Astro-Tech 8″ f/8 Ritchey-Chrtien Astrograph, Steel Tube

Description

Despite the new more rigid focuser design, the weight of very heavy equipment trains (camera, plus filter wheel, plus temperature-compensated electric focuser, etc.) may cause the 50mm long focuser drawtube to tilt slightly when fully extended, affecting the focus. Three threaded extension rings (two 1″ in length and one 2″) are provided to install singly or in combination between the AT8RCS rear cell and the focuser. These provide a flex-free solid metal extension that changes the distance between the 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, while minimizing the need to extend the focuser drawtube. Additional optional 1″ and 2″ long threaded extension rings are available to fine-tune the back focus as needed, as well as optional Astro-Tech 2″ compression ring extension tubes that fit into the focuser drawtube.

The image plane is located 10″ behind the rear cell. With the standard dual-speed Crayford focuser installed on the scope, there is 159.71mm of back focus available from the top surface of the 2″ accessory holder to the image plane.

For exceptionally long and heavy imaging equipment trains, the standard Crayford focuser can be user-replaced by an optional 1.5″ travel Feather Touch focuser from Starlight Instruments, #FT-1.5BC. This requires a 90mm x 1mm pitch rear cell thread to Feather Touch adapter, #M90X1. Optional MoonLite focusers from MoonLite Telescope Accessories can also be used.

For even more impressive coma-free imaging with the AT8RCS, consider adding the Astro-Tech AT2FF field flattener. While not specifically designed to work with Ritchey-Chrtien astrographs, images taken with the field flattener by Astro-Tech R-C owners have shown that the Astro-Tech 2″ field flattener works remarkably well with these advanced coma-free reflectors as well as with refractors. 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. An optical analysis and ray tracing of the field flattener was done in ZEMAX and applied to the R-Cs by Roger Ceragioli, the noted optical designer who did the final optimization of the Astro-Tech Ritchey-Chrtien optics. Here is what he had to say about the #AT2FF, “My conclusion, which seems supported by what users are saying, is that you don’t need any other field flattener. This one performs well over a 40mm image circle in all three small RCs (6″, 8″, and 10″).”

Two 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″ accessory holder on the drawtube, the 1.25″ adapter also uses a non-marring soft brass compression ring.

Two dovetail mounting rails: Two dovetail rails are provided for installing the AT8RS on an equatorial mount, as well as for mounting optional accessories (such as a photoguide scope) on top of the AT8RCS. One is a Losmandy-style “D-plate” dovetail rail that runs the full length of the underside of the optical tube, for installing the AT8RCS on a Losmandy-style equatorial mount. The second is a Vixen-style dovetail rail that runs the full length of the top of the tube. This can be used for installing a photoguide ring set, piggyback camera adapter, or any other accessory that attaches to a scope by means of Vixen-style dovetail adapters. If the AT8RCS is rotated 180, it will bring the Vixen-style rail to the bottom of the tube so it can be used to install the AT8RCS on a Vixen-style equatorial mount. Competitors provide only one Vixen-style dovetail. Providing two dovetail rails on the Astro-Tech AT8RCS does not limit your choice of mounts or accessory mounting options, as can happen with similar scopes provided with only one mounting rail.

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.

Tech Details:

Supplied Accessories:

2″ and 1.25″ compression ring accessory holders
two dovetail rails – one Vixen-styleandone Losmandy-style “D-plate”