Dismiss Notice
New Cookie Policy
On May 24, 2018, we published revised versions of our Terms and Rules and Cookie Policy. Your use of AstronomyConnect.com’s services is subject to these revised terms.

More Views

GSO 1.25" 90-deg Erect Image Amici Prism Diagonal # PD90

4.7 (18 Reviews) Read Reviews Write Review
Brand GSO
Part Number PD90
Availability available

Product Info

Manufacturer Description

GSO's 1.25" 90-degree diagonal features an image erecting Amici prism that allows you to look into a telescope at a 90 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 astronomy applications, especially when you would like to match the orientation of the star field with "correct" star maps and charts.

The clear aperture is 20.0mm. The prism housing is made of precision molded ABS plastic and the two end barrels are precision machined from aluminum. The 1.25" nosepiece is threaded to accept standard 1.25" astronomy filters. The diagonal comes with 2 end caps and weighs 3.8oz. 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.

Reviews

4.7 (Based on 18 Reviews) 100% of reviewers recommended this product.
Rating Distribution
Sort by
  • Date
  • Rating
  • Useful
    1. Guest

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      Review by PERIE J.
      1532765598
      Jul 28, 2018

      4.0 Prisme redresseur à 90°/1,25" = 90/1,25" Erecting prism

      Pros:

      Cons:

      Comments:

      Un bon produit qui remplace le modèle de la marque Celestron, que l'on ne trouve plus qu"en occasion.
      The Celestron erecting prism (same function) is no longer avalaible. This model is a solution of replacement ; it works correctly and i don't see remarkable defaults.

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

      This review was provided courtesy of AgenaAstro.com

    2. Guest

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      Review by Jeff J.
      1525802294
      May 8, 2018

      5.0 Use it in my main scope

      Pros:

      Cons:

      Comments:

      I live in heavy LP and thought a RACI might make finding objects a bit easier while starhopping. Since I have a good quality finder, I thought I'd try this diagonal before buying a RACI. After I received it, I figured I'd try it in my 120mm F/5 refractor as an experiment since I can't see very deeply with my finder anyway. I like using it so much in my main, that I haven't even tried it in my finder.

      I don't see any ghosts, lines, additional aberration, etc. This is a good quality diagonal 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

    3. Guest

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      Review by Carl K.
      1523554161
      Apr 12, 2018

      5.0 Right Angle Erect Image Prism

      Pros:

      Cons:

      Comments:

      I needed a right angle erect image prism for my 70mm 2 element EDS grab-and-go refractor. Cost was a driving factor in my choice. For the price, this item met all my requirements and provides excellent images for both terrestrial and astronomical images.

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

      This review was provided courtesy of AgenaAstro.com

    4. Guest

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      Review by Michael D.
      1517453199
      Jan 31, 2018

      5.0 GSO 1.25 90° ERECT

      Pros:

      Cons:

      Comments:

      GSO optical products offer excellent craftsmanship, and value.

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

      This review was provided courtesy of AgenaAstro.com

    5. Guest

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      Review by JOSEPH D.
      1513566216
      Dec 17, 2017

      5.0 Excellent correct image diagonal

      Pros:

      Cons:

      Comments:

      Some correct image diagonals, because of the Amici prism corner show a line in the image. Not this diagonal. The image is very clean and sharp with no artifacts to be seen. A very good price too!

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

      This review was provided courtesy of AgenaAstro.com

    6. Guest

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      Review by Ross S.
      1502275956
      Aug 9, 2017

      5.0 90 degree Right Image Diagonal

      Pros:

      Cons:

      Comments:

      A good value for the money. Works good for bird watching and even show a good image when stargazing.

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

      This review was provided courtesy of AgenaAstro.com

    7. Guest

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      Review by Michael R.
      1483024785
      Dec 29, 2016

      5.0 Excellent value!

      Pros:

      Cons:

      Comments:

      Great diagonal for a beginning stargazer! Bought for a scope I was giving to my grandson. The quality and performance was excellent for the money.

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

      This review was provided courtesy of AgenaAstro.com

    8. Guest

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      Review by Robert of Chicago.
      1475280000
      Sep 30, 2016

      5.0 GSO Diagonal prism

      Pros:

      Compact,Easy to Use,Lightweight,Strong Construction

      Cons:

      Comments:

      After trying this prism, I can say it is quite good quality and usable for astronomy too (like Jupiter) . I didn't see that this correcting left and right prism would degrade quality of observation more than regular one ..

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

      This review was provided courtesy of AgenaAstro.com

    9. uplateagain

      uplateagain New Member

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      Review by Tom of Rockland, NY.
      1457740800
      Mar 11, 2016

      5.0 Very well made

      Pros:

      Quality Lenses

      Cons:

      Comments:

      I bought this for use with my Travel Scope 70. The images are amazingly crisp. I have another brand of Amici prism that I purchased decades ago for more money than I just paid for the new one.
      The old Amici prism shows flare on bright objects. The new prism has yet to display any such flare.
      I was so impressed that I ordered a second unit. The second unit proved to be just as good as the first. I'm using that one on another small scope. This is a high quality unit despite it's modest price. I'm very pleased.

