Astro-Tech 6" f/9 Astrograph with 2" Crayford Focuser and Vixen style dovetail
Ritchey Chretien astrograph telescopes are exclusively used in many professional fields, including the HST. Astro-Tech has designed a 6" f/9 Astrograph that is based upon this design to the amateur astronomer at affordable prices. Due to its weight of 12.9 pounds and a supplied Vixen style dovetail, this astrograph is primarily compatible with virtually any equatorial mount, so there is no need to buy an additional mount specific for this scope.
The astrograph works great with many DSLR cameras. In fact, Sky & Telescope magazine stated in 2009 this telescope was "a superb match" to many astrophotography cameras featuring APS-C chips.
Since Ritchey Chretien designs bring wide coma-free photographic fields that are tougher to achieve in Schmidt-Cassegrains and other reflector telescopes. Due to its internal optics of composed of BK-7 glass, purious color that affects the imaging of all but the most costly apochromatic refractors, and it does it with an 6" aperture that equals or exceeds the light gathering of most apo refractors.
So if you have a great passion for astrophotography, this affordable astrograph is just for you. Thanks to Astro-Tech!
Optical design - true Ritchey-Chrétien two-mirror Cassegrain optics, with hyperbolic primary and secondary mirrors. 6" aperture, 1370mm focal length, f/9 focal ratio.
Hyperboloid primary mirror - Made of BK-7 optical glass. Polished to diffraction-limited or better surface accuracy. Unlike catadioptric designs (SCTs, Maksutovs, etc.) that move the primary mirror to focus (which can lead to image shift as the mirror moves), the AT6RC mirror is fixed to eliminate potential image shift. The fixed primary mirror also eliminates the frequent primary mirror collimation requirements of a Newtonian reflector. The December 2009 issue of Sky & Telescope said that the AT6RC 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."
Hyperboloid secondary mirror - Made of BK-7 optical glass. Polished to diffraction-limited or better surface accuracy. Mounted in a four-vane spider and fully collimatable.
Enhanced aluminum optical coatings - Both primary and secondary mirrors have enhanced aluminum mirror coatings, overcoated with a protective layer of quartz for long life. Reflectivity is in the 96% range, the same as the larger multi-thousand dollar R-C scopes.
Optical tube - Painted rolled steel, 7.5" o.d. x 19.25" long, with die-cast and machined aluminum front and rear cells.
Internal light baffles - Computer optimized primary and secondary baffling. Eight contrast-enhancing glare-stop baffles in the optical tube, multiple glare-stop microbaffles in the secondary mirror light shield, and four baffles in the primary mirror baffle tube provide truly dark sky backgrounds during imaging.
Dual-speed Crayford focuser - A 2" Crayford focuser is threaded onto the 90mm x 1mm pitch rear cell of the AT6RC. The 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 CCD imaging. All focus knobs are ribbed, so they are easy to operate, even while wearing gloves or mittens in cold weather.
Focuser travel - Focuser drawtube travel is 34mm, while back focus between the focuser's 2" accessory holder and the focal plane is 150mm. The December 2009 issue of Sky & Telescope said that “the infinity focus of the AT6RC falls 9-3/4" outside the back of the telescope, allowing plenty of room for all types of cameras, filter wheels, and ancillary equipment." Two extension rings (one 1" and one 2") are provided to thread onto the rear of the scope (singly or in combination) between the scope rear cell and the focuser. These form a very rigid and tilt-free extension that moves the focuser out from the rear cell to accommodate the varying back-focus requirements of webcam/Deep Sky Imager-type cameras versus DSLRs. Typically, both rings (one 1" and one 2") will properly position most DSLRs at the AT6RC focal plane. Sky & Telescope pointed out that the “1- and 2-inch extensions, as well as conventional extension tubes that fit in the 2-inch focuser, mean that you can always assemble the system so it comes to focus with whatever camera and accessories you're using." While it is possible to use large CCD cameras and filter wheels with the AT6RC, these cameras are generally not recommended. The weight of a standard CCD camera and the color filter wheel typically used with it may cause the focuser drawtube to tilt slightly when fully extended, affecting the focus. Adding an optional Feather Touch focuser (our part #FT-1.5BC), plus an optional 90mm x 1mm pitch adapter (our part #M90X1), will eliminate any such potential problem, although at significant additional cost (more than double the cost of the AT6RC itself). For even more impressive coma-free DSLR imaging with the AT6RC, consider adding the Astro-Tech AT2FF field flattener. 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 of the APS-C size imaging chip used in most DSLR cameras. Use of the #AT2FF with a DSLR will typically requires one optional #AT2EXT 2" extension ring, instead of the two rings generally needed with a DSLR camera alone.
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" eyepiece holder on the eyepiece holder rotation mechanism, the 1.25" adapter also uses a non-marring soft brass compression ring.
Mounting dovetail - A Vixen-style dovetail bar runs the length of the underside of the optical tube. The dovetail can be removed, if desired, so the AT6RC can be installed in optional user-supplied mounting rings for piggyback mounting on a larger scope. An optional #AT6SDP Vixen-style dovetail is available for mounting on top of the AT6RC to allow you to mount an optional photoguide scope on top of the astrograph.
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.