This dark forest green Astro-Tech 90mm finderscope/photoguide ring set allows you to mount a 60mm to 70mm refractor optical tube on an optional Losmandy D-series 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 #ATDA Dovetail accessory adapter for Losmandy-style "D-plate". A single hand-tighten knob clamps each Astro-Tech #ATDA adapter firmly in place on your Losmandy-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. A three-legged lever on each adapter’s knob shaft lets you tighten the knob securely when wearing gloves or mittens. The levers also make it easier to loosen the clamps should cold weather cause the metal to contract and the adapter to bind on the dovetail.
There are three hand-tighten brass adjustment screws 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. close to 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 2.15 lbs. Each ring weighs 17.2 ounces, including its attached #ATDA adapter. The height from the top of the main scope’s dovetail plate (not the top of the #ATDA adapter) to the center of the rings is 2.55” (65mm). The rings and adapters are anodized a handsome dark forest green for good looks and durability.
Short (7.9”) Astro-Tech Losmandy-style “D-plate” dovetails are available in forest green (#ATDUPS) and black (#ATDUPSB). Long (13”) Astro-Tech Losmandy-style “D-plate” dovetails are available in forest green (#ATDUP) and black (#ATDUPB). The forest green versions will let you color-coordinate your ring set with your dovetail.
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