Astro-Tech 16" f/8 truss tube Ritchey-Chrétien optical tube

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This newest Version A of the Astro-Tech AT16RCT Ritchey-Chrétien astrograph (named a Sky & Telescope Hot Product for 2015) has:
  • 16” f/8 true Ritchey-Chrétien hyperbolic mirror optical design
  • carbon fiber Serrurier truss tube design with CNC-machined stainless steel and aluminum components
  • 18-point primary mirror floatation system
  • low thermal expansion quartz primary and secondary mirrors 
  • 99% reflectivity non-tarnishing multi-layer dielectric mirror coatings 
  • three built-in cooling fans in rear cell 
  • 3" dual-speed compression ring Crayford focuser with 2" and 1.25" adapters 
  • focuser attachment collar isolated from the primary mirror and baffle tube
  • dust covers and finder shoe
  • two-year warranty
    This newest version of the Astro-Tech AT16RCT makes 16" coma-free true Ritchey-Chrétien imaging available to the DSLR and large format CCD astrophotographer at a price less than that of some large format CCD cameras by themselves. 
    Featuring first-quality 99% reflectivity dielectric mirror coatings and premium low thermal expansion quartz mirrors, rather than aluminized Pyrex glass mirrors, this reasonably-priced 16” Astro-Tech truss-tube R-C 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 AT16RCT 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 a 16” aperture that dwarfs the light gathering of 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, your wait is over. The 16” Astro-Tech AT16RCT 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 AT16RCT Ritchey-Chrétien . . .
  • Optical design: true Ritchey-Chrétien Cassegrain-type two-mirror optics, with hyperbolic primary and secondary mirrors. The 288mm usable back focus (from the end of the focuser attachment port) allows for the use of long CCD equipment trains.
  • Optical specifications: 16” aperture, 3250mm 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 and a changing focal length and focal ratio as the mirror position changes) the AT16RCT's non-moving primary mirror is supported by an 18-point floatation system. Primary mirror collimation is pre-set at the factory, but can be adjusted if needed using three heavy-duty spring-loaded 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 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 AT16RCT 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." 
        The secondary mirror's baffle obstruction is 7.44" (189mm) in diameter (46.5% by diameter and 21.62% by area of the 16" primary.)
  • 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: Four 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 middle and rear support rings have a hexagonal outer shape, measuring 22.5" (572mm) across at their widest point. They are 10mm thick. The dual front support rings provide attachment points for the secondary mirror spider vanes, as well as for the truss tubes. They measure 19.75" (502mm) across at their widest point of their hexagonal outer shape and are 5mm thick. The AT16RCT measures only 45.5" long (1156mm) from the front support ring to the end of the focuser mounting collar, and weighs only 77 pounds (35kg) without focuser.
        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 when 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.  
        The Astro-Tech AT16RCT is 45.5" (1156mm) long without focuser. The supplied focuser adds 5.25" (133mm) with the drawtube retracted. The center and rear truss tube support rings are 22.5" (572mm) in diameter, while the front support ring is 19.75" (502mm) in diameter. The AT16RCT weighs 77 lbs (35 kg) without focuser. The supplied focuser adds 2.2 lbs (1 kg).
  • 3" Crayford focuser: The AT16RCT is supplied with a basic light duty 3" Crayford focuser to get you started imaging. While acceptable for casual imaging with a light imaging train (such as a light DSLR body by itself), upgrading to a high quality rack-and-pinion focuser is required for serious imaging with an optical tube of this quality. A heavy multi-component imaging train will 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 light-duty Crayford focuser can be unthreaded from the focuser attachment collar, 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 newly designed focuser attachment collar is bolted to the back plate of the AT16RCT, completely independent of the primary mirror and baffle tube. This allows it to be collimated separately from the primary and secondary mirrors if needed, using the built-in focuser collimating ring. The focuser attachment collar has a male 117m x 1mm pitch thread for attaching the focuser. The Feather Touch #3015 normally comes with a 109mm threaded collar for connecting to a scope. Using the #3015 with the AT16RCT therefore requires an optional #M117x1 Feather Touch adapter. The #M117x1 adapter threads onto the focuser attachment port. The #3015 focuser (without its 109mm collar) then 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 16" Astro-Tech AT16RCT 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. 
  • Back focus: To fine-tune the 288mm usable back focus of the AT16RCT 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 focuser attachment collar and the focuser of your choice. These rings 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. 
  • Mount requirements: Because of the 77 pound (35 kg) weight of the AT16RCT, plus the weight of your focuser, ancillary camera equipment, and any photoguide scope, installing the AT16RCT on a German equatorial mount with a 90 to 100 pound minimum payload capacity (preferably greater) is recommended. Such mounts include the 90 pound capacity Celestron CGE Pro; the Losmandy 100 pound capacity Losmandy HGM Titan; the 220 pound capacity Astro-Physics 1600GTO; and the 90 pound Software Bisque Paramount MX or 240 pound capacity Paramount ME II. Other suitable mounts are also available.
  • Cooling fans: To allow the AT16RCT to reach ambient temperatures more quickly for optimum imaging performance, there are three low vibration/high CFM primary mirror cooling fans built into the AT16RCT's rear support truss/back plate. The high speed DC fans are powered by a standard equipment battery pack that plugs into a 5mm female jack in the back plate. 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 16.2” (412mm) 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 AT16RCT 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.
  • Other accessories: This new version of the AT16RCT includes slip-on dust covers for both the primary and secondary mirrors. Also included is a mounting shoe for a red dot-type finder, such as the Astro-Tech ATF. The mounting shoe can also accept the Astro-Tech ATF50QRB 50mm finderscope quick release bracket.
  • Two year warranty: As an expression of the confidence Astronomy Technologies has in the quality of their products, the Astro-Tech AT16RCT is protected by a two-year limited warranty against flaws in materials and workmanship.

