GSO 8" f/12 Classical Cassegrain Reflector Telescope OTA (Black)
- Classical Cassegrain telescope with 8” aperture and focal ratio of f/12, ideal for visual observation and imaging of the Moon, planets, double stars, planetary nebulae, and other compact celestial objects
- Fully and rigorously tested on site for cosmetics, mechanical operation, and optical performance by a master optician to ensure excellent overall telescope operation and customer satisfaction (an Agena exclusive service)
- Folded optical design packs a focal length of 2436mm into an optical tube only 535mm long
- No glass corrector plate means no chromatic aberration and fast temperature equalization and resistance to dew formation
- Lightweight tube and optics for a total weight of just 20lbs, ideally matched to many mid-capacity alt-az and equatorial mounts
- Dual-speed 2” Crayford focuser with 1.25” adapters with non-marring compression rings
- Includes Losmandy and Vixen-style dovetail rails, shoe for finder scope, and dust cap
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 nebulae, compact globular clusters, and other objects that are well framed in the tighter fields of view that result from a long focal length.
This 8” f/12 Cassegrain from GSO is a modern take on the classical Cassegrain design. With a focal length of 2436mm, it’s ideal for crisp and razor-sharp visual observation and imaging. This telescope offers views that rival apochromatic refractors at a tiny fraction of the price. Despite its long focal length, the optical tube of this telescope is just 21.1” (536mm) long, not including the focusers.
Compared to more complex Maksutov-Cassegrain designs, which include a thick and heavy corrector plate, this scope is not only far less expensive, it’s also much lighter, just 20bs, which matches well with nearly any mid-capacity altazimuth or equatorial mount. And unlike compound telescopes like Maksutov or Schmidt-Cassegrains, these telescopes have fixed mirrors so there’s no image shift during focusing, even at very high magnifications. And since they are purely reflecting telescopes, there is no chromatic aberration visible, even when observing bright objects.
2. Full Optical and Mechanical Testing and Quality Control - An Agena Exclusive!
GSO telescopes are available from several suppliers, but Agena does much more than simply move the telescope boxes from GSO to you. Each GSO Cassegrain telescope at Agena undergoes a thorough physical, mechanical, and optical evaluation at our shop upon arrival - all at no extra cost to you.
Every GSO telescope is inspected to ensure good cosmetics, collimation, smooth focuser operation, and to make sure all parts are included by the manufacturer. Most importantly, we also check each telescope using a rigorous three-point on-site optical evaluation consisting of the following optical bench tests to ensure that your telescope delivers excellent optical performance:
- Foucault Test: This knife-edge test is performed at the center of curvature of the primary mirror to evaluate the quality of the mirror's conic shape.
- Ronchi Test: We then perform a Ronchi grating test at the center of curvature at inside focus, at focus, and outside focus. This is a qualitative method to evaluate primary mirror smoothness and figure.
- Star test: Finally, we collimate the complete telescope and perform an autocollimation test with an artificial star to examine the overall system's image performance at the focus point.
This testing is performed and certified by a master optician to make sure each GSO instrument sold by Agena has excellent optical performance before it ships. Each telescope should be ready to go out of the box when you receive it.
3. Diffraction Limited Quartz Primary and Secondary Mirrors for Razor-Sharp Views
Both the parabolic primary and hyperbolic secondary mirrors are made from thermally-stable 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 visual observation, but they can also be used for lunar and planetary imaging. 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.
4. Internal Light Baffles for High Contrast
Computer optimization results in eight contrast-enhancing glare-stop baffles in the optical tube as well as multiple glare-stop micro baffles in the secondary mirror light shield. The scope also includes four baffles in the primary mirror baffle tube, resulting in excellent dark sky backgrounds and extremely good image contrast.
5. 2” Dual-Speed Crayford Focuser and Accessories
The telescope comes with a 2” dual-speed Crayford focuser mounted to the M90x1 threads on the rear cell. There’s a coarse focusing knob on each side, and a single 10:1 fine focusing knob. 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. The scope also comes with adapter rings to help achieve focus in a wide range of visual and imaging configurations.
6. Open Metal Tube for Fast Temperature Equalization and Reduced Dew Formation
The black tube of this classical Cassegrain is made from rolled steel for good thermal stability and it's capped with end rings of die-cut aluminum for protection and rigidity. The scope includes a dust cap, a shoe for a finder scope, and Losmandy and Vixen-style dovetail rails for mounting.
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.
- Optical design: Classical Cassegrain Reflector
- Aperture: 203mm (8")
- Focal ratio: f/12
- Focal length: 2436mm
- Primary mirror: Parabolic primary mirror; minimum 96% reflectivity; protective quartz layer
- Secondary mirror: Minimum 96% reflectivity; protective quartz layer; center dot marked for collimation
- Secondary mirror holder: 4-vane all metal spider; 3 collimation screws for alignment of secondary mirror
- Secondary Mirror Obstruction: 58mm
- Tube outside diameter: 9.1"
- Tube length = 21.1"
- Tube Material: Steel
- Focuser: 2" dual-speed Crayford
- Tube weight (with all included accessories): 20lbs
- Shipped size / weight = 30"x14"x14" / 26lbs 10oz
- Inspected for collimation and focuser operation at Agena AstroProducts
- Warranty: 2 years
- Country of Manufacture: Taiwan