GSO 6" f/9 Ritchey-Chretien Astrograph Telescope - Steel OTA (White) # RC6

 1. Overview

Widely used in professional telescopes around the world, the Ritchey-Chretien (RC) design offers many advantages for serious astronomical imagers. RC scopes use primary and secondary mirrors ground to precise hyperbolic figures, which means they suffer little of the aberration and coma that plague Newtonian telescopes. They have flat fields that provide pinpoint star images right to the edge of the field of view. They do not require a corrector lens, so they have no chromatic aberration like refractors or Schmidt-Cassegrain telescopes. And they have open optical tubes, which can make it easier to come to thermal equilibrium with their surroundings.

The difficulty of figuring the optics of an RC meant that these telescopes have, in the past, been prohibitively expensive for amateur astronomers. But modern materials, design, and manufacturing techniques have enabled the production of high-quality RC scopes that are remarkably affordable.

The RC Ritchey-Chretien assemblies produced by Guan Sheng Optical (GSO) offer superb value to intermediate and advanced astronomical imagers who are looking for a longer focal length scope to use with cropped sensor and full-frame DSLR cameras, as well as nearly all dedicated astronomy cameras, including video cameras for electronically-assisted astronomy (EAA). These scopes come with precisely figured optics, baffled tubes for superior image contrast, and rugged and precise dual-speed focusers to securely hold your camera and accessories.

(NOTE: This telescope is equipped to accept 1.25” and 2” eyepieces for visual use, and it does produce acceptable visual images. However, it is primarily intended for imaging applications. Observers looking for a compact and longer focal-length OTA for visual observing should consider a Schmidt-Cassegrain or Newtonian telescope.)

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 imaging Ritchey-Chretien 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. Optics and Tube

This RC OTA comes with a 153mm (6”) mirror with a 1377mm focal length, giving it a relatively fast focal ratio of f/9 in a compact steel tube that’s only 15.4” long.  

The precisely figured hyperbolic primary mirror is made from quartz for enhanced thermal stability. Quartz is superior in thermal characteristics to BK7 glass or soda lime float glass - the glasses used in consumer products like cookware - that's used in many reflectors in this price range. The mirror is coated with enhanced aluminum to a nominal reflectivity of 95% and coated with a protective layer of SiO₂ (quartz) for durability.

The primary mirror is mounted in a fixed cell that’s heavily baffled to prevent stray light from reducing contrast. While Schmidt-Cassegrain telescopes come to focus by moving the position of the primary mirror, RCs scopes have a fixed primary mirror and an external focuser. This avoids image shift while focusing, a big benefit for astrophotgraphers. The secondary mirror is also held in a baffled cell that’s fixed to the outer tube with a 4-vane spider. The secondary cell has adjustable screws to aid in collimating the optics.

The white enameled steel tube of this OTA includes internal baffles to keep stray light to a minimum so that image contrast is maximized.

4. Focuser and Accessories

The solid 10:1 dual-speed 2" Crayford-style dual-speed focuser handles small to mid-sized astronomy cameras and webcams. It also has a tension knob and a locking knob to allow you to secure the cameras and accessories.

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 minimal flexure with most cameras. It offers 41mm 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 telescope includes two 1” and one 2” extension rings for situations when you need further backfocus, or when you want to reduce the extension of the focuser’s drawtube to minimize its deflection under heavier loads. It also comes with a 1.25"-2” adapter with compression ring so you can use cameras with 1.25" nose pieces.

To enable you to solidly and securely mount the telescope, there’s a 12.75” Vixen-style rail along the length of tube. There’s also a Vixen-style finder base for mounting mini guide scopes or red-dot or magnifying finders. A front dust cover is included with the telescope.

With all accessories, the total weight of the tube is 12lbs-6oz. For astronomical imaging applications, this RC scope should be paired with an equatorial mount with at least 30lbs capacity for best results.

5. What’s in the Box

• Ritchey-Chretien optical tube assembly with 6” f/9 primary mirror
• Dual speed 2" Crayford-style focuser with 1.25" eyepiece adapter
• Vixen-style dovetail rail
• 1x2" and 2x1” extension rings
• Vixen-style finder base (finder not included)
• Front dust cover
• No instruction manual is included

 6. Specifications

• Optical design: Ritchey-Chretien reflector
• Aperture: 153mm (6”)
• Focal ratio: f/9
• Focal length: 1377mm
• Primary mirror: Hyperbolic primary mirror with enhanced Al coating to 95% reflectivity, with protective quartz layer
• Secondary mirror: Hyperbolic with enhanced Al coating to 95% reflectivity; protective quartz layer; O.D. of secondary cell is 3”
• Available back focus (distance of focal plane from surface of 1.25" eyepiece adapter, with focuser full racked in): 147.2mm (5.79"). Add 10mm if 1.25" eyepiece adapter is removed.
• Secondary mirror holder: 4-vane all metal spider
• Tube outside diameter = 7.6” including end rings
• Tube length = 15.4”
• Tube weight (with included accessories): 12lbs-6oz
• Shipped size / weight = 24"x11.5"x13" / 17lbs-9oz
• Warranty: 2 years