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NexStar 9.25 GPS

In the fall of 2003, Celestron introduced a new NexStar GPS model using their highly regarded 9.25 inch Schmidt-Cassegrain optical tube  The general description of NexStar 8/11 GPS as found in the book applies equally to this scope, in particular, the mount is virtually identical, although the 9.25 does not include vibration suppression pads.

The main differences optically, other than an aperture about midway between the 8 and 11 inch models, are not immediately apparent.  First of all, the NexStar 9.25 GPS does not have the Fastar option found the 8/11.  Imaging is limited to the rear of the scope, either at f/10 or faster ratios using focal reducers as discussed in chapter 11 of The NexStar User's Guide.

NexStar 9.25 GPS at a Glance

Optical System: Schmidt-Cassegrain Catadioptric
Approximate Street Price: $2,700 (includes tripod)
Aperture: 9.25 inches (235mm)
Focal Length: 2350mm
Focal Ratio: f/10
Supplied Eyepieces: 40mm Plössl (59x)
Maximum Magnification: 470x
Maximum Field of View: 0.74°
Magnitude Limit: 13.5
Resolution Limit: 0.6 arcsecond
Finder: 9x50 straight through
Objects in HC Database: 40,000
Weight: 58 lbs. (26.4 kg)
Tripod Weight: 26 lbs. (11.8 kg)

Secondly, the primary mirror has a lower focal ratio and thus the secondary mirror magnifies less than on the other models.  This produces a flatter field which is most noticeable when imaging.  Stars at the edge of a large CCD camera or the edge of a 35mm film field will be in sharper focus.

Finally, the baffle design is optimized to reduce vignetting (darkening) at the edges of the field, again, when using large CCD cameras or 35mm film photography.

The Celestron 9.25 inch SCT optical tube has almost a cult-following among catadioptric lovers.  The announcement of this model caused quite a stir.  Past Celestron 9.25 inch models have been sold either with no mount or on a mount that was barely adequate for visual use and nearly impossible to use for astrophotography.  With the NexStar 9.25 GPS, Celestron has matched their wonderfully unique optical tube with a mount ready for astrophotography action.

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Michael Swanson
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