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William Optics diagonal

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Hello. I have just recently got my first proper refractor. By this I mean I mean a refractor of  decent size aperture and quality lenses. The skywatcher 120 Ed apo maybe a Chinese offering but I consider it a quality refractor that punches well above its price tag. This scope was mounted on a steel leg AZ4 mount , for me a great set up as I am a big fan of the AZ for both the quickness and the solid build quality of this mount. As I do not like extension of the AZ legs to much , I prefer low to the ground approach. Then using this set up of the 120apo and AZ4 mount I had been spending a lot of my time on my knees to observe anything to high up in altitude. Obviously this was getting a bit wearing on both my poor knees and my viewing position in general. Your back also starts feeling it. And trying to observe through the eyepiece while operating the AZ was getting beyond a joke. Time to spend some more money and get a diagonal.

There is a big choice of diagonal on the market, skywatcher, baader, Televue everbight to name a few. And the prices vary considerably. My first decision was 1.25 or 2". Obviously initially financially the 1.25  was very tempting and recently buying the 120 apo and then even more recently the AZ then I was very tempted to go down this route. But as I had already got a 2" eyepiece then i thought I would love to use it in the refractor, plus I was planning to buy more of the 2" for the benefit of fov. Therefore I started to look at the 2" obviously far more expensive but the benefits will out way this. Next what manufacturer to go for, you obviously have the top Televue everbight at around the 240 pounds mark. Then you have the cheaper end of the market at a fraction of this. But a lot of time in life you get what you pay for ,and having a quality frac and decent mount I did not want to undermine this combination by going for an unsatisfactory diagonal. After looking at the market for performance, budget and availability I came up with the decision to go for a middle of the road diagonal.

The diagonal I decided on was the William Optics 2" diagonal. At around 140pounds new, I felt money for performance then hopefully this should be a good diagonal for the 120 apo combination. I did feel though that I may be able to save a few pounds and have a look on the used market. To be quite honest one or two Televue everbight did come up used but still over 200 pounds so they where left alone(even though I was tempted as I do like Televue). Lucky soon after a William Optics 2" dielectric came up for sale. So I was happy something was now on the market that I wanted and was around the price range that I would like to pay. Lucky I managed to purchase the William Optics  dielectric diagonal at around just under 50% of the new price ,so all in all I was very happy. Just had to wait for the post now.

When the parcel did come my first impression was seemed quite heavy for a small box( heavy to me means quality ,hopefully). On opening the box and getting the diagonal out  I was p!eased to see like a black casing with two shinny metal plates on either side. On closer inspection the main body seems to be made out of high quality aluminium ,coated with an enamel type finish. The two shinny plates on either side seem to be of a quality stainless steel. In each side of the diagonal main body ,was a 2" scope end opening with a plastic protection cap on it , and on the other side a 2" opening with a 1.25 eyepiece converter with a protective white cap inserted in it. Therefore from an aesthetic point of view and quality of build point of view I was very happy. I then decide to take the caps off and have a look at the heart of the diagonal which was the mirror of course, my understanding of these dielectric mirrors are they have 99% reflectivity and the mirrors are polished to 1/12. On taking the caps off and aiming to diagonal towards light outside objects how clear and crisp and bright this image was in the mirror, a lot better than expecting. Again all looked good, but on a closer look at the diagonal mirror surface there did seem an amount of fine dust on it. Therefore some home surgery required. I got the Henry out attached the plastic fine attachment hose to it and carefully held this just above the mirror surface. About 10minutes later most of the offending dust was gone. My overall impression was very positive, but as they say the proof of the pudding is in the eating. I do like to try kit in the daytime at first if at all possible, saves possible damage to the set up IMO. So I did set up the AZ and 120apo for a short time one afternoon. And targeted a neighbour chimney and chimney pot. The views of the brick work and pointing was very sharp and to the point I could even see stamp marks on the chimney from the manufacturer ,very crisp and sharp, the diagonal certainly seemed a top performer. Very happy so far.


The next time I got the diagonal out was a few nights later (clouds had stopped play). After setting up the AZ 4 and putting the 120 apo into position it was time to try my new toy. Taking the 2" caps out of the apo I slide the WO diagonal into its place , a nice close fit but smooth into position. I then took the plastic white protective 1.25 cap out the other end of the WO diagonal ,to expose the cooper type eyepiece mating ring and placed a Televue 32mm plossel in. Again a nice close fit but smooth into place nicely. All still looking good I thought and nice close but smooth fitting connections . I do like using the moon initially for testing kit out, big bright easy to find and a lot to see. So first target was the moon. On location of the moon the seeing conditions where reasonably good ,and the view that greeted me was a sharp crisp image and I was well impressed, the diagonal did not seem to take anything away from the view at all, and as my poor knees where intact they were more than happy. Sitting in a chair now going up the magnifications I really was getting more and more impressed with the quality of this WO offering. I was now up to a 8mm plossel and the views were still crisp and very sharp. Again from my eyes this diagonal was not taking anything away from the view compared to an un diagonal scope set up. I have since used this diagonal on Jupiter, Saturn, star clusters, with eyepieces such as nagler and more recently xw and I have been very impressed with the overall quality of build as well as well as the crisp sharp, and comfortable   viewing experience it gives you. So overall I am very pleased with my purchase.







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I hope you like this review and have found it helpful. Thank you 

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Excellent and helpful review :icon_biggrin:

Having had an ED120 for a few years now and having tried a few diagonals with it I've been through similar experiences. The William Optics 2" is an excellent diagonal and I've not noticed any obvious performance differences between it and more expensive mirror diagonals from Tele Vue and Astro Physics.

I also found the Revelation Dielectric 2" a good performer for a very reasonable price.


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Hello John and thank you very much for your input on this matter. As a far more advanced member than me with far more experience of equipment also, then it is nice to hear your view. As obviously we all have different eyes and sometimes we do not see things in quite the same way. But I was really impressed with the William optics diagonal and for you to back up and support my findings is nice to know for me and I am sure other members will find it helpful. 

By the way I do apologise for not putting any pictures into the review of the William Optics diagonal. But I am doing this review from a tablet and i am not sure how to hook up and insert pictures. Maybe if a moderator reads this they may be kind enough to put a picture or two in for me under the review. Or maybe a fellow member who wishes to reply could post a picture. As we do all love to see a picture of the item mentioned in a review ?. Thank you

I have managed to sort out the tablet settings and get some pictures up now of my William Optics diagonal, so hope you like

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Very nice review.  While I have never undertaken a serious side by side comparison of all the diagonals I own, many of which are dielectrics, I have not been aware of any diagonal diminishing the view offered by any of my scopes by any noticeable amount.  However, I recently acquired a Baader BBHS Sitall diagonal and it has started to change my view on the importance of this piece of the optical chain.  I need to run some dedicated side by side tests to confirm some early observations, but I have noted an appreciably lower amount of scatter around bright objects such as Jupiter and colours appear more vibrant.  Jupiter’s red spot was far more obvious and the colour far richer (like it was 15 years ago) compared to my dielectrics.  Downside, the Baader diagonal is about £400 so quite a bit more expensive than even the Tele Vue.


Why might this be?  Some suggestions that while the mirror blank for a diagonal might be 1/10 wave or better, this is pre the depositing of the dielectric coating which may leave the final optical surface far worse than the marketing numbers.


As it stands, I think you have chosen well.  I own the same diagonal and happily use it on a regular basis.

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