12" f/8 Ritchey-Chrétien optical tube

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Astro-Tech Ritchey-Chrétien optics
Optical features of this Optical Tube . . .
  • 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.
Mechanical features of this Telescope’s Optical System . . .
  • 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.
Developed by Astro-Tech to make large-aperture/large-format CCD imaging affordable for the dedicated amateur astronomer and educational institution, the 12” Astro-Tech AT12RC is priced thousands of dollars less one competitor’s 10” R-C! The Astro-Tech AT12RC is truly the first affordable 12” true Ritchey-Chrétien astrograph.
Sky and Telescope Hot Product for 20011
This Astro-Tech AT12RC Ritchey-Chrétien astrograph has:

• 12” f/8 true Ritchey-Chrétien hyperbolic mirror optical design
• 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" Crayford-style focuser with 2" and 1.25" compression ring adapters
• two 2” focuser extension rings to fine-tune the back focus of your focuser
• rolled steel tube with cast and machined aluminum front and cell
• split hinged tube rings with two Losmandy-style “D-plate” dovetail rails
• mounting bracket for optional finder
• two-year warranty

    The 12” Astro-Tech AT12RC is the first 12” true Ritchey-Chrétien astrograph available at a reasonable price. It joins the line-up of other Astro-Tech Ritchey-Chrétien astrographs that have previously been honored by Sky & Telescope (the original Astro-Tech 6” R-C was named a Hot product for 2009 and the Astro-Tech 8” and 10” R-Cs were named Hot Products for 2010).

    And we are serious when we say the 12” aperture AT12RC is “reasonably priced.” The Astro-Tech AT12RC is priced many thousands of dollars less than one competitive 12.5” R-C, and still thousands less than another manufacturer’s smaller 10” R-C. As Sky & Telescope pointed out in their Hot Product citation in their January 2010 issue, “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 Ritchey-Chrétiens.” (The AT12RC is priced at only a third of that per inch of aperture figure.)

    The Ritchey-Chrétien optical design is used in virtually every recent large mega-million dollar professional observatory telescope – including the Hubble Space Telescope. The Astro-Tech AT12RC astrograph makes the coma-free imaging of true Ritchey-Chrétien imaging 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 premium low thermal expansion quartz mirrors, rather than aluminized Pyrex glass mirrors, this reasonably-priced 12” Astro-Tech 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 AT12RC provides the coma-free photographic field that large format CCD astrophotographers crave, but can’t get from conventional reflectors and Schmidt-Cassegrains. Likewise, as a pure two-mirror system, the AT12RC 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 12” aperture that dwarfs the light gathering of virtually every commercially-available apo refractor.

    In the “Feature Images” section below is a first light AT12RC image of M51 taken by Preston Starr, Observatory Manager of the Astronomy Program at the University of North Texas. Preston shared these comments with us when he sent in the image: “The conditions for imaging were really bad . . . but I thought the AT12RC really performed! Location: ‘Astronomers by Necessity’ observatories; 5 miles southeast of Necessity, TX; 100 miles west of Ft. Worth, TX. Conditions: Poor seeing, poor transparency due to 25~45 mph winds and dust. Telescope: AT12RC F8 (No Field Flattener). Camera: Atik 4000 OSC CCD. Exposure Details: 10 minutes each, 31 images, cooled -15c. Capture with MaxImDL, processed using bias, darks. IP processing by Don Waid.” Our congratulations to both Preston and Don for their very good first light image, despite the very poor imaging conditions.

    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 12” Sky & Telescope Hot Product for 2011 Astro-Tech AT12RC astrograph 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 AT12RC 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 288mm of available back focus allows for the use of long CCD equipment trains.

  • Optical specifications: 12” aperture, 2432mm focal length, f/8 focal ratio.

  • Hyperboloid primary mirror: Made of low thermal expansion quartz. Ground and polished under precision computer control. 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 AT12RC primary mirror is fixed to eliminate both a catadioptric’s image shift and 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. 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, the AT12RC secondary mirror is fixed and focusing is done externally by means of an optional external focuser, discussed below.

        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 rear cell of the AT12RC has a male 117m x 1mm pitch threaded port for attaching a focuser. The standard Feather Touch 3” diameter 1.5” travel focuser normally comes with a 109mm threaded collar for connecting to a scope. However, in its #3015 configuration (Feather Touch part #FTF3015B-A), it requires an optional #M117x1 Feather Touch adapter to be used with the AT12RC. The #3015 focuser (without 109mm collar) slips into this adapter and is held in place by three large brass Delrin-tipped retaining knobs. Only focusers with a maximum 1.5" drawtube travel, such as the Feather Touch #3015, are recommended for use with the Astro-Tech AT12RC 12" Ritchey-Chrétien. 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 rear of the scope.

        To fine-tune the 288mm back focus of the AT12RC to the requirements of your camera and equipment train, two threaded extension rings (each 2" in length) are provided to thread singly or in combination between the 117mm port on the AT12RC 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. Each 2” x 117mm diameter threaded ring weighs 14.4 ounces.

  • Recommended mounts: Because of the 41.5 pound (18.9 kg) weight of the AT12RC (without rings, dovetails, or focuser), plus the weight of your ancillary camera equipment and any photoguide scope, installing the AT12RC on a German equatorial mount with a 60 pound (minimum) to 100 pound or greater payload capacity is recommended. Such mounts include the 90 pound capacity Celestron CGE Pro and the Losmandy 60 pound capacity G11 or G11 go-to and 100 pound capacity Losmandy HGM Titan. Other suitable mounts are also available.
  • Cooling fans: To allow the AT12RC 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.

