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GSO 1.25" 90-deg 99% Dielectric Mirror Diagonal with Compression Ring

4.8 (33 Reviews) Read Reviews Write Review
Brand GSO
Part Number ODIA-GS-125D-COMP
Availability available

Product Info

Manufacturer Description

New, improved model - Now with compression ring in eyepiece barrel

GSO's new 1.25" dielectric diagonal continues their tradition of offering superb products at excellent prices. This beautiful1.25" mirror diagonal features a BK7 mirror that is precision ground to a flatness of 1/10th wave (minimum) and features dielectric 6-layer enhanced silver coatings with an over-coat protective layer for a light efficiency of over 99% reflectivity across the entire visual spectrum. The clear aperture is 27.9mm.

The mechanics of this diagonal are also top-notch. The robust aluminum body and attachment barrels are precision CNC machined to strict tolerances from solid stock throughout. The 1.25" nosepiece is threaded to accept standard 1.25" astronomy filters. The 1.25" eyepiece holder features a brass compression ring to protect your eyepieces and accessories. It also includes an internal safety stop to prevent long barrels on some eyepieces and barlows from hitting the diagonal mirror. The diagonal sports a handsome anodized finish.

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.

The diagonal comes in a nice foam-fitted storage box and includes 2 end caps. Weight = 5.9oz.. Made in Taiwan.

Reviews

4.8 (Based on 33 Reviews) 100% of reviewers recommended this product.
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    1. Guest

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      Review by Mark W.
      1546953661
      Jan 8, 2019

      5.0 Bright

      Pros:

      Cons:

      Comments:

      Needed a new better diag than what came with the scope. Tried the GSO and it is every bit as bright as my Televues.

      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 Charles W.
      1545496642
      Dec 22, 2018

      5.0 Well made diagonal

      Pros:

      Cons:

      Comments:

      In getting an old Jaegers refractor back into service I needed a diagonal that would do justice to the optics, but that I wouldn't mind having the barrel scuffed by the friction-fit focuser, and this hit a perfect price point for that goal. This barrel on this diagonal has a milled finish that engages well with the focuser and keeps the diagonal from rotating, even with fairly heavy eyepieces. The inside of the barrel is fully ribbed and blackened, and the mirror is very bright and clear. Its body is nicely machined from a single block, with the back held on by four screws. The compression band on the ocular end prevents scratching of eyepieces. So, in addition to being a very good diagonal for the price, it can also be considered as an adapter between a scratch-inducing vintage focuser and a scratch-free modern eyepiece holder.

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

    3. Guest

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      Review by David R.
      1534216672
      Aug 13, 2018

      5.0 A good diagonal

      Pros:

      Cons:

      Comments:

      Works good and fits nicely on my smaller scope

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

      This review was provided courtesy of AgenaAstro.com

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      Review by Mihai R.
      1530085289
      Jun 27, 2018

      5.0 Not bad at all :)

      Pros:

      Cons:

      Comments:

      I am quite satisfied with this diagonal, which is clearly one class over the standard "kit" ones (i'm talking about the metal case diagonals, not plastic ones, of course), in terms of build quality and optical performance.
      I'm yet to test it against other models, but so far the improvement is hard not to notice.
      The compression ring does a great job at holding the eyepiece or binoviewer straight centered without marking the barrel.
      At this price (except shipping and duty) it was quite affordable, so no big hole in the budget for such welcome upgrade.

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

      This review was provided courtesy of AgenaAstro.com

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      Review by Michael C.
      1527843812
      Jun 1, 2018

      5.0 GSO diagonal

      Pros:

      Cons:

      Comments:

      Definitely an improvement from the stock diagonal on my Nexstar

      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 Konstantin P.
      1526468080
      May 16, 2018

      5.0 Nicely made, sturdy and strong

      Pros:

      Cons:

      Comments:

      Nicely made, sturdy and strong construction.

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

      This review was provided courtesy of AgenaAstro.com

    7. Guest

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      Review by liav c.
      1522726828
      Apr 2, 2018

      5.0 sturdy and convenient

      Pros:

      Cons:

      Comments:

      sturdy and convenient

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

      This review was provided courtesy of AgenaAstro.com

    8. Dave In Vermont

      Dave In Vermont Well-Known Member

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      1513829325
      Dec 20, 2017

      5.0 1.25" GSO Dielectric 90° Diagonal

      Pros:

      Low price an added attraction.

