TPO 2" SCT Quartz Dielectric Diagonal

  • Quartz offers greater thermal stability, resulting in minimal thermal expansion & contraction.
  • The TPO 2-inch SCT Quartz Dielectric Diagonal features a deluxe 1/10th wave mirror diagonal with 99-percent reflectivity and dielectric coatings.
  • Attaches to rear cell thread of Schmidt-Cassegrain/Maksutov-Cassegrain telescopes.
  • The Third Planet Optics 2-inch SCT Quartz Dielectric Diagonal has a fully CNC machined housing and includes 1.25-inch adapter.
  • Brass compression rings on both diagonal and adapter assure gentle treatment of your eyepieces.


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  • Regular price $138.95
  • $125.10
  • You save: $13.85 (9%)

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TPO 2" SCT Quartz Dielectric Diagonal

The Third Planet Optics 2-inch quartz dielectric mirror diagonal features 99-percent reflectivity, dielectric coatings to maximize light output of your telescope system. Our custom made diagonal uses a quartz mirror polished to 10th wave accuracy (or better) for best possible image quality.

TPO's quartz dielectric diagonal provides greater thermal stability than comparable glass diagonals, resulting in minimal thermal expansion and contraction. This quartz diagonal, therefore, minimizes focus position change due to variations in temperature.

The TPO 2-inch quartz dielectric diagonal also features a CNC machined housing. The CNC machined aluminum housing will provide outstanding results for years of use. This housing is designed to retain the optimal collimation of the mirror and assure the best optical performance.

This diagonal also includes brass compression rings on both diagonal and the included 2-inch to 1.25-inch adapter, assuring gentle treatment of your eyepieces. Some other brands use set screw locking mechanisms that can mar the barrel surface of your favorite eyepieces. Avoid scratches and gouges by using the OPT dielectric series diagonals.

TPO quartz dielectric diagonals provide outstanding performance with low prices to provide amateur astronomers with excellent value! OPT backs this diagonal with a full 2 year warranty against manufacturer defects.

This diagonal attaches to the rear cell of SCT's. It is not meant for SC telescopes that have an electric focuser attached to the rear cell. In these instances, we recommend the OPT 2 inch Quartz Dielectric Diagonal.


OPT Product Number: OS-D2SCTDIQ


Angle90 degree
Diagonal Barrel Size2"
Free ShippingYes
Warranty2 Year Warranty
Customer Reviews
4.8 Based on 4 Reviews
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Mike E.
United States United States
Good for the price

If you are looking for an affordable 2 inch diagonal this is a good one, good quality and good to the eyepieces thanks to compression rings, only downside is that its not treaded for filters

United States
Great helpful staff

Very happy with the purchase.

Money well spent!

I am using this diagonal on a Celestron Nexstar 8SE. I wanted to make my scope able to accept wide field 2 eyepieces and this diagonal does that and more. I like the fact that it threads onto where the visual back normally would go. It ******* on just perfectly. I have a Feathertouch microfocuser installed on my tube, and the diagonal leaves plenty of room to get your hand on the focuser. I have pointed it to zenith with a giant Explore Scientific 100 degree 9mm eyepiece in and it clears the goto mount just as well as the 1.25 diagonal did. The fit and finish of the diagonal is top notch. The compression ring on both the diagonal and the adapter work flawlessly. It barely takes a quarter turn or so to release or lock your eyepiece. Very happy with this purchase.

Excellent Quality but BEWARE

This diagonal is of EXCEPTIONAL quality both optically and with respect to fit and finish. However owners of older SCT's BEWARE! The threads on this diagonal were not compatible with the threads on the rear cell of my old orange tube C8. This caused the diagonal threads to lock on my rear cell when i attempted to fit it to my scope. As a result the baffle ring on my scope was irretreviably damaged and my scope has been rendered unusable. I am now having to hunt for a new baffle ring to restore my telescope to a usable state. While this is an excellent product, be VERY careful when first fitting it to your scope. If the threads are incompatible it will thread one or tow turns then lock. Be very careful when testing fit for the first time!!