  • Split rings and two dovetail mounting rails: Dual split hinged mounting rings, joined by two 14” Losmandy-style “D-plate" dovetail rails bolted to the rings, are provided for installing the AT12RC on an equatorial mount, as well as for mounting optional accessories (such as rings for a photoguide scope) on top. The rings are positioned so that one rail is underneath the 14.3" diameter by 34” long (without focuser) optical tube for installing the AT12RC directly on a suitable payload capacity 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. The rings and dovetails have a combined weight of 7.8 pounds (3.55 kg).

  • Finderscope dovetail: A Vixen-style finderscope bracket dovetail base is installed on the upper left side of the optical tube rear cell. 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.

  • Two year warranty: As an expression of the confidence Astronomy Technologies has in the quality of their products, the Astro-Tech AT12RC is protected by a two-year limited warranty against flaws in materials and workmanship.
Focal Length:
This is the length of the effective optical path of a telescopeor eyepiece (the distance from the main mirror or lens where the lightis gathered to the point where the prime focus image is formed). Focallength is typically expressed in millimeters.

The longer the focallength, the higher the magnification and the narrower the field of viewwith any given eyepiece. The shorter the focal length, the lower themagnification and the wider the field of view with the same eyepiece.

2432mm
Focal Ratio:
This is the ‘speed’ of a telescope’s optics, found by dividing the focal length by the aperture. The smaller the f/number, the lower the magnification, the wider the field, and the brighter the image with any given eyepiece or camera.

Fast f/4 to f/5 focal ratios are generally best for lower power wide field observing and deep space photography. Slow f/11 to f/15 focal ratios are usually better suited to higher power lunar, planetary, and binary star observing and high power photography. Medium f/6 to f/10 focal ratios work well with either.

An f/5 system can photograph a nebula or other faint extended deep space object in one-fourth the time of an f/10 system, but the image will be only one-half as large. Point sources, such as stars, are recorded based on the aperture, however, rather than the focal ratio – so that the larger the aperture, the fainter the star you can see or photograph, no matter what the focal ratio.

f/8
Resolution:
This is the ability of a telescope to separate closely-spaced binary stars into two distinct objects, measured in seconds of arc. One arc second equals 1/3600th of a degree and is about the width of a 25-cent coin at a distance of three miles! In essence, resolution is a measure of how much detail a telescope can reveal. The resolution values on our website are derived using the Dawes’ limit formula.

Dawes’ limit only applies to point sources of light (stars). Smaller separations can be resolved in extended objects, such as the planets. For example, Cassini’s Division in the rings of Saturn (0.5 arc seconds across), was discovered using a 2.5” telescope – which has a Dawes’ limit of 1.8 arc seconds!

The ability of a telescope to resolve to Dawes’ limit is usually much more affected by seeing conditions, by the difference in brightness between the binary star components, and by the observer’s visual acuity, than it is by the optical quality of the telescope.

0.38 arc seconds
Aperture:
This is the diameter of the light-gathering main mirror or objective lens of a telescope. In general, the larger the aperture, the better the resolution and the fainter the objects you can see.
12"
Back Focus:
11.33” (288mm) from rear cell
Weight:
The weight of this product.
49.3 lbs.
Heaviest Single Component:
The weight of the heaviest component in this package.
41.5 lbs.
Telescope Type:
The optical design of a telescope.  Telescope type is classified by three primary optical designs (refractor, reflector, or catadioptric), by sub-designs of these types, or by the task they perform.
Ritchey-Chrétien
Warranty:
2 years
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General Accessories
Finderscopes (2)
Illuminated multiple reticle finder
by Astro-Tech
Quantity:  
$59.95 
Vixen-Style mounting bracket for Astro-Tech multiple reticle finder
by Astro-Tech
Quantity:  
$10.00 
Visual Accessories
Dewcaps and Lens Shades (1)
Flexible black plastic for the 12" Astro-Tech AT12RC Ritchey-Chrétien astrograph
by Astrozap
Quantity:  
$64.95 
Photographic Accessories
Collimating Tools (1)
Focuser collimating ring for 10" and 12" Astro-Tech Ritchey-Chrétiens
by Astro-Tech
Quantity:  
$99.95 
Focusers (2)
Feather Touch 3" Manual rack and pinion focuser and 117mm adapter
by Starlight Instruments
Quantity:  
$794.00 
117mm x 1mm pitch adapter to put Feather Touch Focuser on Astro-Tech 10" and 12" RC astrographs
by Starlight Instruments
Quantity:  
$164.00 
Tele-Compressors (Photo/Visual) (1)
2" Field flattener for imaging with Astro-Tech and TMB refractors
by Astro-Tech
Quantity:  
$150.00 
• two 2” focuser extension rings to fine-tune the back focus of your optional focuser
• rolled steel tube with cast and machined aluminum front and cell
• split hinged tube rings with two Losmandy-style “D-plate” dovetail rails 
• mounting bracket for optional finder 
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Astro-Tech - 12" F/8 Ritchey-Chrétien astrograph body only

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Astro-Tech - 12" F/8 Ritchey-Chrétien astrograph body onlyImage showing the rear cell of the Astro-Tech AT12RC, showing the cooling fans and the 117mm x 1mm port for attaching your optional focuser.AT12RC first light image, taken under
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The 12” Astro-Tech AT12RC Ritchey-Chrétien astrograph is a large aperture true Ritchey-Chrétien astrograph, an astro-imager’s dream, a Sky & Telescope Hot Product for 2011, and – at a price thousands below competitive scopes – the first reasonably-priced 12” true R-C available.





. . . our 34th year