      Cons:

      None.

      Comments:

      I have 4 - 2" dielectric William Optics diagonals, but needed a good 1.25" diagonal. So I 'rolled the dice' and got a GSO model. The low price was an added incentive. It arrived quickly, as always with Agena, and I plugged it into my 150mm Maksutov. I was amazed! This excellent diagonal worked every bit as well as the higher-cost 2" WO cousins. I resolved that if I needed another 2" model - it would be a GSO. Everything just says 'Quality' - from the aesthetic appeal and up to the excellent optics manifest at high-power viewing through the powerful Maksutov telescope.

      Bottom Line: Would you recommend this item? Yes
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      Review by James J.
      1513790332
      Dec 20, 2017

      4.0 GSO 1.25" Dielectric diagonal

      Pros:

      Cons:

      Comments:

      I got this to update an old Celestron C5. This diagonal is well made, nicely finished and packaged and a bargain at its selling price. I was able to compare it to others I have that are similar, but more expensive. My Tele Vue is clearly better optically--less scatter and a bit brighter, but it costs twice as much as the GSO. My problem with the GSO is not that eyepieces 'bottom out' when inserted, as I have read in some reviews; but that the compression ring hangs up several of my eyepieces, making them hard to place in the diagonal. Maybe this will wear in over time, but I don't know. Maybe mine is unusual, but don't know that either. In any event, it beats most of the stock diagonals that come with scopes today.

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

      This review was provided courtesy of AgenaAstro.com

    10. Zen Kletsyn

      Zen Kletsyn Member

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      Review by Randal D.
      1513769475
      Dec 20, 2017

      5.0 Nice diagonal

      Pros:

      Cons:

      Comments:

      Arrived quickly from Agena, this diagonal looks as good as any I have ever had. Appears to be well made, much better than the stock diagonals that are shipped with scopes. Always a great experience buying from Agena Astro, I always look to them first for my astronomy gear.

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

      This review was provided courtesy of AgenaAstro.com

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      Review by Carlos L.
      1512964577
      Dec 10, 2017

      5.0 Buy it

      Pros:

      Cons:

      Comments:

      It is a good diagonal mirror for that price. It is good for short tubes in which chromatic aberration with diagonal prisms is seen.

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

      This review was provided courtesy of AgenaAstro.com

    12. Guest

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      Review by Rick S.
      1502197446
      Aug 8, 2017

      4.0 good improvement over stock

      Pros:

      Cons:

      Comments:

      good improvement over stock

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

      This review was provided courtesy of AgenaAstro.com

    13. Guest

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      Review by Gus G.
      1499348193
      Jul 6, 2017

      5.0 Excellent Optical Upgrade With Great Build Quality

      Pros:

      Cons:

      Comments:

      This was an excellent upgrade for the prism diagonal which came with my telescope, a Celestron Omni XLT 102AZ. Images are brighter and sharper, with higher contrast. More detail can clearly be seen on the moon and Jupiter. Build quality is excellent - this is very well made, solidly built, and nicely finished. This is a very good value!

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

    14. Guest

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      Review by J C.
      1498690562
      Jun 28, 2017

      5.0 Superb upgrade for my refractors

      Pros:

      Cons:

      Comments:

      A huge improvement over the stock Prisms of my two Meade refractors; 80mm and 102mm. Better sharpness, contrast and darker fields, this diagonal is an exceptional performer and an excellent value.

      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 John C.
      1493749018
      May 2, 2017

      5.0 Good Value. Great Views.

      Pros:

      Cons:

      Comments:

      As has been my experience with GSO products in the past, this diagonal exceeded my expectations. Workmanship, the diagonal body and the mirror were top notch. It even has the "heft" of a high-end diagonal. I have had numerous high-end diagonals over the past few years. This GSO diagonal is the equal of each of them. Would I recommend it to a friend? My best friend is me and this is my third diagonal from the GSO lineup.

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

      This review was provided courtesy of AgenaAstro.com

    16. Guest

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      Review by Adam T.
      1491407775
      Apr 5, 2017

      4.0 Very good replacement

      Pros:

      Cons:

      Comments:

      Nicely made diagonal with brass compression fitting instead of thumbscrews. I really like that it has a smooth nosepiece intead of being tapered or having a safety groove. Minus one star for its eyepiece safety stop being a bit shallow - all but one of my eyepieces "bottoms out" before registering at their shoulder.

