The CGEM 925 HD Computerized Telescope from Celestron features a Schmidt-Cassegrain optical system. This compound, or catadioptric, light-path employs a combination of mirrors and lenses that offer a large high-resolution aperture, a shorter tube length, and enhanced portability. This telescope has a 9.25" aperture, a focal length of 2350mm, and a focal ratio of f/10.
EdgeHD optics are designed to produce aberration-free images across a wide visual and photographic field of view. With a significant reduction in off-axis star coma and a flat astrograph quality image plane to the edges of the field of view, the EdgeHD system is capable of imaging diffraction limited stars of 1 arcsec at the edges of a full-frame sensor.
The optical path consists of a zero power corrector plate, a spherical primary mirror, and a secondary mirror along with a set of field flattening lenses integrated into the baffle tube. The resulting imaging plane is 3 times flatter than the field produced by a standard SCT telescope. EdgeHD telescopes feature StarBright XLT multi-layer coatings on the primary and secondary mirrors for increased reflectivity. The corrector is also fully coated to ensure enhanced anti-reflection characteristics.
EdgeHD optics optimize an image circle with a 42mm diameter around the center of the field of view. This circle completely covers the diagonal of a full-frame DSLR camera or a standard 35mm film frame. As such, it extends significantly beyond the diagonal of APS-C sensors and the smaller chips used in specialized astrophotography CCD cameras. Further refinement of the system's imaging capabilities is managed via mirror tension clutches that help support the primary mirror and minimize its lateral movement. This enhancement is particularly important to astrophotographers as it significantly improves the system's stability and imaging clarity.
The OTA comes with a very useful 9x50 finderscope fitted to its exterior. Due to its wider field of view, this ancillary optic can offer assistance in locating objects to view with the main scope.
Normal image orientation for the EdgeHD design is upside-down and reversed. The 2.0" diagonal inverts this image so it is right-side-up, but reversed. This diagonal directly accepts 2.0" eyepieces, and a 1.25" eyepiece adapter is included. This kit comes packaged with Celestron's Axiom LX 23mm eyepiece. This ocular is ideal for deep-space viewing - its wide field of view produces a feeling of being surrounded by the viewing experience.
The CGEM Mount is fully computerized with a database of over 40,000 celestial objects, and it offers numerous design advantages. While not quite as robust or refined at the CGE PRO, the CGEM is a sturdy and reliable mount with an excellent feature set that fills the niche between the advanced and pro series mounts. The software-assisted all-star polar alignment feature allows the viewer to choose any bright star for alignment of the mount's polar axis, even if the North Star isn't visible. Large hand-knobs offer easy altitude and azimuth adjustments. With just a standard hand-control alignment, the CGE PRO is capable of centering a star to within 5 arc minutes. The NexStar program's advanced pointing feature further improves the pointing accuracy to as low as 1 arc minute. Additionally, extended meridian tracking, fast slew speeds, and a precision tracking system all combine to provide outstanding tracking and viewing performance.