Astro-Tech 8" f/8 Ritchey-Chrétien Astrograph, Carbon Fiber Tube

The Astro-Tech AT8RC is the first affordable 8" Ritchey-Chrétien with premium features available to the amateur astronomer who is into serious deep space imaging. Sky & Telescope agrees, as they named the AT8RC a Sky & Telescope Hot Product for 2010. A review in the December 2009 Sky & Telescope said "it was how nicely this scope is matched to APS-C and 35mm formats that really wowed me."

This Astro-Tech AT8RC Ritchey-Chrétien astrograph has:
• 8" f/8 true Ritchey-Chrétien hyperbolic mirror optical design
• High strength/low thermal expansion carbon fiber optical tube
• ten contrast-enhancing main tube knife edge light baffles
• dual-speed linear Crayford focuser
• 2" and 1.25" compression ring accessory holders
• two dovetail rails - one Vixen-style and one Losmandy-style "D-plate"

The AT8RC that Astro-Tech helped develop was the first sensibly-priced 8" true Ritchey-Chrétien astrograph available from a U. S. company. Sensibly-priced? Yes - but with premium features like a carbon fiber body, quartz mirrors, two dovetail mounting rails, and more that led Sky & Telescope magazine to name it a Hot Product for 2010, along with its larger 10" brother, the AT10RC.

As Sky & Telescope pointed out in their Hot Product citation, "Ritchey-Chrétien reflectors are highly regarded among today's elite astrophotographers, and premium instruments often carry price tags starting at about $1,000 per inch of aperture. So it's the best kind of "sticker shock" to see the prices for Astro-Tech's 8- and 10-inch f/8 Ritchey-Chrétiens, which pack features too numerous to list here. Our review of the 8-inch scope appears in December 2009, and our initial hands-on look at the 10-inch suggests that it will be equally exciting for deep sky astrophotographers." Excerpts from the December Sky & Telescope review are quoted below.

The Ritchey-Chrétien optical design is used in virtually every recent large mega-million dollar professional observatory telescope - including the Hubble Space Telescope. And more "affordable" true coma-free Ritchey-Chrétien optical systems made for schools and individuals by commercial R-C manufacturers typically come only in large apertures and often start at well over $1,000 per inch of aperture, as noted above by Sky & Telescope. Their size and cost put them out of the reach of most amateur astronomers. At least, true Ritchey-Chrétiens used to start at out-of-reach prices.

The Astro-Tech AT8RC astrograph (a telescope designed specifically for photographing comparatively wide areas of the sky) makes the coma-free imaging of true Ritchey-Chrétien imaging optics available to the DSLR and CCD astrophotographer at a price less than that of most CCD cameras. It is not designed for digiscoping through an eyepiece. 

The December 2009 issue of Sky & Telescope said "during the 1970s, there was an explosive growth of the hobby of deep-sky astrophotography that parallels the growing popularity of 8-inch f/10 Schmidt-Cassegrain telescopes. These scopes delivered decent star images across a field about 3/4° in diameter with enough resolution to record detail in small star clusters, nebulae, and galaxies. The AT8RC does that with even better star images, a larger field, and a slightly faster focal ratio. What's not to like?!"

Designed for exceptional imaging, the Astro-Tech AT8RC provides the wide coma-free photographic field that DSLR and CCD astrophotographers crave, but can't get from conventional reflectors and Schmidt-Cassegrains. Likewise, as a pure two-mirror system, the AT8RC is totally free from the spurious color that affects the imaging of all but the most costly apochromatic refractors, and it does it with an 8" aperture that dwarfs the light gathering of most apo refractors.

The AT8RC photo of IC1396, the "Elephant's Trunk Nebula," is from AT8RC owner John O'Neill. It was taken with an SBIG ST-10XME CCD camera and Astrodon filters. According to John, "This scope performed exceptionally well, but the camera distance from the back of the scope may be a bit unsettling for some users." (See the section on the Crayford focuser, below, for an explanation.)

"The performance of the 8" surprised me," John continued. "I was able to get plenty of data using relatively short exposures of 400 sec x 4 through LRGB filters. Can't wait for the 12" RC to be produced." 

If serious astrophotography is your goal, but the price of most true Ritchey-Chrétien optics has been keeping you from the optical design most modern professional observatories and the Hubble Space Telescope use for their imaging, your wait is over. The Astro-Tech AT8RC astrograph can bring the world of professional DSLR/CCD deep space imaging to your backyard observatory at a truly affordable price.

