GSO 12" f/8 Ritchey-Chretien Astrograph Telescope - Truss Tube OTA
Availability: No ETA available from the manufacturer at this time
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- Ritchey-Chretien OTA with 304mm f/8 primary mirror for astronomical imaging with DSLRs or dedicated CMOS or CCD astronomy cameras
- 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)
- Carbon-fiber truss tube design minimizes flexure and temperature shifts
- Quartz mirror for superior thermal stability coated to 95% reflectivity and overcoated with protective quartz layer
- Linear bearing 3.25” 10:1 rotatable EAF-compatible Crayford focuser with 50mm focus travel and tension and locking knobs to hold heavy loads; includes 1.25” adapter and two 1” and one 2” extension rings to enable focus with a wide range of cameras and accessories
- Updated focuser now terminates in an M56x0.75 male thread and includes a built-in removable camera rotator
- Includes top and bottom Losmandy mounting plates and two Vixen-style finder mounting bases for optional finder scope or guide scope
- Dust caps for primary and secondary mirrors
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. Note that these tests are qualitative and no detailed test reports are included. The telescope will be in perfect collimation when it leaves our warehouse but may require some adjustment after delivery.
3. Optics and Tube
This RC OTA comes with a 304mm (12") mirror with a 2432mm focal length, giving it a relatively fast focal ratio of f/8 in a compact steel tube that's only 34" 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 rear mirror cell includes three built-in cooling fans to help equalize the temperature of the primary to ambient as quickly as possible (requires 8xAA batteries, not included).
The secondary mirror is 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 secondary mirror includes a center dot to aid in collimation.
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.
4. Focuser and Accessories
The solid 10:1 rotatable dual-speed 3.25" Crayford-style linear bearing focuser handles DSLR's, mirrorless cameras, astronomy video cameras, and large sensor CCD and CMOS cameras for astrophotography. 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 linear bearing focuser on this telescope is compatible with the ZWO EAF out of the box without the need for other 3rd party brackets. The bottom of the focuser now incorporates a flat surface with additional screw holes to allow users to easily and directly mount the ZWO EAF using the standard mounting bracket that the EAF comes with. The housing in this version is also made of aluminum for a secure, robust fit. Imagers can now use the EAF to precisely control the focuser's movement for visual or critical imaging applications. Note that the EAF is not included.
This latest incarnation of focusers from GSO also has two more major feature upgrades: 1) The focuser drawtube now terminates in an M56x0.75 male thread. This allows you to directly mate other optional M56x0.75 threaded adapters and accessories (such as a Baader clicklock visual back) for a more secure connection. 2) The focuser now includes a built-in, removable camera rotator with its standard 2" holder. A large thumbscrew lets you lock the rotator at the desired angle to perfectly frame your imaged targets. The camera rotator features M56x0.75 threads on both sides (female on the telescope end and male on the camera side). You can adjust the stiffness of the rotation by tweaking the 3 small screws provided for this with a small flat-head screwdriver.
The focuser on this telescope is decoupled from the primary mirror cell and baffle. It attaches to its own collimating cell on the OTA's backplate so that you can tweak the primary mirror or the focuser alignment independently.
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, a 14" long Losmandy D-plate is included. There are also two Vixen-style bases for mounting a mini guide scope or red-dot or magnifying finders. A primary mirror dust cover and a secondary mirror dust cover are included with the telescope.
With all accessories, the total weight of the tube is 52lbs-12oz. For astronomical imaging applications, this RC scope should be paired with an equatorial mount that has sufficient capacity for this weight, along with cameras and other accessories.
5. What's in the Box
- Ritchey-Chretien truss-tube optical assembly with 304mm f/8 primary mirror
- Dual speed rotatable 3.25" Crayford-style linear bearing focuser with 2" eyepiece holder and 1.25" eyepiece adapter
- Losmandy D-plates for mounting (2x)
- 1x2" and 2x1" extension rings
- Two Vixen-style finder bases (finder not included)
- Front dust cover
- No instruction manual is included
6. Specifications
- Optical design: Ritchey-Chretien reflector
- Aperture: 304mm (12")
- Focal ratio: f/8
- Focal length: 2432mm
- 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 5.9"; center-dot for collimation
- Secondary mirror holder: 4-vane all metal spider
- Available back focus (distance of focal plane from surface of 1.25" eyepiece adapter, with focuser # DW403 full racked in: 150.9mm (5.94in). Add 10mm if 1.25" eyepiece adapter is removed.
