SaratogaSkies Jim Solomon's Astropics


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M13 (Great Cluster in Hercules)

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Image Details:

Camera: SBIG ST-2000XM
Mount: Astro-Physics 1200GTO
Scope: AP Starfire 160EDF APO Refractor
Configuration: Prime Focus
Additional Optics: n/a (Prime)
Filter: Astrodon LRGB I-Series Filters
Effective Focal Length: 1200mm
Effective Focal Ratio: f/7.5
Exposure: 8 × 8min @ -20°C (Lum); {16R, 16G, 16B} × 2min @ -15°C
Total Exposure: 2hrs, 40min
Date: 6/10/2008 11:37 PM; 7/25/2009 11:12 PM (Start)
Location: Saratoga, CA, USA
Acquisition: CCDSoft
Focus: FocusMax
Dithering: None
Guiding: Self Guided


  • IRIS: Registration, gradient removal
  • JimP: Dark subtraction, Flat field, Kappa-Sigma Stacking, White balance, LRGB combining, ASINH stretching
  • Photoshop: JPG conversion

Image Description:

This is M13, aka, the Great (globular) Cluster in Hercules. I captured the RGB of this image with the TMB 152/1200, and the Luminance about a year later with the AP 160 Starfire EDF. The RGB was taken on a pretty icky night, with FWHMs well over 3”. But the Luminance exposures, taken through the exquisite AP 160, all had sub-2” FWHMs, on a night of fantastic seeing, at least for my location. Some minor DEC backlash adjustments resulted in pretty good guiding too. Note that there is absolutely no sharpening applied to this image. All in all, this is by far my best image of a globular cluster. A higher-resolution image is also available. North is up.