William Optics 2" 90° Erecting Prism Diagonal

4.0 WO 2" Erecting Diagonal

Excellent build/optical quality; will seal scope from elements if left in place

A bit pricey; suffers in comparison with star diagonal

This product allows correctly oriented views of both sky and land in all directions, up/down and right/left, as opposed to the eyepiece view of a (mirror or prism) "star diagonal", which is rightside up but reversed-looking, right to left. Some backyard astronomers feel the eyepiece view should match that of most star charts and maps (which are often printed as if viewed with the naked eye), and is especially important for lunar work. Birders also prefer to follow the critter with proper right/left movement of the telescope, relative to the target. In the past, most of these devices were optically abysmal, rife with distortion, aberration and cloudy views.

The William Optics 2" erecting prism diagonal is a very solid affair, well machined with close tolerances and an overall feel of quality. Included is a 2"-1 1/4" adaptor. Inspection of the prism disclosed nice multicoatings. One issue that was encountered was the shorter than expected clearance between the prism face and the top rim of the eyepiece insertion barrell. At least one of my oculars, possessing the dual 2"/ 1 1/4" design (think Orion Stratus/Baader Hyperion) actually made contact with the prism if inserted without the 1 1/4" adaptor in place. Be sure and check the mating of eyepiece/diagonal before proceeding in the field.

Optically, this prism teamed with a 17 mm Orion Stratus/ WO 80mm Flourite Doublet, while not waterproof and a tad bulky, absolutely blows the doors off most (if not all) other birding scopes. I compared this setup to a friend's Swarovski 65mm HD spotter, and the WO combo showed better resolution of fine detail, a brighter image, greater color saturation, and overall a more relaxing, easier view. This at about 1/2 the price of the Swar.

While undoubtably one of the better products of it's type on the market, it did suffer in a head-to-head showdown with the same company's Dielectric Mirror Star diagonal. In high contrast situations (bare twigs or limbs against a bright cloudy background) , even at relatively modest power (32X), some purple or green etching around the limbs and twigs was noted. That tack-sharp apo scope "wow factor" was not as great thru the erecting prism diagonal as thru the star diagonal. There is also a small, but present, form of field distortion (curvature?) that is most apparent when panning or adjusting the scope. Although I have not used the erecting prism extensively for astronomy, these shortcomings would still remain, but I cannot directly comment on their bothersome-ness.

All this is understandable when considering a mirror diagonal asks the light to make one surface reflection, and a prism is contorting the beam more severely thru a hefty thickness of glass.

So do you need one? For the moon, utilizing a book, map or software program (there are several) that provide landmark orientation as seen thru a refractor/star diagonal pairing can be used. I would certainly avoid employing this erecting prism with high powers.
Star-hoppers would benefit most from an erecting prism/ short-focus refractor, when using the main scope as both primary instrument and finder- as long as the power is kept low to low-moderate when wishing to examine an object in greater detail.

For terrestrial use, if you are acustomed to a regular birding scope with correct reading right-left eyepiece fields, the choice is a bit tougher, but having experience with both I would recommend a star diagonal. I submit it's easier to get used to reverted images than to lesser optical performance. After a while, it's just second nature to move the scope right if your quarry is moving left. In fact, having used a mirror diagonal for many years on my trusty 'ol Pronto as a birding/astronomy scope, I was constantly disoriented and moving the "wrong" direction when I switched to the erecting prism!!!

Having said all that (really more than anyone probably wanted to read!), if you must have a eyepiece field that is both rightside up and rightway 'round, this is one of the few products that won't give you a headache when looking thru it.