- Tube outside diameter = 17.3" (at middle of truss)
- Tube length = 34"
- Tube weight (with included accessories): 52lbs-12oz
- Shipped size / weight = 44"x24"x25" / 73lbs-6oz
- Warranty: 2 years
| Free Shipping Graphic | Free Contiguous US Shipping |
|---|---|
| Telescope Focal Length | 2001 - 2500mm |
| Hide Found a better Shipped Price on Frontend? | Yes |
- Ritchey-Chretien OTA with 304mm f/8 primary mirror for astronomical imaging with DSLRs or dedicated CMOS or CCD astronomy cameras
- 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)
- Carbon-fiber truss tube design minimizes flexure and temperature shifts
- Quartz mirror for superior thermal stability coated to 95% reflectivity and overcoated with protective quartz layer
- Linear bearing 3.25” 10:1 rotatable EAF-compatible Crayford focuser with 50mm focus travel and tension and locking knobs to hold heavy loads; includes 1.25” adapter and two 1” and one 2” extension rings to enable focus with a wide range of cameras and accessories
- Updated focuser now terminates in an M56x0.75 male thread and includes a built-in removable camera rotator
- Includes top and bottom Losmandy mounting plates and two Vixen-style finder mounting bases for optional finder scope or guide scope
- Dust caps for primary and secondary mirrors
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. Note that these tests are qualitative and no detailed test reports are included. The telescope will be in perfect collimation when it leaves our warehouse but may require some adjustment after delivery.
3. Optics and Tube
This RC OTA comes with a 304mm (12") mirror with a 2432mm focal length, giving it a relatively fast focal ratio of f/8 in a compact steel tube that's only 34" 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 rear mirror cell includes three built-in cooling fans to help equalize the temperature of the primary to ambient as quickly as possible (requires 8xAA batteries, not included).
The secondary mirror is 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 secondary mirror includes a center dot to aid in collimation.
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.
4. Focuser and Accessories
The solid 10:1 rotatable dual-speed 3.25" Crayford-style linear bearing focuser handles DSLR's, mirrorless cameras, astronomy video cameras, and large sensor CCD and CMOS cameras for astrophotography. 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 linear bearing focuser on this telescope is compatible with the ZWO EAF out of the box without the need for other 3rd party brackets. The bottom of the focuser now incorporates a flat surface with additional screw holes to allow users to easily and directly mount the ZWO EAF using the standard mounting bracket that the EAF comes with. The housing in this version is also made of aluminum for a secure, robust fit. Imagers can now use the EAF to precisely control the focuser's movement for visual or critical imaging applications. Note that the EAF is not included.
This latest incarnation of focusers from GSO also has two more major feature upgrades: 1) The focuser drawtube now terminates in an M56x0.75 male thread. This allows you to directly mate other optional M56x0.75 threaded adapters and accessories (such as a Baader clicklock visual back) for a more secure connection. 2) The focuser now includes a built-in, removable camera rotator with its standard 2" holder. A large thumbscrew lets you lock the rotator at the desired angle to perfectly frame your imaged targets. The camera rotator features M56x0.75 threads on both sides (female on the telescope end and male on the camera side). You can adjust the stiffness of the rotation by tweaking the 3 small screws provided for this with a small flat-head screwdriver.
The focuser on this telescope is decoupled from the primary mirror cell and baffle. It attaches to its own collimating cell on the OTA's backplate so that you can tweak the primary mirror or the focuser alignment independently.
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, a 14" long Losmandy D-plate is included. There are also two Vixen-style bases for mounting a mini guide scope or red-dot or magnifying finders. A primary mirror dust cover and a secondary mirror dust cover are included with the telescope.
With all accessories, the total weight of the tube is 52lbs-12oz. For astronomical imaging applications, this RC scope should be paired with an equatorial mount that has sufficient capacity for this weight, along with cameras and other accessories.
5. What's in the Box
- Ritchey-Chretien truss-tube optical assembly with 304mm f/8 primary mirror
- Dual speed rotatable 3.25" Crayford-style linear bearing focuser with 2" eyepiece holder and 1.25" eyepiece adapter
- Losmandy D-plates for mounting (2x)
- 1x2" and 2x1" extension rings
- Two Vixen-style finder bases (finder not included)
- Front dust cover
- No instruction manual is included
6. Specifications
- Optical design: Ritchey-Chretien reflector
- Aperture: 304mm (12")
- Focal ratio: f/8
- Focal length: 2432mm
- 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 5.9"; center-dot for collimation
- Secondary mirror holder: 4-vane all metal spider
- Available back focus (distance of focal plane from surface of 1.25" eyepiece adapter, with focuser # DW403 full racked in: 150.9mm (5.94in). Add 10mm if 1.25" eyepiece adapter is removed.
- Tube outside diameter = 17.3" (at middle of truss)
- Tube length = 34"
- Tube weight (with included accessories): 52lbs-12oz
- Shipped size / weight = 44"x24"x25" / 73lbs-6oz
- Warranty: 2 years
| Free Shipping Graphic | Free Contiguous US Shipping |
|---|---|
| Telescope Focal Length | 2001 - 2500mm |
| Hide Found a better Shipped Price on Frontend? | Yes |