William Optics 1.25'' 90 degree Erecting Prism

William Optics 1.25" 90 degree Erecting Prism

  (11 Reviews)
✓ 2 year warranty


1x in stock shipped 1-2 working days

New upgraded version from William Optics

About this product

Model:  WOerectPrism1.25-90
Part Number:  D-EP90-125II

William Optics 90° Erecting Prism with helical focuser for fine adjustment.

Turns upside-down views right-side-up and gives correct left-to-right orientation. 

Works with 1.25" eyepieces and telescopes with 1.25" eyepiece holders.

  • Large, high-quality BAK4 erecting prism with anti-reflection coating for brightness and contrast.
  • Internally matte blackened to reduce internal reflections.
  • Threaded for 1.25" filters.
  • Made from black anodised aluminium.

NOTE: This erecting prism works very well for terrestrial observing but can occasionally produce a vertical diffraction spike when used for astronomy. 

Customer reviews

Average Rating (11 Reviews):  
Write a Review and share your opinions!

Rating (max 5):  
William Optics (WO)1.25" 90-degree Erecting Prism with helical focuser for fine adjustment
30 June 2023  | 

Used on Skywatcher Star Travel Refractor 120 (f/5) Telescope.
I obtained the WO 90-degree Erecting Prism to give true orientation for less painless star hopping with existing erecting finder scope and star maps, that now all match up in the same orientation for object hunting. The helical focuser provides very fine adjustment and is a plus with this f/5 telescope which at times on high power can be a little bit fiddley to focus. It is a solid well-made product.
This is not to say that the original telescope star diagonal was not any good, but the WO Erecting Prism gives crisp contrast and brighter views and along with the Apo filter, brings out the best this telescope can provide. It occasionally produces a vertical diffraction spike on brightest objects like Venus which is inherent with this kind of device, but not a problem for the observed target. I still use the original star diagonal for the bright moon, since being familiar with reversed view moon landscapes.
FLO service was excellent and the advice very helpful. Will use again.

Rating (max 5):  
William Optics 90 degree erecting prism diagonal
30 August 2022  | 

Very quick delivery of a good product. The helical focusing eyepiece holder is very useful.

Rating (max 5):  
Great service. fantastic product.
17 June 2021  | 

I am a devotee of simple visual astronomy and the fewer the gadgets the better. ( My wife agrees) My red dot finder is fairly essential but apart from that it's a case of hopping from star to star. This is where my new purchase has made life so much easier. Left is left and right is right . I think the prism quality is probably good enough to match mirror diagonals at the same price with the advantage of true orientation and also the bonus of a focussing ring built into the collar. The moon is a much friendlier place as well now: as straightforward as using binoculars but with 120 x magnification on my ST120 with 5mm eyepiece. Probably the best money I've spent on my recent journey into astronomy.

Rating (max 5):  
Nice Upgrade
12 January 2021  | 

Very pleased with this erecting prism which was used to replace a stock diagonal on a Star-Watcher 102mm Starquest refractor. Not only is the image sharper but I find it easier to navigate the night sky with this arrangement. The basic star diagonal supplied with the Star-Watcher was disappointing and covered in fine dust from new. The William's Optics prism has the added benefit of the built in helical focusser for fine tuning.

Rating (max 5):  
Perfect upgrade
24 November 2020  | 

This is easliy the best thing I have bought from FLO so far. It is the perfect upgrade for my Skywatcher Evostar 90. Looking at the heavens directly above gets you in some serious yoga positions with the standard 45 degree diagonal so this just makes life a whole lot easier. Highly recommended!

Rating (max 5):  
Incredibly Worthwhile Upgrade
14 September 2019  | 

I recently bought a barely used Sky-Watcher StarTravel 120 achromatic refractor and knew the star diagonal was going to be mediocre at best.

The difference between the stock diagonal and this excellent William Optics diagonal is night and day. Just looking into the stock diagonal away from the 'scope showed light was being refracted with a lot of colour bleed.

The William Optics one? Flawless and very reflective. It is exceedingly well built, and evidently a lot of careful work was put in to give it perfect collimation, and not to become a weak link. Chromatic aberration was notably reduced, and the fine focuser is so smooth and an excellent addition.

Pairing this with a Baader Semi-Apo filter reduces so much chromatic aberration and really helps to get the most out of your optics.

If you are not a fan of dielectric mirror diagonals for their horizontally flipped image on a refractor, seriously consider getting this. You can feel right away that this is a carefully crafted diagonal for optimal viewing for both day and night. If, you really want an Amici prism diagonal, then you simply can't go wrong with this and could give a refractor you haven't used much a whole new lease on life.

Rating (max 5):  
Great alternate to dual focuser!
02 June 2018  | 

This is a great bit of kit if you don't want to upgrade to a 2 inch or 10:1 focuser, helical action is very smooth, and really helps fine focus, and even though its an erect image In think it performs as good as or better that a star diagonal. I use it coupled with my st80 & evo 90 and makes a great addition! Also used with a baader T2 1.25 eyepiece holder which holds it well and prevents slippage and marking on the nose piece. Flo very helpful, and kept me updated on delivery!

Rating (max 5):  
Great Product at a great price
28 February 2018  | 

This is a very good erecting prism. My intention was to use it in my Baader Varifinder, but it is so much more than a simple erecting prism, making it an ideal match for a high quality refractor for terrestrial viewing. Recommended!

Rating (max 5):  
William Optics 90 degree prism
25 January 2018  | 

Very useful piece of well-engineered kit. Optically excellent and the addition of the helical focuser to my telescope has made a huge improvement. Highly recommended.

Rating (max 5):  
25 May 2016  | 

OK not everyone wants an erecting prism but I did! The build quality of this 90 prism is very solid and the optics are good with no perceivable aberrations to my ageing eyes. Much better than the 45 prism that came with my scope. Also the inbuilt helical focuser is a bonus and acts as a fine focus once the scope is focused and does not vibrate the set up. I now don't get back ache with this 90 diagonal in place - highly recommended for quality with ecconomical cost.

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