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The Hole in the Trees Skybox | all galleries >> Deep Sky >> Clusters > M15
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Globular Cluster Messier 15 in Pegasus

M15 was discovered by Jean-Dominique Maraldi in 1746 and included in Charles Messier's catalogue of comet-like objects in 1764. At an estimated 12.5±1.3 billion years old, it is one of the oldest known globular clusters. It’s also one of the most densely packed globulars in the Milky Way galaxy, and may harbor a black hole at its core. It contains multiple x-ray sources, an unsually large number of variable stars and pulsars, and Pease 1, one of only four few planetary nebulae known to exist in globular clusters. It has an integrated spectral type of F3. M 15 is about 33,600 light-years from Earth, and 175 light-years in diameter.

Exposure: Total exposure time about 15 hours, 250:25:24:26 x 2 minutes LRGB. All bin 1x1. Data collected in September and October of 2020.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border.)
Seeing: FWHM of integrated luminance around 2.2 arcsecs
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arcsecs/pixel (50% reduction)

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
Camera: SXVR-H694, connected via SX ASCOM driver (SX 1.2.2 also installed)
Filter wheel: Atik EFW2 with 7x1.25 carousel and Artemis 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
Guide SW: PHD 2.6.7, connected to guide cam via native SXV driver
Platesolving: PlateSolve 2, failover to local 0.19 server
Collimation: Metaguide 3, using ASI120MM connected via ZWO Direct Show driver
Processing Software: Pixinisight, Affinity Photo, Photoshop CS2

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
Dynamic Background Extraction

3. Luminance Linear Processing
(I skipped PSF-based deconvolution – see the next step)

4. Luminance Stretching
STF Transform, copied to Histo Trans and slightly adjusted. This was easier than my standard approach of combining HT and Curves Trans.
Parametric Deconvolution. This gave better results than a PSF-based Deconvolution on the pre-stretched luminance. Stars had sharp edges, while deconvolution on the pre-stretched image resulted in star halos.
TGV Denoise

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

6. RGB Stretching
Histo Trans
Masked Stretch

7. Color Combination
LRGB Combination of Luminance and RGB images, with a boost to saturation

8. Background Subtraction
 a. Create an image of the background:
   1. StarNet++ to create an image without most of the outlying stars. This leaves a bright blotch for the core of the cluster.
   2. Modify the starless image in Photoshop:
     a. Use the Healing Brush and CloneStamp tools to remove halos, leaving only the background
    b. Apply a heavy Noise Reduction filter so that noise is not removed during the subtraction process.
 b. Subtract the background image from the original image (using Image>Apply Image) to remove optical artifacts (rings in the image) and any remaining messy clumps in the background. Use a mask to prevent the bright core of the cluster itself from being subtracted, and adjust the offset to get the right background brightness.
 c. Save as TIFF and move back into PI

9. Final Adjustments
Histo Trans to darken background

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

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