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

      This review was provided courtesy of AgenaAstro.com

    10. Guest

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      Review by John of Plantsville, CT.
      1443312000
      Sep 26, 2015

      4.0 Money well (smartly) spent

      Pros:

      Strong Construction,Quality Build For The,Lightweight

      Cons:

      Comments:

      I have a 400 mm refractor Omni Vista Spotter that I purchased many years ago from Orion. I needed an erect prism to bird watch in my back yard as I an surounded by trees.The prism that I purchased is perfect for daytime use as well as occasional looks at the moon.

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

      This review was provided courtesy of AgenaAstro.com

    11. Guest

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      Review by Sky Muse of Lake Cormorant, MS.
      1439251200
      Aug 10, 2015

      5.0 A Best Buy, Hands Down...

      Pros:

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

      Cons:

      None

      Comments:

      I purchased this 90

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

      This review was provided courtesy of AgenaAstro.com

    12. Jack

      Jack New Member

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      Review by Jack Thomas of Baltimore.
      1436054400
      Jul 4, 2015

      4.0 Better than I had imagined

      Pros:

      Quality Lenses,Lightweight,Compact,Accurate

      Cons:

      Comments:

      Having been an amateur astronomer and bird watcher for many decades, this diagonal far exceeded my expectations for such an inexpensive item. I note no doubling, nor roof artifacts, even at high powers.

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

      This review was provided courtesy of AgenaAstro.com

    13. Guest

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      Review by Froggy of Los Angeles, CA.
      1408320000
      Aug 17, 2014

      4.0 Good value

      Pros:

      Quality Lenses

      Cons:

      Comments:

      The amici line on most low cost amici prisms is commonly visible. This is one low cost amici prism that is cut so well the amici line is not visible.

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

      This review was provided courtesy of AgenaAstro.com

    14. Guest

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      Review by Star Gazer John of Coventry, Rhode Island.
      1385769600
      Nov 29, 2013

      5.0 Great Images at Minimal Cost

      Pros:

      Lightweight,Strong Construction

      Cons:

      Comments:

      Well 40 years ago I bought a war surplus (WW2) prism and made my own Amici diagonal for my astronomical telescopes. The one I made works fine but it is very difficult to change eyepieces. They slide in and out with a lot of friction. Recently I bought a new 150mm RFT and wanted to replace the diagonal mirror that came with it with an Amici. Agena Astro came to the rescue with this modestly priced erect image diagonal. The eyepieces can be changed out easily with the provided thumb screws. Thanks Agena Astro for an excellent product.

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

      This review was provided courtesy of AgenaAstro.com

    15. Guest

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      Review by Bob K of Heart of Georgia.
      1369958400
      May 30, 2013

      5.0 Just right for the range

      Pros:

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

      Cons:

      Comments:

      Using this on a Celestron 52268 C90 Mak Spotting scope with an Alpen table top tripod. All I have to do is raise my head a little, turn it and I can see perfectly down range. This 90 degree prism lets me set the scope/tripod back far enought it no longer crowds my elbow like a 45 degree one does.

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

      This review was provided courtesy of AgenaAstro.com

    16. gazer

      gazer New Member

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      Review by Sepi61 of Sweden.
      1320537600
      Nov 5, 2011

      5.0 Excellent diagonal from GSO

      Pros:

      Quality Lenses

      Cons:

      None

      Comments:

      Very dialectical quality, though it is not. The wavelength is 1 / 10, I am very satisfied with its performance, given the price. Thanks GSO
      Sepi61

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

      This review was provided courtesy of AgenaAstro.com

    17. astro_anthro

      astro_anthro New Member

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      1237026840
      Mar 14, 2009

      4.0 Very nice, lightweight star diagonal

      Pros:

      Very good mirror, lightweight, best value in star diagonals out there.

      Cons:

      Nothing specific, maybe the plastic housing.

      Comments:

      Don't be put off by the no-frills, plain plastic body of the GSO 1.25 star diagonal, it's a great little performer and an excellent value. The real selling point for this little star diagonal is its 1/10th wave mirror. Don't buy a diagonal for its reflectivity measure, buy it for its wave measure, with a 1/10 wave mirror being better than, say, a 1/4 wave mirror. The wave function is a measure of the surface accuracy of the mirror. In practice you are going to hardly notice a difference between a 99% reflectivity diagonal and one that comes in at 92% reflectivity--believe me I've compared them. But you will notice a difference in wave front error, particularly on finer details of planets or the moon, or when trying to split double stars and the like. I have mixed feelings about the plastic housing of the diagonal--I worry a little about its long term durability--but it is lightweight and of course keeps the cost of this otherwise great little performer down. If you are looking for a good starter diagonal this is the one.

      Bottom Line: Would you recommend this item? Yes
      Was this review helpful? Yes / No
    18. mhuston6

      mhuston6 New Member

      Skill Level
      Unknown
      Time with Product
      Unknown
      User Notes:
      1178709360
      May 9, 2007

      5.0 GSO 1.25" 90° Mirror Star Diagonal

      Pros:

      Fine optics, light weight, good buy!!

      Cons:

      No bad reviews!!

      Comments:

      This 90* is much better than the one that comes with your OEM refractor!! The images are more clear and it is about the same weight! The price is one of the best features for me!! All-round nice accessory!!

      Bottom Line: Would you recommend this item? Yes
      Was this review helpful? Yes / No
    Sort by
    • Date
    • Rating
    • Useful

      Specifications

      Manufacturer GSO