The December 2009 issue of Sky & Telescope said about the Astro-Tech AT8IN 8" F/4 imaging Newtonian optical tube, AT6RC, and AT8RC, "of the three scopes, I liked this one (the AT8RC) the best. Its advanced features beyond those of the six-inch RC - carbon-fiber tube, quartz optics, and dual mounting rails (Losmandy and Vixen-style dovetails) - were part of the reason. But it was how nicely this scope is matched to APS-C and 35-mm formats that really wowed me."

Sky & Telescope concluded by saying, "Astro-Tech has done a great job of balancing performance and price on all three of these imaging telescopes. By optimizing them for use with the cameras that many beginning and intermediate-level astrophotographers are using, the company has created affordable instruments that can produce stunning images. It's an exciting time to be entering the field of deep-sky astrophotography."

Features of this Astro-Tech AT8RC Astrograph . . .

Optical design: true Ritchey-Chrétien Cassegrain-type two-mirror optics, with hyperbolic primary and secondary mirrors. 

Optical specifications: 8" aperture, 1625mm focal length, f/8 focal ratio.

Hyperboloid primary mirror: Ground and polished under precision computer control to diffraction-limited or better surface accuracy. Unlike catadioptric designs (SCTs, Maksutovs, etc.) that move the primary mirror fore and aft in the optical tube to focus (which can lead to image shift as the mirror position changes) the AT8RC primary mirror is fixed to eliminate both a catadioptric's image shift and the primary mirror collimation requirements of a Newtonian reflector.

Hyperboloid secondary mirror: Ground and polished under precision computer control to diffraction-limited or better surface accuracy. Mounted in a four-vane spider and fully collimatable using simple standard Cassegrain reflector collimating techniques. Unlike complicated R-C designs that use motors to move the secondary mirror fore and aft to focus, the AT8RC secondary mirror is fixed and focusing is done externally.
The December 2009 issue of Sky & Telescope said that the Astro-Tech R-Cs' fixed primary and secondary mirrors "eliminate image shift, which has been the bane of Cassegrain scopes with moving-mirror focusing systems . . . It also keeps the effective focal length of the system constant, and the infinity focal point remains at a fixed point outside of the telescope, neither of which is the case with moving-mirror systems that change the separation between a Cassegrain's primary and secondary mirrors."

Carbon fiber optical tube: Made of light weight/high strength woven carbon fiber-reinforced composite material with extremely low thermal expansion characteristics to reduce the possibility of temperature-related focus changes that can occur with rolled steel tube systems during extreme temperature swings. Die-cast and machined aluminum front and rear cells. The 9" o.d. x 18" long carbon fiber tube is virtually indestructible. The use of a carbon fiber composite reduces the weight of the optical tube with no loss of strength or rigidity compared to a steel tube. Carbon fiber composites are so strong that the new $200,000,000 Boeing 787 Dreamliner passenger jet will use a wing and fuselage made almost entirely out of carbon fiber. Because of the use of a carbon fiber optical tube, the weight of this 8" R-C is only a little more than two pounds heavier than the steel tube 6" Astro-Tech R-C.

The December 2009 issue of Sky & Telescope said "the carbon-fiber tube did a good job of holding focus over the modest temperature changes occurring during my summer evenings. More telling, perhaps, was the scope's tendency to remain accurately focused after several days of inactivity during which the temperature in my backyard observatory would cycle over a huge range."

Multiple internal light baffles: Computer optimized primary and secondary baffling. Ten contrast-enhancing glare-stop baffles in the optical tube; multiple glare-stop microbaffles in the secondary mirror light shield; and five baffles in the primary mirror baffle tube provide truly dark sky backgrounds during imaging.

Dual-speed linear Crayford focuser: A new design 2" Crayford focuser is threaded onto the 90mm x 1mm pitch rear cell of the AT8RC. The matte black interior of the new longer 50mm travel drawtube has anti-reflection threading for high contrast. The focuser can be rotated to any convenient angle for the sake of photographic composition by simply loosening the collar that secures the focuser to the scope body, rotating the focuser to the desired angle, and tightening the collar to lock the focuser in the new orientation.

The new bearing-less linear focuser has a polished stainless steel drive rail that runs the length of the drawtube, rather than having the stainless steel drive shaft simply press directly on (and wear) the aluminum drawtube as with conventional Crayford focusers. The drive rail rides in a self-lubricating track that extends most of the length of the focuser body. The drive rail and its attached drawtube are thereby supported over most of their length at all times, rather than by a conventional Crayford focuser's two sets of small contact area roller bearings. This system distributes the drive force evenly over the entire drawtube, without concentrating it on a few small contact points. The result is less potential drawtube flexure and no wear (much less uneven wear) on the drawtube.

