SaratogaSkies Jim Solomon's Astropics


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Jupiter and Moons   [obsolete]

Image Details:

Camera: Canon Digital Rebel (300D)
Mount: Celestron AS-GT
Scope: Celestron C8-N (8" f/5 Newtonian)
Configuration: Negative Projection (Barlowed)
Additional Optics: TeleVue 2x Barlow
Filter: None
Effective Focal Length: 2844mm
Effective Focal Ratio: f/54.2
Exposure: 9 x 1/2 second @ ISO 800
Date: 6/22/2004, 9:28pm PDT
Location: Saratoga, CA, USA
Acquisition: Manual
Focus: Manual
Dithering: None
Guiding: None


  • IRIS: Dark subtraction, flat field, registration, stacking
  • Photoshop: Compositing of IRIS moons and Jupiter images, sharpening, saturation, scale, JPG conversion

Image Description:

An interesting shot of Jupiter and the four Galilean moons. The f/ratio is so high because I put the aperture mask on the end of the telescope, effectively turning the 200mm obstructed Newtonian into a 52.5mm unobstructed offset-Newtonian. For those familiar with telescope optics, the Newtonian design has a secondary mirror in the path of the light that hits the primary mirror, and this so-called "obstruction" can cause a loss of contrast, especially on objects like planets that have low contrast to begin with. By putting the aperture mask on, the secondary is out of the path of the light hitting the primary (now effectively only 52.5mm instead of 200mm), preserving all of the contrast. However, the lower effective aperture could conceivably cause a loss of resolution, but since this is a relatively low effective focal length (2844mm), that loss of resolution isn't evident, and is overwhelmed by the seeing conditions in any case. [In retrospect — what was I thinking!?!? <grin>]