Cassegrain GSO 250mm
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1. A Modern Take on a Classic Telescope Design
First conceived in 1672, the classical Cassegrain telescope uses a parabolic primary mirror and a hyperbolic secondary mirror to fold a long focal length into a relatively compact open optical tube. This simple design is ideal for visual observations of the Moon, planets, double stars, planetary nebula, and other objects that are well framed in the tighter fields of view that result from a long focal length.
This 10” f/12 truss-tube Cassegrain from GSO is a modern take on the classical Cassegrain design. With a focal length of 3045mm, it’s ideal for serious lunar and planetary imaging and razor-sharp visual observation. This telescope offers views and images that rival and exceed those of large Schmidt-Cassegrain and premium apochromatic refractors.
Because there’s no corrector plate on this classical Cassegrain design, the optics are far less susceptible to dewing, and they exhibit much faster temperature equalization because of the open optical tube. The absence of a corrector plate also means this telescope is ideal for imaging planets in the IR to pick up additional detail not seen at visible wavelengths.
These classical Cassegrains come with precisely figured optics, center dot marked secondary mirror, light-weight Serrurier truss tubes for superior image quality, and rugged and precise dual-speed focusers to securely hold your camera and accessories. Unlike compound telescopes like Maksutov or Schmidt-Cassegrains, these classical Casses have fixed mirrors so there’s no image shift during focusing, even at very high magnifications. And the 690mm long Serrurier truss tube, which weighs just 38lbs, matches well with nearly any mid-to-high capacity equatorial mount.
2. Diffraction Limited Quartz Primary and Secondary Mirrors for Razor-Sharp Views
Both the parabolic primary and hyperbolic secondary mirrors are made from quartz and ground and polished to a surface accuracy that’s diffraction limited or better. Each mirror has enhanced aluminum coatings overcoated with a protective layer of quartz for long life. And each has a reflectivity of better than 96%, similar to Ritchey-Chretien, Schmidt-Cassegrain, and other scopes that cost far more. The secondary mirror is mounted in a four-vane spider, and the central obstruction is about 33%, comparable to most Schmidt-Cassegrain telescopes.
These telescopes are ideal for serious imaging of the Moon and planets, but they can also be used for visual observation. While these telescopes do not exhibit spherical aberration, like Newtonians they will have some coma especially at the lowest magnifications or with astronomy cameras with large chips.
3. Heavy-Duty Dual-Speed Crayford Focuser
These scopes come with a 3.25” dual-speed linear bearing Crayford focuser with 50mm travel. There’s a coarse focusing knob on each side, and a single 10:1 fine focusing knob. It also has a tension knob and a locking knob to allow you to secure heavier loads. The ridged focus knobs give you a good feel and grip, even with gloved hands in cold weather, and the 10:1 fine focus knob lets you snap your image into perfect focus. The drawtube of the focuser is engineered to have minimal wear and very little flexure with most cameras. It offers 50mm of focus travel and includes a focusing scale on the top of the drawtube to make it easy to return to the correct focus position during each observing session. The drawtube includes a non-marring soft brass compression ring to hold 2” accessories in place. An included 1.25” adapter, also with a non-marring compression ring, slips into the 2” focuser to hold 1.25” eyepieces or imaging accessories.
4. Carbon-Fiber Serrurier Truss Tube Fast Temperature Equalization Minimal Flexure
The Serrurier truss tube components are made of a strong, light-weight carbon fiber composite material that exhibits extremely low thermal expansion to prevent changes in tube length during temperature changes. The tube design is based on two sets of opposing trusses to resist flexure as the tube orientation changes, even as it passes through the meridian. As a result, the optical elements remain in their optimum position to help achieve the sharpest possible images. The scope also includes a cooling fan for fast temperature equalization.
The scope includes dust caps for the primary and secondary mirrors, Losmandy and Vixen-style dovetail rails for mounting, and M117 extension rings.