|Astro-Tech Ritchey-Chrétien optics|Optical features of this Optical Tube . . .
Mechanical features of this Telescope’s Optical System . . .
- Ritchey-Chrétien optical design: This Astro-Tech optical tube is a true Ritchey-Chrétien (R-C) reflector optical system. Unlike a Maksutov-Cassegrain or Schmidt-Cassegrain catadioptric scope (that uses simple spherical mirrors and corrector lenses), or Newtonian reflectors (that use a coma-producing parabolic primary mirror), this Astro-Tech R-C is a Cassegrain-type two-mirror optical system that uses a concave hyperbolic primary and a convex hyperbolic secondary mirror to form its images. These sophisticated and difficult-to-make mirrors combine to produce images at the Cassegrain focus at the rear of this Astro-Tech scope that are free from coma and spherical aberration, with a smaller spot size, over a much wider field than conventional Newtonians or catadioptrics. The images are likewise free from the chromatic aberration found in refractors and some catadioptrics.
Because of this wide coma-free field, small spot size, and relatively fast focal ratio, the Ritchey-Chrétien design is particularly well suited to astrophotography, rather than visual observing. For imaging, the R-C is the optical system of choice for most of the major professional observatory imaging telescopes built in the last half-century. For example, the Hubble Space Telescope, the twin 10-meter Keck telescopes in Hawaii, and the four 8.2 meter telescopes of the Very Large Telescope array in Chile are all Ritchey-Chrétiens. For serious amateur astronomers and astrophotographers without NASA’s optical budget, an Astro-Tech R-C is likewise the imaging system of choice.
- Fully multicoated quartz and BK7 mirrors: The primary mirror of the 6” Astro-Tech is first-quality BK7 optical glass, while the 8” and larger Astro-Tech R-Cs use primary mirrors of low thermal expansion quartz for maximum focus stability during long exposure imaging sessions. Both 6” R-C mirrors are vacuum-coated with enhanced aluminum for high reflectivity and overcoated with a durable layer of silicon monoxide (quartz) for long life. The 8” and larger mirrors are dielectric multi-coated for long life and reflectivity approaching 99%+.
- Computer designed and fabricated optics: To keep the cost of each Astro-Tech R-C so reasonable when compared to competitive R-C scopes, the computer-optimized Astro-Tech hyperboloid mirrors are automatically ground and finished to very high tolerances using custom-made computerized mirror grinding machines. This precision computer control guarantees an exact repeatability of figure from mirror to mirror that is difficult to achieve using more costly conventional hand figuring. After grinding and polishing, each mirror is individually tested multiple times during fabrication using Zygo interferometers to assure that it meets or exceeds its designed performance standards.
- Frill-free design: To further keep its cost reasonable, an Astro-Tech R-C does away with most of the bells and whistles found on competitive scopes that add little to their performance (but much to their cost). For example, Astro-Tech front and rear cells are first die-cast, then CNC machine-finished, rather than completely CNC machined from raw stock at considerably greater expense but no significant improvement in performance as is the case with other R-Cs. Glare stops in many of the optical tubes are a molded insert, rather than machined aluminum, resulting in a significant savings in cost at no appreciable difference in performance. The Astro-Tech scopes use an external manual dual-speed Crayford focuser, rather than the considerably more complicated and much more costly motorized movable secondary mirror system that other manufacturers use for focusing. The result of the Astro-Tech no-frills approach is genuine Ritchey-Chrétien wide-field performance at a fraction the cost of other commercial R-C systems. While the mechanical bells and whistles may be limited in an Astro-Tech R-C, an Astro-Tech scope still has the high precision flat field/coma-free true Ritchey-Chrétien optics that are the most important reason for buying an R-C scope.
- Fixed primary mirror with computer optimized primary and secondary baffling: Unlike traditional Cassegrain designs that move the primary mirror fore and aft along the central baffle tube in order to achieve focus (which can lead to image shift and focal length changes as the mirror position is adjusted) each Astro-Tech R-C primary mirror is fixed at the precise focal length required for optimum sharpness. The Astro-Tech is focused externally by means of a dual-speed 2” Crayford-style focuser on the rear cell, thereby eliminating a Cassegrain’s moving mirror image shift and focal length change during focusing. Molded field stops are installed along the interior of the optical tube to effectively prevent stray off-axis light from reaching the image plane, resulting in improved contrast. In addition multiple glare-stop microbaffles on the inner surfaces of the primary mirror baffle tube and the secondary mirror light shield further prevent off-axis light from reaching the image plane, resulting in still further improved contrast.
