SaratogaSkies Jim Solomon's Astropics


Latest news

December: Test shots with new scopes/mounts

Dec 21: TMB 80/480 Arrives!

Dec 3: AP1200 Arrives!

Nov 30: TMB 152/1200 Arrives!


C/2001 Q4 [NEAT] (Comet)

Image Details:

Camera: Canon Digital Rebel (300D)
Mount: Celestron AS-GT
Scope: Celestron C8-N (8" f/5 Newtonian)
Configuration: Prime Focus
Additional Optics: n/a (Prime)
Filter: None
Effective Focal Length: 1000mm
Effective Focal Ratio: f/5
Exposure: 9 x 30sec @ ISO 800
Total Exposure: 0hrs, 4min
Date: 5/12/2004, ~10pm PDT
Location: Saratoga, CA, USA
Acquisition: Manual
Focus: DSLRFocus
Dithering: None
Guiding: None


  • IRIS: Dark subtraction, flat frame, registration, and stacking
  • Photoshop: Compositing, noise reduction, levels, saturation, image scale (1/4 x), JPG conversion

Image Description:

This was a tour-de-force of image processing. Using IRIS, a "comet only" image was generated by registering (aligning) on the comet nucleus and median-combining the 9 individual frames. Then, also in IRIS, a "star plus smeared comet" image was generated by aligning on the background stars and summing the 9 individual frames. The "star plus smeared comet" image, as you've probably guessed by now, has a smeared comet image, since the comet was moving quite rapidly with respect to the background stars over the course of the 9 individual exposures. Then using Photoshop, the "star plus smeared comet" image was put through a median filter (to remove the stars!), and then that "smeared comet" image was subtracted from the original "stars plus smeared comet" image to yield a "stars only" image. Then the "stars only" image was composited with the "comet only" image to produce this result. Various noise-reduction, levels, saturation, and other fun was done along the path. Pretty cool, huh!?

While this method has the advantage of unsmeared stars, the signal-to-noise of the comet itself is not as good as it would have been by doing a simple addition of the comet-aligned frames. This is becuase a median combine does not improve the signal-to-noise as effectively as addition does.