Visual Limiting Magnitude:
This is the magnitude (or brightness) of the faintest star that can be seen with a telescope. The larger the number, the fainter the star that can be seen. An approximate formula for determining the visual limiting magnitude of a telescope is 7.5 + 5 log aperture (in cm).

This is the formula that we use with all of the telescopes we carry, so that our published specs will be consistent from aperture to aperture, from manufacturer to manufacturer. Some telescope makers may use other unspecified methods to determine the limiting magnitude, so their published figures may differ from ours.

Keep in mind that this formula does not take into account light loss within the scope, seeing conditions, the observer’s age (visual performance decreases as we get older), the telescope’s age (the reflectivity of telescope mirrors decreases as they get older), etc. The limiting magnitudes specified by manufacturers for their telescopes assume very dark skies, trained observers, and excellent atmospheric transparency – and are therefore rarely obtainable under average observing conditions. The photographic limiting magnitude is always greater than the visual (typically by two magnitudes).

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.

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.

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.

0.29 arc seconds
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.
Back Focus:
11.33" (288mm)
The weight of this product.
77 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.
2 years
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General Accessories
Dust Covers (2)
Light shroud for 16" Astro-Tech AT16RCT truss-tube Ritchey-Chrétien
by Astrozap
Dust cover for tube end of Astro-Tech AT16RCT truss-tube Ritchey-Chrétien
by Astrozap
Visual Accessories
Finderscopes (2)
Illuminated multiple reticle finder
by Astro-Tech
Quick release bracket for 50mm Astro-Tech finders
by Astro-Tech
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Astro-Tech 16" f/8 truss tube Ritchey-Chrétien optical tube

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Astro-Tech 16" f/8 truss tube Ritchey-Chrétien optical tubeClose-up of Astro-Tech AT16RCT primary mirror section, showing baffle tube and mirror clip.Front view of Astro-Tech AT16RCT.Close-up of Astro-Tech AT16RCT back plate showing the new focuser attachment collar design.The supplied AT16RCT press-fit dust covers.The Astro-Tech AT16RCT secondary mirror assembly, showing the center marking for collimation.The Astro-Tech AT16RCT center truss, showing the primary mirror light baffle and truss tube attachment method.Close-up of the Astro-Tech AT16RCT spring-loaded secondary mirror baffle and spider vane attachment method.The Astro-Tech AT16RCT on a Losmandy HGM Titan mount.The Sky & Telescope Hot Product for 2015 Award
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Our Product #: AT16RCT
Manufacturer Product #: AT16RCT
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This newest version of the Astro-Tech AT16RCT carbon fiber Serrurier truss 16" f/8 Ritchey-Chrétien Hot Product for 2015 makes high quality large-aperture DSLR and CCD imaging affordable for the dedicated amateur astrophotographer and educational institution.

. . . our 36th year