The precision-made non-vignetting focuser has dual-speed focusing. There are two coarse focusing knobs. The right knob also has a smaller concentric knob with a 10:1 reduction gear microfine focusing ratio. This provides exceptionally precise image control during critical imaging. All focus knobs are ribbed, so they are easy to operate, even while wearing gloves or mittens in cold weather. Multiple internal baffles in the focuser drawtube assure high contrast.

Despite the new more rigid focuser design, the weight of very heavy equipment trains (camera, plus filter wheel, plus temperature-compensated electric focuser, etc.) may cause the 50mm long focuser drawtube to tilt slightly when fully extended, affecting the focus. Three threaded extension rings (two 1" in length and one 2") are provided to install singly or in combination between the AT8RC rear cell and the focuser. These provide a flex-free solid metal extension that changes the distance between the focuser and the rear cell. This lets you accommodate the varying back-focus requirements of DSLR-type camera imaging versus long equipment train CCD imaging, while minimizing the need to extend the focuser drawtube. Additional optional 1" and 2" long threaded extension rings are available to fine-tune the back focus as needed, as well as optional Astro-Tech 2" compression ring extension tubes that fit into the focuser drawtube.

The image plane is located 10" behind the rear cell. With the standard dual-speed Crayford focuser installed on the scope, there is 159.71mm of back focus available from the top surface of the 2" accessory holder to the image plane.

For exceptionally long and heavy imaging equipment trains, the standard Crayford focuser can be user-replaced by an optional 1.5" travel Feather Touch focuser from Starlight Instruments, #FT-1.5BC. This requires a 90mm x 1mm pitch rear cell thread to Feather Touch adapter, #M90X1. Optional MoonLite focusers from MoonLite Telescope Accessories can also be used.

For even more impressive coma-free imaging with the AT8RC, consider adding the Astro-Tech AT2FF field flattener. While not specifically designed to work with Ritchey-Chrétien astrographs, images taken with the field flattener by Astro-Tech R-C owners have shown that the Astro-Tech 2" field flattener works remarkably well with these advanced coma-free reflectors as well as with refractors. This modestly-priced imaging accessory essentially eliminates the residual field curvature inherent in all reflector telescope designs, so that the coma-free star images remain point-like all across the field. An optical analysis and ray tracing of the field flattener was done in ZEMAX and applied to the R-Cs by Roger Ceragioli, the noted optical designer who did the final optimization of the Astro-Tech Ritchey-Chrétien optics. Here is what he had to say about the #AT2FF, "My conclusion, which seems supported by what users are saying, is that you don't need any other field flattener. This one performs well over a 40mm image circle in all three small RCs (6", 8", and 10")."

Two compression ring accessory holders: The focuser drawtube ends in a 2" accessory holder that uses a non-marring soft brass compression ring to hold 2" imaging accessories in place. The compression ring won't scratch the barrel of your accessories as an ordinary thumbscrew can. Also supplied is a 1.25" accessory holder that slips into the 2" compression ring holder to let you use 1.25" imaging accessories. Like the 2" accessory holder on the drawtube, the 1.25" adapter also uses a non-marring soft brass compression ring.

Two dovetail mounting rails: Two dovetail rails are provided for installing the AT8RC on an equatorial mount, as well as for mounting optional accessories (such as a photoguide scope) on top of the AT8RC. One is a Losmandy-style "D-plate" dovetail rail that runs the full length of the underside of the optical tube, for installing the AT8RC on a Losmandy-style equatorial mount. The second is a Vixen-style dovetail rail that runs the full length of the top of the tube. This can be used for installing a photoguide ring set, piggyback camera adapter, or any other accessory that attaches to a scope by means of Vixen-style dovetail adapters. If the AT8RC is rotated 180°, it will bring the Vixen-style rail to the bottom of the tube so it can be used to install the AT8RC on a Vixen-style equatorial mount. Competitors provide only one Vixen-style dovetail. Providing two dovetail rails on the Astro-Tech AT8RC does not limit your choice of mounts or accessory mounting options, as can happen with similar scopes provided with only one mounting rail.

Finderscope dovetail: a Vixen-style finderscope bracket dovetail base is installed on the upper left side of the optical tube. It can easily be removed if not needed. It will accept Vixen-style finderscope brackets as well as red dot-type finders, such as the Astro-Tech #ATF.

Other accessories: A snap-in dust cap is standard.

Tech Details: 

Supplied Accessories: 

• 2" and 1.25" compression ring accessory holders 
• two dovetail rails - one Vixen-style and one Losmandy-style "D-plate"