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William Optics 45-deg Erecting Prism

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Williams Optics 45 Degree Erecting Prism
For Daytime Use Only...

I recently purchased a TeleVue-76 from a friend when he upgraded to another scope. This little telescope is a pleasure to use and the images are nothing short of beautiful when panning through a dark Arizonan sky. The telescope was acquired to be used for daytime as well as nighttime use as my wife and I also enjoy birding. To allow for this I needed to equip the scope with a correct image diagonal. Some research led me to decide on the Williams Optics 2" 45 degree diagonal, or as termed on the William's website a "45 Degree Erecting Prism". I was looking for a unit that matched the TV-76 in quality and hoped it would perform well.

Having no particular preference in which internet vendor I used I chose to order from Astronomics as they are prominently listed as a Cloudy Nights sponsor. Their website was easy to navigate and the order was processed quickly. A few days later the diagonal arrived as promised on my doorstep. Shortly after my arriving home from work the diagonal was quickly unpacked and installed in the TV-76. Focus was reached near the middle of focuser travel with all eyepieces and all looked good with a nice image of the iron sitting on the ironing board at the far end of the hallway.

First Impressions

The construction of the diagonal is strictly first class. The machining beautifully done with deep black anodizing. Sliding into the TV-76 focuser with a tight, close tolerance fit that gave a nice impression. The eyepieces likewise fit into both the open tube or the supplied 1.25" adaptor with a nice tight feel. It was clear that this diagonal could hold any eyepiece, including the big 'terminaglers', it certainly has no issue with the 35mm Panoptic. Looking down the tube of the diagonal you can see the prism with about 1.25" of clear aperture providing a fairly wide light path. Good sized locking knobs retain the eyepieces, with both the main tube as well as the supplied 1.25" adaptor using brass compression rings.

The night the diagonal arrived the weather was quite cooperative. While high cirrus draped most of the sunset sky with beautiful orange streamers the seeing was quite steady, a solid seven or better on Pickering's scale. My wife knew exactly what was up as I disappeared to the back yard with a tripod mounted scope over my shoulder.


I tested the diagonal with several eyepieces, taking advantage of the 2" capability I mounted a 35mm Panoptic. For higher powers I used my 8mm and 4mm Radians. The view at wide angle was quite pleasing with a fully illuminated field. Out of focus stars seemed a little odd but I couldn't quite determine what was wrong, in focus the stars were sharp until near the edge, but that I expected with the short focal length telescope. The difference really became apparent at higher powers...

With Saturn very high in the evening sky it was an obvious first target to try out the new toy. Using a low power eyepiece to locate the view was just fine, it is when I switched to an 8mm Radian that I started to have a little trouble. Saturn would not quite focus, the view seeming a little soft as I rocked back and forth through ideal focus. There was a fair amount of glare in the image as well as a slight purple haze of uncorrected color I would not expect in a full apochromatic telescope like the TV-76. The glare was most noticeable as a bright pair of wide horizontal bars each side of Saturn. Running back inside I stole a decent quality Meade 2" 90 degree mirror diagonal out of my other refractor. While not a premium piece of gear, the diagonal does perform reasonably well. With the 90 degree diagonal installed the views were once again what I would expect from the TV-76. A crisp, clear view of Saturn rewarded me with both the 8mm as well as the 4mm radian eyepieces.

With the scope at a high elevation I found myself lying in the grass to get a good view with the 45 degree diagonal, simply adjusting the legs in on the tripod to allow a reasonably comfortable view. (Fortunately I had mown the lawn recently.) This is the main reason I would recommend a 90 degree diagonal for astronomy use, the 45 degree diagonal is not a great deal better than straight through for viewing angle, while the 90 degree diagonal provided a comfortable sitting viewing angle at all but the highest elevations.

Swinging over to Procyon, likewise quite high in the sky, I examined the focused and out-of-focus star images with both the Williams 45 degree diagonal and the standard 90 degree diagonal at high power. The result was quite surprising at first until I though about the structure of the erecting prism. With the 45 degree diagonal I saw an odd pattern of vertical bars instead of the expected bulls-eye pattern (see figure below), and the star would never form a decent airy disk at focus. When far out of focus there was a thin vertical bar in the out-of-focus image resulting from the prism structure. With the 90 degree star diagonal the airy disks were textbook perfect with the 4mm Radian eyepiece. This result completely agreed with the soft view I had experienced with Saturn.

Daytime Use

To complete the testing I took the scope to a local birding hot-spot. A large wetlands created using the outflow from a sewage treatment plant, creating a lush oasis in otherwise arid Tucson that abounds with all manner of avian life. I observed treats like a Great Horned owl with it's large, fuzzy, awkward chicks and a beautiful American Kestrel posed perfectly about 100ft away on a phone line. I was using both 14mm and 8mm radian eyepieces for 35 and 60x respectively.

As a daytime spotting scope, at lower magnification, the 45 degree erecting prism performs quite well. It provides a sharp and bright image when used at powers from 35x to 60x. For daytime use I had no objective method to use in testing the scope. When evaluating optics at night there are optical references scattered across the sky, these perfect optical points called stars. Daytime provides no such reference targets, all I can do is relay is my general impression of the resulting images. I have nothing negative to note despite trying to give the image a critical appraisal.

With an erect image the TV-76 becomes a high performance spotting scope with images to match any I have seen in purpose built spotting scopes. The only issue somewhat it that it is somewhat heavy and not as compact as a spotting scope with the same capabilities. The prism equipped scope rewarded me with a wonderful day of memorable sights I would have been unable to enjoy without it.


I would consider the 45 degree diagonal for daytime use only. Keep in mind this is the reason I had purchased it in the first place and I am not disappointed with the device. I will need to remember to pack both diagonals when traveling with the scope if I intend to use it for both night and day observing.

I do not believe the trouble is with the quality of the optics used in the diagonal. I have had a chance to use top end Swarovski spotting scopes at night, and was similarly disappointed, noticing some of the same issues. Instead I believe the problem lies in the inherent limitations of the prism design, too many reflections and too many optical surfaces to get just right. For astronomy stick with a good quality 90 degree mirror diagonal.


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