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GSO 2" 90-deg 99% Quartz Dielectric Mirror Diagonal for Refractors

4.4 (17 Reviews) Read Reviews Write Review
Brand GSO
Part Number GS2QST
Availability available

Product Info

Manufacturer Description

If you want a high-end mirror diagonal for your refractor telescope, this GSO 2" Quartz Dielectric Diagonal Mirror is an excellent choice. GSO's 2" refractor diagonal features dielectric coatings with 99% reflectivity across the visible spectrum and a 1/12 wave quartz mirror. The all metal diagonal body is CNC machined. The combined result is a superb diagonal offering excellent mechanical and optical quality at a great price. Dielectric coatings are more durable than regular or enhanced aluminum coatings, so you will enjoy years of excellent performance from this product. Diagonals with comparable quality and specifications often sell for $200 or more in today's astronomy market.

High quality diagonal mirrors are usually made of BK7 glass. Quartz glass goes a notch higher - it offers greater thermal stability, resulting in minimal thermal expansion and contraction as compared to other glass types. The result is a minimal, if any, shift in focus due to temperature changes over long observation sessions, making this quartz diagonal ideal for photographic and imaging applications.

The 2" chrome barrel of this diagonal on the telescope end is threaded to accept standard 2" filters. A 1.25"-2" adapter is included so you can use this diagonal with 1.25" or 2" eyepieces. The 1.25" and 2" holders both feature a brass compression ring, so your eyepiece barrels will not be scratched by the set screw. The 1.25"-2" adapter is also threaded to accept 2" filters. The clear aperture is 46.6mm. The diagonal comes in a very nice foam-fitted storage box and includes 2 end caps. Weight = 1lb-3oz. (with 1.25" adapter); 1lb. (without 1.25" adapter). Made in Taiwan.

Reviews

4.4 (Based on 17 Reviews) 88% of reviewers recommended this product.
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    1. Guest

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      Review by Rolando G.
      1549559594
      Feb 7, 2019

      5.0 GSO 2" diagonal

      Pros:

      Cons:

      Comments:

      Works as advertised. Excellent views. Holds all my 2" eyepieces securely. Nice product at a very reasonable price.

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

      This review was provided courtesy of AgenaAstro.com

    2. HWM

      HWM New Member

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      Review by Henry M.
      1536566147
      Sep 10, 2018

      5.0 GSO 2" dielectric diagonal

      Pros:

      Cons:

      Comments:

      Upon opening the box I was impressed by the size and weight of the diagonal compared to 1.25" diagonals. That heft was the primary reason for the purchase, to give greater security and stability when using heavy eyepieces or camera. I also want to use 2" eyepieces in the future. The high quality suggested by reviews and confirmed in my use, the functionality of the 2" size and the reasonable price made this diagonal a very satisfying purchase.

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

      This review was provided courtesy of AgenaAstro.com

    3. Guest

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      Review by Ronald H.
      1506265199
      Sep 24, 2017

      5.0 Quality unit

      Pros:

      Cons:

      Comments:

      A very nice piece of equipment.

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

      This review was provided courtesy of AgenaAstro.com

    4. pastorg

      pastorg New Member

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      Review by Gary R.
      1501528256
      Jul 31, 2017

      5.0 GSO 2 inch Quartz Dielectric Diagonal

      Pros:

      Cons:

      Comments:

      Received the diagonal in a timely manner in good condition. Works very well in my SW 120ED Pro Refractor. Build quality seems very good.

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

      This review was provided courtesy of AgenaAstro.com

    5. Guest

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      Review by Jason C.
      1500219074
      Jul 16, 2017

      5.0 Excellent value for this 2" diagonal upgrade

      Pros:

      Cons:

      Comments:

      I've noticed darker, and more crisp fields with this GSO diagonal over the stock prisms that came with my scopes.

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

      This review was provided courtesy of AgenaAstro.com

    6. Guest

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      Review by William P.
      1496948943
      Jun 8, 2017

      5.0 One little issue but handled quickly

      Pros:

      Cons:

      Comments:

      One item I received had the wrong fitting on it, evidently from the factory, but I was shipped a new one right away. I do think Agena should check the boxes before shipping, but they were very helpful and friendly.

      Bottom Line: Would you recommend this item? Yes
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      This review was provided courtesy of AgenaAstro.com

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      Review by Brian V.
      1492012606
      Apr 12, 2017

      4.0 Solid performer

      Pros:

      Cons:

      Comments:

      Very nice 2" star diagonal used in a C80EDR. Minimal scatter, very good views.

