In a world abundant with Schmidt-Cassegrainian telescopes (SCT), the GSO (Guan Sheng Optical of Taiwan) 8-inch offering (MSRP: $899) is throwback to the all-mirror classical Cassegrain design that uses a parabolic primary and hyperbolic secondary. Decades ago, this configuration was a favorite of many dedicated lunar and planetary observers.
The f/12, 2436mm EFL specifications seem a good fit to again fill that role. The GSO scope boasts excellent fit and finish with its gloss white (or black) tube, smooth dual-speed focuser, mounting plates for both Vixen and Losmandy-style mounts, a standard Synta shoe for attaching a finder, and two extension tubes to allow focus with a variety of star diagonals, eyepieces, and cameras. The optical tube contains numerous baffles to suppress stray light.
To simplify accurate collimation, the primary mirror is fixed and only the secondary is adjustable.
Achieving perfect alignment is no more difficult than with a Schmidt-Cassegrain. With an overall weight of about 19 pounds with a star diagonal and 50mm finder attached, the complete ensemble is lighter than a Celestron 9.25”, but a fair degree heavier than a C 8. An AVX-class equatorial will hold it with reasonable stability.
Optically, the telescope is excellent. Direct comparison with a high-quality 6-inch ED refractor at magnifications ranging from 145x to 252x showed nearly identical detail on Jupiter. The image with both telescopes was crisp, well saturated with color and able to reveal delicate festoons in the now-yellowish Equatorial Zone.
Likewise, Saturn presented an impressively sharp and pleasing image, the Cassini division wrapping the full circumference of the planet, and subtle belt and zone detail being just a bit less visible than with the 6-inch refractor.
Multiple stars, though somewhat cleaner in the refractor, were very pleasing in the GSO. Epsilon Bootis, Delta Cygni, the Iota Cass trio, and even the seeing-hampered Antares resolved nicely into concentrated pinpoints, only marred slightly by the unavoidable spider diffraction and secondary obstruction. The tightness of stellar points is superior to what the average SCT can produce.
The long focal length of the Cass restricts its use for wide-field viewing, but the scope did perform admirably on deep-sky targets. Good contrast and a dark background allowed some impressive views of the Orion Nebula, the trio of Auriga open clusters, M81 and 82, and some of the brighter planetary nebulae. However, the scope’s image was no brighter than that of the 6-inch refractor.
The optical design and tight baffling of the GSO results in a clear aperture of only 7.34 inches, the light grasp suffering accordingly.
While the telescope is a reasonably capable deep-sky instrument, its true value is in the realm of lunar, planetary, and double star observing. As an alternative to a much pricier 5 or 6-inch ED refractor, or an 8-inch SCT, it strikes me as a very good deal.
Read Larry Carlino’s comprehensive review at either of these links: