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GSO 0.965"/1.25" 45-deg Hybrid Erect-Image Prism Diagonal

3.0 (1 Reviews) Read Reviews Write Review
Brand GSO
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Manufacturer Description

Important Note: Please note that the length of the 0.965" nosepiece, as listed below, is 15mm (0.59"). Before purchase, please make sure that the thumbscrew on your telescope is within this distance from the rear end of your focuser drawtube. Otherwise, the thumbscrew will not be able to engage and lock the diagonal. This is a known issue with some vintage telescopes in which the locking thumbscrew on the telescope is recessed by more than 0.6", and this diagonal will not work for you.

Disappointed by the lack of quality 0.965" eyepieces for your vintage telescope? Missing a diagonal for that small telescope you picked up in a garage sale? Wish you could use wider angle 1.25" eyepieces with your small spotting scope? Here's the solution you need!

Many vintage telescopes sold up to the 1960s, cheap introductory telescopes sold today, and many spotting scopes use a smaller, older, almost obsolete 0.965" diameter format for diagonals and eyepieces. Having a 0.965" barrel severely limits the number of eyepieces and accessories you can buy and use in the after-market.

This popular hybrid diagonal from GSO allows 1.25" eyepieces, diagonals, and other accessories to be used in 0.965" focusers, and at once allows hundreds of more common 1.25" format astronomy items to be used with your telescope.

GSO's hybrid 0.965"-1.25" 45-degree diagonal features an image erecting Amici glass prism that allows you to look into a telescope at a 45 degree angle. It produces an image that is oriented properly, which means that the image is upright and correct from left to right (i.e., a correct, non-inverted, non-reversed image). This is a great diagonal for daytime terrestrial use with small telescopes or spotting scopes. It can also be used for nighttime astronomy applications, especially when you would like to match the orientation of the star field with "correct" star maps and charts.

A unique feature of this hybrid diagonal is the use of a dual step 0.965"/1.25" chrome barrel on the telescope end. While designed primarily for use with 0.965" focusers, the step barrel also allows you to use this diagonal in 1.25" focusers (although your field of view might vignette in some situations if used in this manner). Total length of the nosepiece is 25mm (0.98"); length of 1.25" portion of nosepiece = 10mm (0.39"); length of 0.965" portion of nosepiece = 15mm (0.59").

The prism housing is made of high grade ABS plastic and the end barrels are made from precision machined aluminum. Weight: 2.8oz; 2 end caps included. Made in Taiwan.

Please Note: This diagonal can only be used with refractor telescopes and some catadioptric telescopes like Schmidt-Cassegrain, Maksutov-Cassegrain, etc. It not meant for use with Newtonian / Dobsonian reflector telescopes or other variants of Newtonian telescopes.


3.0 (Based on 1 Reviews) 0% of reviewers recommended this product.
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    1. woodylarkin

      woodylarkin New Member

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      User Notes:
      Review by Woody of Indianapolis, IN.
      Mar 28, 2016

      3.0 Has a problem


      Quality Lenses


      Limited Device Use


      This hybrid diagonal is intended to be used with telescopes that require 0.965 eyepieces and accessories. It is supposed to allow the use of your 1.25" eyepieces with scopes that require 0.965 eyepieces and also can be used in telescopes that use 1.25" eyepieces. The fitting that goes in the scope has a raised shoulder that supposedly permits this dual use.
      Now there are many classic scopes that should be able to use this diagonal, and I do not have access to them to judge whether it works or not.
      I can, however, report that it is not a good choice for an early orange tube Celestron C90 (like the black telephoto version) with the added finder, that focuses by turning the barrel rather than a knob. These scopes were shipped with 0.965" backs.
      Here is the problem: when you insert this diagonal into the scope; because of the truncated barrel with the 1.25" shoulder, there is insufficient area for the screw to engage the barrel and the connection is very iffy at best (only the tip). If the folks at Agena have access to one of these scopes, they should try it.
      In other words, it doesn't work. Trying to do two things, it fails at the main purpose for which I bought it.

      I have other adapters (which I won't list here) to use 1.25" eyepieces with this scope, but all of them compromise the length of the light path. I also have other Japanese hybrids that have a decent 0.965 length the the barrel.

      Worse yet, there seems to be no way to use available parts from other diagonals or eyepieces to fix this hybrid. The 2 step part screws into the prism at 1.25" so no other barrels can be substituted. If anyone has a part that can be substituted, I would be happy to buy it.

      Bottom Line: Would you recommend this item? No
      Was this review helpful? Yes / No

      This review was provided courtesy of AgenaAstro.com

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      Manufacturer GSO