- Collimatable secondary mirror: Since the primary mirror of an Astro-Tech R-C is fixed in position, only the secondary mirror can (or needs to) be collimated. This makes it easy to keep the Astro-Tech RC optics aligned for peak performance. Collimation adjustments to the secondary mirror are made by adjusting the three collimating screws in the back of the secondary mirror holder.
- Cooling fan: The open tube R-C design allows for fast cool-down of the primary and secondary mirrors. Built-in fans on the rear cell of the 10” and larger scopes increases the air-flow around the optics to achieve still quicker “cool down” times of the larger primary mirrors. The 6” and 8” scopes do not have primary mirror cooling fans, as their mirrors are small enough to cool down quickly without any external aid.
This Astro-Tech AT10RCT Ritchey-Chrétien astrograph (named a Sky & Telescope Hot Product for 2015) has:
• 10” f/8 true Ritchey-Chrétien hyperbolic mirror optical design
• carbon fiber truss tube design with CNC-machined stainless steel and aluminum components
• low thermal expansion quartz primary and secondary mirrors
• 99% reflectivity non-tarnishing multi-layer dielectric mirror coatings
• center spotted secondary mirror
• three built-in cooling fans in rear cell
• 3" dual-speed Crayford focuser with 2" and 1.25" compression ring adapters
• three focuser extension rings to fine-tune the back focus
• two Losmandy-style “D-plate” dovetail rails
• two-year warranty
The Astro-Tech AT10RCT astrograph makes the coma-free imaging of true Ritchey-Chrétien optics available to the DSLR and large format CCD astrophotographer at a price less than that of many large format CCD cameras by themselves and even less than some DSLR camera bodies. Featuring first-quality 99% reflectivity dielectric mirror coatings and interferometer-tested premium low thermal expansion quartz mirrors, rather than aluminized Pyrex glass mirrors, this reasonably-priced 10” Astro-Tech truss-tube R-C makes you wonder just what competitive high-price R-Cs have that makes them cost so much more.
Designed for exceptional imaging, the Astro-Tech AT10RCT provides the coma-free photographic field that large format CCD and DSLR astrophotographers crave, but can’t get from conventional reflectors and Schmidt-Cassegrains. Likewise, as a pure two-mirror system, the AT10RCT has a wide spectral response and 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 10” aperture that dwarfs the light gathering of virtually every commercially-available apo refractor.
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 use for their imaging, your wait is over. The 10” Astro-Tech AT10RCT truss-tube R-C can bring the world of professional DSLR/CCD deep space imaging to your backyard observatory at a truly affordable price.
Features of this Astro-Tech AT10RCT Astrograph . . .
Optical design: true Ritchey-Chrétien Cassegrain-type two-mirror optics, with hyperbolic primary and secondary mirrors. For more details, click on the “optics” icon above. The 12" available back focus allows for the use of long CCD equipment trains.
Optical specifications: 10” aperture, 2000mm focal length, f/8 focal ratio.
Hyperboloid primary mirror: Made of low thermal expansion quartz. Ground and polished under precision computer control and tested for accuracy multiple times during fabrication on Zygo interferometers. 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 and focal length changes as the mirror moves), the AT10RCT primary mirror is fixed to eliminate both image shift and focal length changes, as well as the frequent primary mirror collimation requirements of a Newtonian reflector. Focusing is done by means of an optional external focuser, discussed below. Primary mirror collimation is pre-set at the factory, but can be adjusted if needed using three traditional push/pull locking adjustment screw sets.
Hyperboloid secondary mirror: Made of low thermal expansion quartz, rather than Pyrex. Ground and polished under precision computer control and Zygo interferometer tested. Mounted in a CNC-machined four-vane spider and fully collimatable using simple standard Cassegrain reflector collimating techniques. The secondary mirror is precisely center-spotted to make collimation easier. Unlike complicated R-C designs that use motors to move the secondary mirror fore and aft to focus, which can lead to focal length changes, the AT10RCT secondary mirror is fixed and focusing is done externally by means of an optional external focuser, discussed below.
