Astro-Tech 2" 99% Reflectivity Dielectric Mirror Diagonal For Refractors


Availability: More on the way

This 2” dielectric mirror-type star diagonal is an outstanding value. At this low price, if there ever was a bargain in astronomy, this is it . . .

Our Product #: AT2D

Product Description

The current quick lock version of this diagonal will not work with Astro-Tech AT72EDII versions manufactured prior to October 2022.  The top of the diagonal is too tall and will not allow some eyepieces to focus in the AT72EDII.

This 2” Astro-Tech first-surface mirror star diagonal has state-of-the-art 46-layer dielectric mirror coatings for a full 99% reflectivity, higher than the 88% of a conventional aluminum diagonal or the 96-97% of an easily-tarnished silver diagonal.

The durable ion-deposited dielectric coatings on the mirror’s finely-polished 10mm thick 1/10th wave BK7 optical glass substrate also provide higher contrast and resolution during critical planetary, lunar, and binary star observations.

The supplied 1.25” eyepiece adapter uses a compression ring to hold eyepieces in place. The scope-side barrel of the 1.25” adapter has a subtle slanted groove to keep it from accidentally slipping out of the 2” diagonal. Dust covers are provided for the 1.25” adapter and the diagonal’s 2” barrel.

The interior of the diagonal body and the inside of the focuser-side barrel are treated with light absorbing materials. The focuser-side barrel is also internally anti-reflection threaded for even higher contrast. The barrel is threaded to accept standard 2” filters.

A subtle slanted groove cut into the outside of the focuser-side 2” barrel engages with your telescope’s focuser thumbscrew to act as a safety stop. This keeps the diagonal from falling should the scope’s thumbscrew loosen slightly from cold or use.

This diagonal is a visible improvement over the conventional 2" diagonal provided with many refractors capable of using 2” optics. It can also be used with a Schmidt-Cassegrain or Maksutov-Cassegrain telescope with a suitable adapter, such as the #7785C. It is an absolutely premium 2” 99% reflectivity dielectric self-centering star diagonal at a fraction the price of competitive models with equivalent performance and build quality. The same performance, for as little as half the price. If there ever was a bargain in astronomy, this is it.

Tech Details

Weight 17 oz.
Warranty 1 year


Review by:
I wanted a 2" Dielectric diagonal for my AT80ED and Orion 120ST and this fit the bill with quality and value. A great 2" diagonal that is nicely made. (Posted on 3/4/2023)
Review by:
This diagonal is a wonderful addition to my AT102ED. It's fit, form, and function are first rate. It provides wonderful images and it's construction is beyond reproach. This stands to reason as being a product from Astro Tech. (Posted on 2/13/2022)
Review by:
Love this diagonal. Amazingly crisp and flat views with no optical aberrations. (Posted on 2/18/2021)
Review by:
I ordered this diagonal to couple it to my AT80ED. I prefer 2” optical trains for wider views. This diagonal is fantastic! Well constructed and sturdy, it offers razor sharp views of objects that I observe. The diagonal lives up to it’s quality for a much less price than many premium diagonals. A great addition to any refractor scope. (Posted on 2/10/2021)
Review by:
I have used this and also the Baader two-inch prism (with Zeiss specs); Baader's 1 1/4 inch version of this is top rated in William Paolini's excellent online review of multiple mirror and prism diagonals, for contrast and capture of detail, and for vividness of color. I can see no difference between the Astro-Tech and the Baader, except that the Astro-Tech costs one fourth of the latter. (Posted on 1/2/2020)
Review by:
I've owned this diagonal for several years and it is hands down the best one I've ever owned.
It is solid and very well machined. The mirror has crisp sharp images. Great reflectivity.
It still performs and looks like new after many years. (Posted on 10/31/2019)
Review by:
Initially bought this thinking I'd improve the views in my Celestron SCT. Unfortunately, not enough of a win there (better to use 1.25" eyepieces on that), but this wound up on my short focus refractor where it is simply outstanding with 2" oculars. Really a great bargain. Incredible views - can detect no optical faults with this. Mechanically very nice as well - has a compression ring to hold the eyepiece securely but without any dings or dents on the tube. (Posted on 10/6/2019)
Review by:
I purchased this diagonal a couple of years ago to replace the 1.25" one that came with my C8. It is an excellent value that has many features for the price. (Posted on 7/24/2019)
Review by:
What I like about this diagonal is the design of the mirror housing, because it's either machined from a single piece of aluminum or it's an extrusion, with no access or external alignment under the mirror. The grippy plastic sideplates are plus for me because they conduct less heat when use them to nudge my telescope on its alt-az mount. Nice rubber dust cap too. (Posted on 7/22/2019)
Review by:
I've had this 2" diagonal about 1.5 years now and it's my go to one to use. At this price point, the features and quality are excellent. (Posted on 1/14/2019)
Review by:
A nicely machined housing gives a sense of quality. I leave this diagonal in my 120 Orion ED for viewing at a moments notice! (Posted on 1/3/2019)
Review by:
I have several 2" dielectric diagonals and two things set this AT diagonal apart from similar priced units. First, the twist lock is smooth and works very well. Second, the interior of this diagonal is noticeably better blackened than others in this price range. Overall, a great value 2" diagonal. (Posted on 11/1/2018)
Review by:
Bright and clear views with this diagonal. I also own the 1.25" and both perform wonderfully! (Posted on 10/7/2018)
Review by:
I've owned this diagonal for 10 years and it's one great diagonal. I can find no significant flaws in it. It's well designed, very well built and has never given any trouble. The coatings are as good as brand new and after several AB comparisons, the performance is comparable to the 2" TV Everbrite, which I sold to fund other accessories after getting the AT. The clear aperture is large enough to match my 31T5 with no vignetting at all. I highly recommend this product. (Posted on 10/6/2018)
Review by:
The build quality of the Astro-Tech 2" Diagonal was apparent the moment it was in hand. The fit and finish are first rate. Its optical performance is superb. I have owned more expensive 2" diagonals but none have presented better views. (Posted on 10/6/2018)
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