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

      This review was provided courtesy of AgenaAstro.com

    8. Guest

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      Review by Adam T.
      1487895900
      Feb 23, 2017

      5.0 I like so much I bought another

      Pros:

      Cons:

      Comments:

      Machined from solid aluminum. Smooth barrel (a good thing). Unique zero-height eyepiece adapter. Great value.

      Bottom Line: Would you recommend this item? Yes
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      This review was provided courtesy of AgenaAstro.com

    9. MDF

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      Review by C6R User of OH.
      1428019200
      Apr 2, 2015

      4.0 Solid Upgrade

      Pros:

      Quality Optics,Strong Construction

      Cons:

      Dust And Dirt On Mirror

      Comments:

      The unit is well machined with good fit and finish. All of my 2" filters (from several different manufacturers) screwed in smoothly. I had to tighten down the clamp ring a little more than expected on a few of my eyepieces, but I was able to get a solid grip on everything in my collection.

      The views with this diagonal are noticeably better than with the stock diagonal that came with my Celestron C6R. I tested the 2 diagonals on the Moon and Jupiter with 6mm, 8.8mm, 12.5mm, 18mm and 20MM 1 1/4" eyepieces from Zhumell and Explore Scientific. All the images seemed a bit crisper in the 2" GSO diagonal. The view seemed a little brighter with the 6mm Zhumell and 2" diagonal on Jupiter.

      When I unboxed my unit there was a good bit of dust and a some large black particles on the mirror. The black particles looked to be flakes of the diagonal's interior blackening and were likely knocked loose in shipping. The amount of dust was annoying, though. Fortunately a few puffs of air from a squeeze bulb cleaned things up.

      Overall- a solid upgrade.

      Bottom Line: Would you recommend this item? Yes
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      This review was provided courtesy of AgenaAstro.com

    10. Guest

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      Review by Navegante of Tenerife Canary Islands.
      1422662400
      Jan 30, 2015

      4.0 Really useful

      Pros:

      Strong Construction,Quality Lenses,Accurate

      Cons:

      Heavy

      Comments:

      Good quality for a cheap price

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

      This review was provided courtesy of AgenaAstro.com

    11. spongebob

      spongebob New Member

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      Review by Spongebob of NJ.
      1365206400
      Apr 5, 2013

      2.0 Good Mirror, Poor Mechanicals

      Pros:

      QUALITY MIRROR,Strong Construction,Quality Lenses

      Cons:

      Poor Brass Strip,Difficult to Use

      Comments:

      It is extremely difficult to get E.P.s into the diagonal. The Brass collar slips out of place and either stops the EP from going in or almost locks it in place. I have to check the placement of it all the time which in the dark, is sub optimal.
      In addition, the 1-1/4" adapter sits way too far into the diagonal. Meaning, the set screw for the 1-1/4" EP is recessed in the wall of the diagonal and its difficult to figure which to loosen.

      Bottom Line: Would you recommend this item? No
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      This review was provided courtesy of AgenaAstro.com

    12. Guest

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      Review by Doug of Finger Lakes, NY.
      1335744000
      Apr 29, 2012

      4.0 Great product, price and service!

      Pros:

      Strong Construction,Easy to Use,Reliable optical quality,Accurate

      Cons:

      Heavy

      Comments:

      GSO is Taiwan-based company that has long had a very good name in Astronomy. 10 years ago, they supplied the dobsonian telescope for companies like Orion and the reputation for functional, solid build and decent quality control has remained solid. Mainland China has caught up, but I still look to GSO for reliable optics at a good price. I bought this diagonal to use with a Mewlon 210--a very high quality instrument--and this diagonal fits in quite well. The machining is very well done, and the optics appear to perform very well. I have no doubt that it's built to last. I have purchased several things from Agena-astro and the service is bar-none. Shipping is fast, issues are resolved, they stand behind their product.

      Bottom Line: Would you recommend this item? Yes
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      This review was provided courtesy of AgenaAstro.com

    13. Guest

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      Review by Bone-Yard SD of Rapid City, SD.
      1330732800
      Mar 2, 2012

      5.0 Great!

      Pros:

      Easy to Use,Accurate,Strong Construction

      Cons:

      Comments:

      Really good SCT diagonal. I used it for visual and with my mallincam to do AP. The images are very bright and I noticed no degradation from the mirror at all. The reason I bought this was that it just didn't make sense to save 30 bucks on a piece of equipment you'll most likely hold onto for the rest of your life. I think it could definitely last that long!

