This Astro-Tech 90mm finderscope/photoguide ring set allows you to mount a 60mm to 70mm refractor optical tube on an optional Vixen-style dovetail plate that has been installed piggyback on a larger telescope. The set lets you use the small refractor as a high power/variable power finderscope for your larger telescope; as a photoguide scope for long exposure deep sky photography; or as a wide field astrograph that is guided through the main scope.
The Astro-Tech 90mm ring set consists of two 90mm i.d. rings mounted on an Astro-Tech #ATVA Dovetail accessory adapter for Vixen-style dovetails. A single hand-tighten knob clamps each Astro-Tech #ATVA adapter firmly in place on your Vixen-style dovetail, but allows the rings to be quickly repositioned along the dovetail so you can balance your smaller scope fore and aft as needed with no tools required.
There are three hand-tighten adjustment screws with non-marring Delrin tips in each ring. These allow you to collimate the small refractor with the main scope 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 rings have a nominal 90mm (3.54”) internal diameter and will hold small refractor optical tubes with an o.d. of 65mm to 80mm. This size range allows the small refractor to be collimated with the main scope (if the refractor is being used as a finderscope), while still allowing for a reasonable amount of off-axis aiming to find a guide star (if the refractor is being used as a photoguide).
The larger the o.d. of your small refractor, however, the less off-axis aiming capability you will have for guiding. If your small refractor has an o.d. of 75-80mm, you might be best served by going to the next larger guidescope ring size (110mm) to give you more off-axis aiming capability and therefore a wider area of sky in which to find a guide star.
The ring set has a total weight of 1.7 lbs. Each ring weighs 13.6 ounces, including its attached #ATVA adapter. The height from the top of the main scope’s dovetail plate (not the top of the #ATVA adapter) to the center of the rings is 2.55” (65mm). The rings and adapters are hard anodized matte black for good looks and durability.
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