luminance and chrominance
TGV Denoise
PixelMath to remove blue halos
7. Color Combine
LRGB merge
8. Background Removal
In Photoshop, create background image and export it back to PI
PixelMath to subtract background from image
9. Final
Save as TIFF
Downsize 50% and save as JPG
