Astro-Tech 8" Ritchey Chretien Astrograph Telescope
A telescope with high quality optics is key to great observing. However, a high quality telescope priced reasonably is less common. The Astro-Tech 8" Ritchey Chretien is a fabulous optical tube with an excellent reputation for being astrophotography friendly. The telescope features internal optics such as a carbon fiber external body, quartz mirrors, dielectric mirror coatings, and two dovetail mounting rails.
The 8" Ritchey Chretien astrograph has been named by Sky & Telescope as one of its hottest products in their January 2010 release! Since Ritchey Chretien designed optical tubes are very common for professional research fields, there's no question this optical tubes are excellent for detailed observing. The problem is most of these instruments are highly priced and availability can be limited. This 8" telescope from Astro-Tech is an exception.
Another aspect that makes this telescope unique is its fantastic compatibility between many CCD and DSLR astrophotography cameras. Due to its coma-free qualities in which the optics are designed to inhibit aberration of pinpoint stars, this is one important aspect that is crucial in getting images of great detail. This astrophotography friendly features is less present in reflectors and Schmidt-Cassegrain. If you enjoy astrophotography, this is a telescope to consider!
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 - Made of low thermal expansion quartz, rather than the ordinary optical glass used by competitors. 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 - Made of low thermal expansion quartz, rather than ordinary optical glass. 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.”
99% reflectivity dielectric coated optics - Both primary and secondary mirrors have non-tarnishing state-of-the-art dielectric mirror coatings. These have a full 99% reflectivity for the brightest possible images. This is substantially higher than the 88% reflectivity of competitors’ conventional aluminum coatings or the 94-96% reflectivity of enhanced aluminum coatings.
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 lesser rolled steel tube systems. 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")."
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