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

      This review was provided courtesy of AgenaAstro.com

    17. Guest

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      Review by Don A.
      1490387956
      Mar 24, 2017

      5.0 GSO 1.25" 90-deg 99% Dielectric Mirror Diagonal with Compression

      Pros:

      Cons:

      Comments:

      Great product. You can pay more but I think this diagonal offers a big bang for the buck.

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

      This review was provided courtesy of AgenaAstro.com

    18. Guest

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      Review by Jerry P.
      1484031682
      Jan 9, 2017

      5.0 Really Good Diagonal

      Pros:

      Cons:

      Comments:

      Not having much to compare against except the stock diagonals that came with my telescopes, this diagonal just blew them away. What imaging problems I had attributed to shoddy telescope manufacturing are now corrected for the most part and now I can tell what is at the limits of my equipment versus everything looking, well..., like crap. I would rather not have a bad or cheap eyepiece or diagonal than be severely disappointed by them.

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

      This review was provided courtesy of AgenaAstro.com

    19. Guest

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      Review by none of Phoenix, AZ.
      1469059200
      Jul 20, 2016

      5.0 Good choice

      Pros:

      Accurate

      Cons:

      Comments:

      I bought this diagonal because I was not sure how good the prism diagonal I had purchased years ago was. I am impressed. The image quality is very good and does not need refocusing to view other portions of the image. This tells me that the image in this mirror diagonal is 90 degrees from point of entry to the exit point. I live 4 miles from the center of a major light pollution source and it allows to see very deep in the sky. Would I recommend it? Yes. Would I buy another one if I needed it? Another yes. If you don't have big bucks to spend, this is worth the money you will spend.

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

      This review was provided courtesy of AgenaAstro.com

    20. Guest

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      Review by Sam of Portland.
      1465603200
      Jun 10, 2016

      4.0 Very good

      Pros:

      Strong Construction,Quality Lenses

      Cons:

      Mechanical

      Comments:

      This diagonal puts up very good images and feels nice and solid. My only issue is the set screw/ retainer isn't all that smooth. It catches some but still operates as intended. At this price that is probably what you can expect.

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

    21. Guest

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      Review by Refractor nut of Toronto.
      1457308800
      Mar 6, 2016

      5.0 Well collimated & well built

      Pros:

      Lightweight,Strong Construction,Accurate

      Cons:

      Comments:

      I asked for this for Christmas after doing some research. I bought it to replace stock diagonal that came out of collimation with my ST80. I plugged it in on Jupiter and finally, spot-on collimation. I also use it with C102 and noticed the same difference was noticed. Don't spend 100$ on fancy dielectrics when this one does the same job, albeit with less flare.

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

    22. Guest

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      Review by GaryS of Davis, CA.
      1453680000
      Jan 24, 2016

      5.0 Works as advertised.

      Pros:

      Accurate,Lightweight,Easy to Use,Compact,Quality Lenses,Strong Construction

      Cons:

      Comments:

      Works great! Mirror is well centered, good construction.

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

      This review was provided courtesy of AgenaAstro.com

    23. Guest

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      Review by Bob of Roseville, CA.
      1440028800
      Aug 19, 2015

      5.0 My favorite!

      Pros:

      Quality Lenses,Compact,Strong Construction,Lightweight,Easy to Use,Accurate

      Cons:

      Comments:

      I have other preminum diagonals that cost much more, yet this is still my favorite due to its low profile construction which gives me a little more back focus with my refractors.

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

      This review was provided courtesy of AgenaAstro.com

    24. Guest

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      Review by AstroStater of Mx.
      1435449600
      Jun 27, 2015

      4.0 Nice Product

      Pros:

      Quality Lenses,Easy to Use,Lightweight,Strong Construction

      Cons:

      Flips Image Left To Right

      Comments:

      Nice Product

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

      This review was provided courtesy of AgenaAstro.com

    25. Guest

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      Review by Stella of Creta, Greece.
      1433635200
      Jun 6, 2015

      5.0 Sharp images

      Pros:

      Compact,Strong Construction,Easy to Use,Accurate,Quality Lenses

      Cons:

      Comments:

      I used this to replace the stock diagonal that came with my SWE ED80. Wow! What a difference. Much brighter images. I highly recommend this product to anyone who wants high quality optics. If you are going to spend the money on a high quality telescope and eyepieces, don't forget to get a great diagonal. This is the one I recommend. You can spend more money than the price for this diagonal, but you will not get any better performance in my opinion!

