New from William Optics, this 1.25" 90-degree erecting-prism diagonal # D-EP90-125 gives correct right-side up images for terrestrial or celestial observing with any telescope (except Newtonian reflectors). The erecting prism in this diagonal is made of high quality BAK4 glass polished to a high surface accuracy and with anti-reflection coatings that give bright images and reduced ghosting.
The diagonal features a helical focusing mechanism for fine adjustment of up to 15 mm in the eyepiece holder. The inside of the erecting diagonal is matte blackened to reduce internal stray light reflection and keep image contrast high. Precision machined from aluminum and black anodized with a smooth finish, this diagonal is threaded for standard 1.25" filters and features a tapered barrel design.
Clear aperture = 20.6mm; Weight = 5.5oz; Includes two end caps.
Aug 9, 2018
Very useful with good views over mirrors for planetsPros: Cons: Comments:
I bought this on a whim because I read another comment about this diagonal's helical focuser being helpful in keeping the main focuser from extending too far out and throwing off balance. Well, it does that to some degree, and certainly helps me. But the view of the planets through this prism diagonal are so much better than views through my dielectric mirror diagonals, which just leave me with bright blobs of light. I can see Venus' phases and Mars surface details details much, much better through my ST80. I use this diagonal with my WO XWA 3.5mm and 9mm eyepieces and the views of the planets are very good. I have another prism diagonal which also works well on planets, but not as good as this WO diagonal. And Agena is a great store to buy from.
This review was provided courtesy of AgenaAstro.comJul 30, 2018
Two Defective UnitsPros: Cons: Comments:
Regrettably, I can't give this diagonal a good review. It is well designed and very solid, as all Williams Optics products are. The eyepiece holder is a high quality compression ring type with a nice large thumb screw, and the helical focuser is an outstanding addition. I wish more diagonals had this smart feature.
Unfortunately, the first one I purchased was badly miscollimated and severely vignetted about 25% of the FOV on one side for any eyepiece over 18mm. The replacement shipped directly from Williams arrived with the prism loose in the housing. Both were not usable.
Agena's customer service was stellar, immediately exchanging the original defective diagonal, and taking swift action to provide a return and refund for the second.
This review was provided courtesy of AgenaAstro.comApr 2, 2018
Great QualityPros: Cons: Comments:
As usual, the fit and finish is Wiliams Optics awesome. I don't care for the grease selection for the helical focuser. It is a little too thick for my liking and that is at room temperature. It may be a little more stiff in cold temps. Hopefully it will loosen up as it is used. I don't really like torqueing the draw tube a lot, especially on lower end telescopes, and this grease increases the pressure on the entire focuser unit when adjusting the helical focuser. I still like it though. The diagonal is well baffled and blacked out internally. The prism is spotless - no dust or specks on it.
This review was provided courtesy of AgenaAstro.comSep 19, 2017
ASTRO-BALLER...is what you are when you put this in your scope...Pros: Cons: Comments:
This Williams 90 is one badass piece of optical sexiness! Outstanding build quality. My friend and I were blown away at the side-by-side comparison between the stock diagonals on our 90mm mak, 90mm refractor, and this beauty. This takes planetary viewing to a whole new level, and is one serious upgrade! The 15mm of fine helical focus is just what the doctor ordered…oh baby!!! Buy this now, and throw your stock diagonal in the trash!
This review was provided courtesy of AgenaAstro.comDec 24, 2016
Five stars from me!Pros: Cons: Comments:
An excellent quality erecting diagonal. Speedy shipping. Very useful item for my achro telescope. I benefitted from its helical focusing feature too much. It provides a clear image. There is no problem even for high magnifications. Thanks William Optics and Agena AstroProducts.
This review was provided courtesy of AgenaAstro.comSort by
Manufacturer William Optics