12" f/8 truss tube 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.
This Astro-Tech AT12RCT Ritchey-Chrétien astrograph (named a Sky & Telescope Hot Product for 2015) has:
  • 12” f/8 true Ritchey-Chrétien hyperbolic mirror optical design
  • carbon fiber Serrurier 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 1.25" adapter 
  • two-year warranty
    The original steel body Astro-Tech AT12RC was named a Sky & Telescope Hot Product for 2011. It joined the line-up of other Astro-Tech Ritchey-Chrétien scopes 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). The carbon fiber truss-tube AT12RCT is the latest addition to that lineup, featuring the same high quality R-C optical system used in the award-winning 12" steel body version.   

    The Astro-Tech AT12RCT 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 truss-tube R-C makes you wonder just what competitive high-price R-Cs have that makes them cost so much more.

    The Astro-Tech AT12RCT 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 AT12RCT 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.

    If serious astrophotography is your goal, but the price of 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 12” Astro-Tech AT12RCT 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 AT12RCT 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 available back focus allows for the use of long CCD equipment trains.
  • Optical specifications: 12” aperture, 2432mm focal length, f/8 focal ratio. Multiple Zygo interferometer tests, during every stage of optics manufacturing (after rough figure generation, after fine figure generation, after final polish before coating, and after dielectric coating), assure premium optical performance. 
  • 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 AT12RCT 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 AT12RCT secondary mirror is fixed and focusing is done externally by means of an optional external focuser, discussed in the focuser section 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.
  • Carbon fiber truss-tube design: Three CNC-machined aluminum support rings form the basic structure of the optical tube. Light-weight and rigid carbon fiber tubes connect the support rings in a Serrurier truss design, using CNC-machined stainless steel ball and socket hardware.

    • The Serrurier truss solves the problem of optical tube flexure by supporting the primary and secondary mirrors with two sets of opposing trusses mounted before and after the center support ring. The trusses are designed to have an equal amount of flexure, which allows the optics to stay on a common optical axis. When flexing, the "top" truss resists tension and the "bottom" truss resists compression. This has the effect of keeping the optical elements parallel to each other. The net result is that the optical elements stay in collimation regardless of the orientation of the telescope, including passing through the meridian during imaging. 

    The truss tubes are made of a light weight/high strength woven carbon fiber-reinforced composite material with extremely low thermal expansion characteristics. This reduces the possibility of temperature-related focus changes that can occur with steel or aluminum optical tube scopes during extreme temperature swings.

  • 3" Crayford focuser: A good-quality 3" Crayford focuser with a 2" compression ring accessory holder and a 1.25" compression ring adapter is standard equipment for use with light imaging trains (such as just a DSLR body). However, you may plan on using a very heavy multi-ecomponent imaging train which may require a rack-and-pinion focuser to support the extra weight without focus shift. Alternatively, you may already have a premium focuser being used on another scope that you would like to use for imaging.

    • The supplied AT12RCT Crayford focuser can be unthreaded from the rear cell, allowing you to use another focuser. 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 AT12RCT has a male 117m x 1mm pitch threaded port for attaching the focuser. The Feather Touch #3015 normally comes with a 109mm threaded collar for connecting to a scope. However, using the #3015 with the AT12RCT also requires an optional #M117x1 Feather Touch adapter. The #3015 focuser (without its 109mm collar) slips into the #M117x1 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 AT12RCT 12" Ritchey-Chrétien. For other focusers, such as a 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 AT12RCT to the requirements of your camera and equipment train, three threaded extension rings (one 2" and two 1" in length) are provided to thread singly or in combination between the 117mm port on the AT12RCT 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” threaded ring weighs 14.4 ounces, the 1" rings weigh 7.2 ounces each. 

  • Recommended mounts: Because of the 52 pound (23.6 kg) weight of the AT12RCT, plus the weight of your ancillary camera equipment and any photoguide scope, installing the AT12RCT on a German equatorial mount with a 90 to 100 pound or greater payload capacity is recommended. Such mounts include the 90 pound capacity Celestron CGE Pro, the Losmandy 100 pound capacity Losmandy HGM Titan, and the 90 pound capacity Software Bisque Paramount MX. Other suitable mounts are also available.
  • Cooling fans: To allow the AT12RCT 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 14” Losmandy-style “D-plate" dovetail rails are bolted to the top and bottom of the center and rear truss-tube support rings. These allow you to install the AT12RCT directly on an equatorial mount and mount optional accessories (such as rings for a photoguide scope that attach 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 AT12RCT 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.
52 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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1. John on 2/15/2014, said: AstronomicsAstronomicsAstronomicsAstronomicsAstronomics
This is just a preliminary review since the weather has not been cooperating since I received the scope (so what else is new). First, let me tell you that this is my fifth Astro Tech RC, starting with the 6" and moving up in aperture Each one was exceptionally made and a joy to use. Once I heard that Astro Tech was coming out with a carbon fiber truss design, I knew I had to have one. The advantages are many; from fast cool down, to focus and forget it, to thinner spider vanes (nicer star images) and lighter weight. One of the first things you will notice is the excellent workmanship and the fit and finish of the scope. It rivals scopes costing five times as much. Optically I have only had a chance to do some limited testing. The ronchi test showed beautiful straight lines with no turned edge. Visually Jupiter was razor sharp and full of color. I did have a chance to put my ST-10XME camera on the scope which revealed beautiful star images and spikes before the clouds rolled back in. This scope will require some maintenance, since the secondary has to be dead on to get best performance and will require tweaking after a long transport. My scope arrived slightly out of collimation but is a breeze to adjust. Five stars nails this scope. As for value there is nothing on the market that even comes close.
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General Accessories
Dew Heaters (1)
Heater for Astro-Tech 8", 10", and 12" Ritchey-Chrétien secondary mirrors
by Kendrick
Quantity:  
$57.50 
Dust Covers (2)
Light shroud for 12" Astro-Tech AT12RCT truss-tube Ritchey-Chrétien
by Astrozap
Quantity:  
$69.00 
Dust cover for tube end of Astro-Tech AT12RCT truss-tube Ritchey-Chrétien
by Astrozap
Quantity:  
$19.00 
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12" f/8 truss tube Ritchey-Chrétien optical tube

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12" f/8 truss tube Ritchey-Chrétien optical tubeRear cell of AT12RCT.Front view of AT12RCT.Close-up of D-plate for accessories. Also shown is one of three pairs of primary mirror collimating screws.Close-up of center support ring truss tube ball and socket mounts.Close-up of rear cell ring truss tube ball and socket mounts.Close-up of front support ring truss tube ball and socket mounts.The Sky 7 Telescope Hot Product for 2015 award.
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Our Product #: AT12RCT
Manufacturer Product #: AT12RCT
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The Astro-Tech AT12RCT carbon fiber Serrurier truss Ritchey-Chrétien Hot Product for 2015 makes high quality large-aperture/large format DSLR and CCD imaging affordable for the dedicated amateur astrophotographer and educational institution.





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