William Optics 1.25'' 90 degree Erecting Prism

William Optics 1.25" 90 degree Erecting Prism

  (6 Reviews)
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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.
  • Helical focuser with 15 mm of travel for fine focusing adjustment.
  • 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 (6 Reviews):  
Write a Review and share your opinions!

Rating (max 5):  
Incredibly Worthwhile Upgrade
Saturday, 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!
Saturday, 2 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
Wednesday, 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
Thursday, 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):  
Wednesday, 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.

Rating (max 5):  
Alternative to dual focusers
Friday, 8 January 2016  | 

Wonderfully built and with smooth action. A lovely alternative solution if you have a stock rack and pinion focuser. Especially so if you want to keep the original look of a classic refractor.
It gives very fine focussing equivalent to dual speed. The prism provides correct views with minimal effect on observing. We compared it with a WO dielectric on the double star Wasat and found it hard to find any difference in views.
Will definately recommend this as value for money and an essential bit of kit.


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