In a review of Astro-Tech's original 8" and 10" R-Cs, Sky & Telescope said that the Astro-Tech R-C’s 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 multi-layer dielectric mirror coatings. These have a full 99% reflectivity for the brightest possible images. This is higher than the often unspecified (but typically about 96%) reflectivity of the enhanced aluminum coatings used by competitors.
3" focuser: A 3" Crayford-style focuser with a 2" compression ring accessory holder and a separate 1.25" compression ring adapter is standard equipment for use with light imaging trains (such as a DSLR body by itself).
This linear bearing 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 supports the drive rail and its attached drawtube 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. There is a focusing scale, in inches and mm, on the top of the drawtube.
The robust construction of the supplied 3" focuser is designed primarily for imaging with a DSLR body. If you plan on imaging with a heavy CCD imaging train, however, you might want to substitute a heavy-duty rack and pinion focuser (from Feather Touch or Moonlite, for example) for additional stability and precision. One popular choice is the dual-speed 3” diameter 1.5” travel Feather Touch #3015 rack and pinion focuser, available from your Astro-Tech dealer.
The focuser attachment port extending through the center of the rear cell of the AT10RCT has a male 117m x 1mm pitch thread for attaching a focuser. Focusers with a maximum 1.5" drawtube travel, such as the Feather Touch #3015, are recommended for use with the Astro-Tech AT10RCT. The standard Feather Touch #3015 3” focuser (Feather Touch #FTF3015B-A) comes with a 109mm threaded collar for connecting to a scope. It therefore requires an optional #M117x1 Feather Touch adapter to be used with the 117mm threads of the AT10RCT. The #M117x1 adapter threads onto the focuser attachment port. The #3015 focuser (without 109mm collar) slips into this adapter and is held in place by three large brass Delrin-tipped retaining knobs. For brands of focusers other than Feather Touch, such as MoonLite, contact the focuser manufacturer for an adapter to fit the 117mm x 1mm port on the AT10RCT.
The image plane is located 10” behind the rear cell (233mm from the top of the focuser attachment port). To fine-tune the back focus of the AT10RCT to the requirements of your camera and equipment train, three threaded extension rings (one 2" in length and two 1" in length) are provided to thread singly or in combination between the 117mm port on the AT10RCT rear cell and the focuser of your choice. These provide a flex-free solid metal extension that changes the distance between your chosen 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. The 2” x 117mm diameter threaded ring weighs 14.4 ounces; each 1" ring weighs 7.2 oz.
Equatorial mount recommendations: Because of the 33.5 pound (15.2 kg) weight of the AT10RCT (without focuser), plus the weight of your focuser, camera equipment, and any photoguide scope, installing the AT10RCT on a German equatorial mount with a 45 pound (minimum) to 100 pound or greater payload capacity is recommended. Such mounts include the 45 pound capacity iOptron iEQ 45 or 60 pound capacity CEM60, the 50 pound capacity Celestron CGEM DX or 90 pound capacity CGE Pro and the Losmandy 60 pound capacity G11 or 100 pound capacity HGM Titan. Other suitable mounts are also available from Software Bisque and Astro-Physics.
Cooling fans: To allow the AT10RCT to reach ambient temperatures more quickly for optimum imaging performance, there are three small low vibration/high CFM primary mirror cooling fans built into the rear cell. The high speed DC fans are powered by a standard equipment battery pack that plugs into a jack on the rear cell. The battery pack uses eight user-supplied AA batteries. An optional external DC power supply, such as a rechargeable 12VDC battery pack can also be used to power the fans.
Two dovetail mounting rails: Two 9.8” Losmandy-style “D-plate" dovetail rails are provided for installing the AT10RCT on an equatorial mount, as well as for mounting optional accessories (such as rings for a photoguide scope). One dovetail, positioned underneath the 15.75" maximum diameter by 24.4” long (without focuser) optical tube, connects the AT10RCT to your equatorial mount. The second rail, 180° away from the first, will then be on top of the tube for installing a photoguide scope or any other accessory that attaches to a scope by means of Losmandy-style “D-plate" dovetail adapters. The undersides of the dovetails have been hollowed out to lighten their weight without compromising their strength.
Two year warranty: As an expression of the confidence Astronomy Technologies has in the quality of their products, the Astro-Tech AT10RCT is protected by a two-year limited warranty against flaws in materials and workmanship.