90mm finder/photoguide ring system

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This large Astro-Tech finderscope/photoguide telescope mounting ring system will mount a short focal length refractor optical tube on a Celestron or Meade Schmidt-Cassegrain or Ritchey-Chrétien telescope rear cell. It lets you use the refractor as a high power/variable power finderscope for your larger telescope; as a photoguide scope for long exposure deep sky photography with the main scope; or as a wide field astrograph.

The photoguide system has two parts. One part consists of a pair of guidescope mounting rings bolted to a 3.56” long dovetail rail. The second part is a 3.56” long dovetail shoe that holds the ring system to your scope. This shoe bolts onto a catadioptric scope’s rear cell, using the telescope manufacturer’s predrilled finderscope/accessory holes. Celestron and Meade catadioptric telescopes 7” in aperture and larger generally have two pairs of mounting holes on 1.25” centers, located at the 10 o’clock and 2 o’clock positions on their rear cells. The 10 o'clock pair normally holds the telescope’s factory-equipped finder, while the second pair is for mounting accessories, such as this photoguide ring set. The cosmetic bolts filling the unused pair of holes on the rear cell are removed and the 3.56” shoe is bolted into the exposed holes, using the new chrome-finish bolts and hex head wrench supplied with this ring assembly.

There are three pairs of mounting bolt slots in the base of the mounting shoe, spaced along the shoe at approximately 1” intervals. This allows you to position the mounting shoe fore and aft on the telescope rear cell to put your photoguide scope in the most convenient position to avoid bumping it with your head during use. These 3/16” x 5/16” slot pairs allow for the minor differences in the nominal 1.25” mounting hole spacing sometimes encountered with older telescopes. The base of the mounting shoe is curved to fit a 7”-8” telescope’s rear cell, but the shoe will work on the rear cells of catadioptric scopes up to 20” in aperture, although with a minor gap under the center of the mounting shoe that does not affect the stability of the mounting rings/guidescope assembly.

There are also five countersunk holes along the centerline of the shoe. These allow you to mount the rings on those scopes that have only a single mounting hole available.

The ring system can be used on other scope types as well as catadioptrics, provided that the scope has a pair of accessory mounting holes on the industry standard 1.25” centers (or can be drilled with such holes, as in the case of a Sonotube or aluminum optical tube reflector.) It cannot be used on Takahashi or Vixen scopes, which use proprietary finderscope mounting bolt patterns. Proper installation of the mounting ring system on scopes other than Celestron or Meade 7” and larger catadioptrics is the responsibility of the buyer.

The mounting rings themselves have a nominal 95mm (3.75”) internal diameter and will hold optical tubes with an o. d. of 60mm to 90mm. We recommend scopes in the 66mm-70mm range, as larger scopes may have to be partially disassembled to fit into the rings if their lens shade o. d. is larger than 95mm. There are three large chrome finish hand-tighten adjustment bolts with non-marring Delrin tips in each ring. These allow you to collimate the refractor with the main telescope optics when it is used as a finderscope, or to aim the refractor off-axis to locate an appropriate guide star or photographic target when it is used as a photoguide or astrograph. The 11/16” wide rings are mounted at the opposite ends of the 3.56” long dovetail rail. This puts the centerlines of the collimating bolts 2-7/8” apart, allowing considerable freedom in lining up the refractor on off-axis objects for guiding and photography. The knurled heads of the collimating bolts are nearly 3/4” in diameter, providing a sure and easy grip, even while wearing gloves in cold weather. The rings hold the centerline of a photoguide scope approximately 3.75” above the surface of the main telescope on which the rings are mounted.

The dovetail rail holding the mounting rings has a safety groove machined into one side. This groove engages with the two chrome-finish ball head bolts in the mounting shoe that hold the mounting ring system in place on the main scope. This prevents the mounting ring portion from slipping out of the mounting shoe should the bolts in the shoe accidentally loosen during use. In addition, there is a brass safety bolt in the mounting shoe. This engages with a notch in the base of the dovetail rail to absolutely prevent the mounting ring system and photoguide scope from sliding backwards out of the mounting shoe when the main scope is pointed at the sky.

The complete ring system and mounting shoe weighs only 17 ounces. All parts are machined aluminum, anodized flat black to match the finish of most catadioptric scope rear cells. The machined mounting hardware is finished in non-rusting chrome for long life. New hex head mounting hardware is provided to install the system on a Celestron or Meade catadioptric scope, as is the appropriate size hex head installation wrench.

The weight of this product.
17 oz.
2 years
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Astro-Tech - 90mm finder/photoguide ring system

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Astro-Tech - 90mm finder/photoguide ring systemImage showing the ring system mounted on a scope, with an Astro-Tech AT66 ED refractor being used as a finder or photoguide scope.Another view of the system, showing the six large Delrin-tipped collimating bolts. Also visible is the notch in the base of the dovetail rail that engages the brass safety bolt in the mounting shoe.Image showing the mounting shoe with its three sets of slots for positioning the shoe on a scope's rear cell, the two hand-tighten bolts to hold the rings in place, and the brass safety stop.
Our Product #: AT90GSR
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This economical Astro-Tech ring system mounts a small refractor on a Celestron or Meade catadioptric scope for use as a finderscope, photoguide, or astrograph . . .

. . . our 37th year