SaratogaSkies Jim Solomon's Astropics


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NGC 6992 (Network Nebula)   [obsolete]

Click to see high-res version.

Image Details:

Camera: Canon Digital Rebel (300D)
Mount: Celestron AS-GT
Scope: Celestron C8-N (8" f/5 Newtonian)
Configuration: Prime Focus
Additional Optics: Celestron/Baader Multi Purpose Coma Corrector (MPCC)
Filter: None
Effective Focal Length: 1000mm
Effective Focal Ratio: f/5
Exposure: 14 x 8min @ ISO 400
Total Exposure: 1hrs, 52min
Date: 8/25/2004, ~1:30am PDT
Location: Saratoga, CA, USA
Acquisition: DSLRfocus
Focus: DSLRFocus
Dithering: None
Guiding: GuideDog via Philips ToUcam Pro II (840k) through Orion ST80 w/ Celestron 2x "Kit" Barlow


  • IRIS: Dark subtraction, flat field, registration, Kappa-Sigma stacking
  • Photoshop: Levels, image size, JPG conversion

Image Description:

This is NGC 6992, the Network Nebula, the Eastern portion of the Veil Nebula in the constellation of Cygnus. The Network/Veil is the remnant of a supernova explosion. The entire object takes up nearly 4 degrees of sky. For comparison purposes, the full moon takes up only 1/2 a degree. Better framing would have captured more of this object, but to capture the entire object requires a shorter focal length (such as that provided by the ST80 ... hmmmm) or a mosaic of several shots. Since my main mission was to test out my new focuser, the new coma corrector, and to experiment more with GuideDog, I wasn't so much worried about the framing of the object. I regret this decision, because the shot turned out to be one of my best to date.

Anyway, a coma corrector attempts to minimize or eliminate "coma", the primary form of aberration of paraboloid optics, such as those in a Newtonian reflector. Without such a device, the stars and other details in the outer portion of an image tend to "blur outward", not unlike the tail of a comet, which is how the term "coma" came into being. ("Like I'm in coma" is pretty much how I feel the day after going to sleep at 4am after a night of astrophotography, but I digress.) Anyway, the MPCC seems to have done a good job correcting the coma in the image, as stars are much more "pinpoint" from edge-to-edge of the frame.

But to reach focus with the camera and the MPCC in place, I needed to upgrade my focuser to a "low profile" one. The JMI NGF-DX3 seemed to be a good choice, and I'm pretty happy with it so far. Also, I tried cranking the "aggressiveness" setting in GuideDog which resulted in better tracking than previous attempts. Finally, I chose ISO 400 instead of ISO 100 based on the results of a noise analysis I performed on the Digital Rebel. ISO 400 looks to be a good choice, but more experimentation is needed.

This is a (very) slight crop of the full frame, scaled for display on the web. North is up.