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

    26. Guest

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      Review by JHolland of Abilene.
      1422662400
      Jan 30, 2015

      5.0 Top notch diagonal at a good price

      Pros:

      Easy to Use,Strong Construction

      Cons:

      Comments:

      I replaced the diagonal that came with my 6" SCT with this diagonal. The images are much brighter. The diagonal is of good quality and is well constructed. This is a must in my opinion if your new scope takes 1.25" eyepieces and doesn't have a quality diagonal. You won't be sorry, I highly recommend.

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

      This review was provided courtesy of AgenaAstro.com

    27. Guest

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      Review by retired of Miami.
      1422662400
      Jan 30, 2015

      5.0 A must for small telescopes

      Pros:

      Compact,Strong Construction,Accurate,Quality Lenses,Easy to Use

      Cons:

      Comments:

      I use this on a small 80mm telescope. It brightens the image a lot better than the diagonal that came with scope.

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

      This review was provided courtesy of AgenaAstro.com

    28. Guest

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      Review by The Bus Driver of Bloomington, IN.
      1413849600
      Oct 20, 2014

      5.0 Great!

      Pros:

      Cons:

      Comments:

      This diagonal is great. It's a definite improvement over the stock diagonal that came with my Celestron 102SLT. While the differences between the two mirrors are subtle I feel that I can actually tell a difference. Fainter objects are just a little easier to see with the dielectric mirror. I have read some complaints about the compression ring (vs a set screw) but as long as one uses common sense it's not a problem. I am glad I bought it.

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

      This review was provided courtesy of AgenaAstro.com

    29. Guest

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      Review by dwmedic of Kansas.
      1408147200
      Aug 15, 2014

      4.0 Great Diagonal

      Pros:

      Strong Construction,Lightweight,Easy to Use,Quality Lenses

      Cons:

      Comments:

      SOOO much better than the stock diagonals. Should be one of your first upgrades.

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

      This review was provided courtesy of AgenaAstro.com

    30. Guest

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      Review by JimBr SFAAA of Miami, FL.
      1385769600
      Nov 29, 2013

      5.0 No need to pay more elsewhere

      Pros:

      Strong Construction,Lightweight,QUALITY MIRROR,Easy to Use,Compact

      Cons:

      Gso Labeling Will Fade

      Comments:

      Big improvement over Diagonal supplied new as standard equipment on my Celestron Nexstar 5SE SCT

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

      This review was provided courtesy of AgenaAstro.com

    31. Guest

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      Review by desertlens of NM.
      1369958400
      May 30, 2013

      4.0 A winner from GSO

      Pros:

      Lightweight,Compact,Strong Construction

      Cons:

      Comments:

      This is a well made component. It was bought to replace a flimsy diagonal that came with an SCT. The GSO product is far superior to the stock part. The compression ring and 99% coatings are well executed. Fit and finish are excellent and the price is reasonable.

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

      This review was provided courtesy of AgenaAstro.com

    32. Guest

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      Review by bjmaster of BridgePort, CT.
      1363996800
      Mar 22, 2013

      5.0 Fine quality.

      Pros:

      Compact,Lightweight

      Cons:

      Comments:

      Rather than the conventional thing, since the fixed method has been improved, accuracy is maintained at attachment of an eyepiece. It is wonderful quality.

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

      This review was provided courtesy of AgenaAstro.com

    33. Guest

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      Review by Max Headroom of Vail, AZ.
      1360368000
      Feb 8, 2013

      5.0 Excellent diagonal

      Pros:

      Accurate,Lightweight,Easy to Use,Compact

      Cons:

      Comments:

      Excellent lightweight diagonal. Best feature is it is square with the optical path so collimation is not negated. This applies to the 2" model as well.

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

      This review was provided courtesy of AgenaAstro.com

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      Specifications

      Manufacturer GSO