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

      This review was provided courtesy of AgenaAstro.com

    14. Guest

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      Review by Nan of Japan.
      1319932800
      Oct 29, 2011

      4.0 Nan

      Pros:

      Cons:

      Comments:

      This is as good as the diagonal of TV. However, since the 31.7mm adapter is low profile design, if I use Ng zoom 2-4mm, I need an extension tube when I combined with TSA-120.
      In addition, when I plug the adapter into the hole, or out of the hole, it is often trapped.
      This diagonal has been subjected to a stop. 2-inch barrel do not hit the mirror. I can remove the barrel, so I am easy to put a flocked paper in the barrel, and easy to clean.

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

      This review was provided courtesy of AgenaAstro.com

    15. Guest

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      Review by NickH of Oregon Coast.
      1289347200
      Nov 9, 2010

      4.0 Excellent Diagonal for the money

      Pros:

      Strong Construction,Quality Lenses

      Cons:

      Comments:

      I bought this diagonal as my newly purchased refractor did not have one. This one caught my eye and I checked other people's reviews. It was well received and the price is very competitive. I will second that sentiment here.

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

      This review was provided courtesy of AgenaAstro.com

    16. Guest

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      Review by albireo of Baltimore, MD.
      1272672000
      Apr 30, 2010

      5.0 Clear and Sharp!

      Pros:

      Accurate

      Cons:

      Comments:

      I used this diagonal mirror for my 5 inch refractor (BORG 125SD). I fully enjoyed the star gazing by this mirror without any deficits such as los of brightness, decrease of contrast and artificial color.

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

      This review was provided courtesy of AgenaAstro.com

    17. DigitalDon

      DigitalDon New Member

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      1249469880
      Aug 5, 2009

      4.0 GSO 2" Dieletric Quartz Diagonal for refractors

      Pros:

      Quality fit and finish. High-end features. Great price!

      Cons:

      Three-piece design.

      Comments:

      I recently decided to replace the non-compression ring style 2” diagonal on my CPC 1100. Many manufacturers offer numerous variations today – enhanced coatings, dielectric coatings, Pyrex, quartz, etc.

      A well-known ‘East Coast’ company made my original diagonal and since I was pleased with its performance I naturally checked their product line first. However, their current 2” dielectric diagonal retails for nearly $300!

      Further research ultimately led me to Agena AstroProducts whose product line includes several 2” diagonals. I was interested in the 'refractor version' of the 2” GSO compression ring version. It features 99% reflectivity dielectric coatings, a 1/12th wave quartz mirror, 1¼” eyepiece adapter, and sells for $129! Since this is less than half of the price of the ‘famous’ diagonal it would be a true bargain if it was any good at all.

      I placed an order and received the diagonal quickly. The diagonal itself was in a nice foam-lined box. The GSO 2” dielectric, quartz diagonal includes a plastic 2” cap for the telescope end, a 2” to 1¼” adapter, and a plastic 1¼” cap for the eyepiece end. The adapter is an interesting ‘recessed’ design. If additional in-focus is needed, this type of adapter allows smaller eyepieces so sit about ¼” ‘lower’ than the standard style adapters. Like the diagonal itself, the adapter also includes a compression ring.

      The GSO diagonal is a three piece design. It consists of the main body, the tube that holds the eyepiece, and the tube that’s inserted into the focuser. The body and eyepiece tube are black anodized. The focuser tube contains a ‘safety under-cut’ and is chromed or polished. Interior surfaces are anti-reflection blackened. While the one piece style of the ‘famous’ diagonal insures the mirror is accurately positioned, a three-piece design can perform just as well if properly executed.

      Cosmetically, the GSO diagonal looked great! This is nice, but optical performance is what really counts so the first clear night, I installed the GSO diagonal in my CPC 1100. I can’t honestly say that I saw a dramatic difference between the GSO and my original diagonal, but it certainly performed very well. Since my goal was simply to obtain a diagonal with a compression ring, the dielectric coatings and quartz mirror were nice extra features. A great product for a great price!

      If you’re considering a 2” diagonal, be sure to check out the GSO 99% Dielectric Quartz Diagonal.

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

      Manufacturer GSO