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William Optics Zoom II Eyepiece (7.5mm-22.5mm)

Posted by Timm Bottoni   11/22/2008 13:16:PM

William Optics Zoom II Eyepiece (7.5mm-22.5mm)
Zoom eyepieces have long been classified by many users as inferior, or inadequate for most astronomy use due to their inherent limitations, and subpar quality. The reviews I had read of the various models available all seemed to come up short, with the exception of the high magnification Televue 2-4mm and 3-6mm zoom models, suited mostly to planetary viewing with refractors, and some very expensive other brands. When William Optics came out with the Zoom II eyepiece promising wider fields of view, more eye relief, and higher optical quality, it got my interest. I already own the WO UWAN eyepieces in sizes 4mm, 7mm, and 16mm, and the WO SPL eyepieces in 3mm, 6mm, and 12.5mm, as well as a WO SWAN 33mm, so why would the WO Zoom II be of any interest to me? Well, because I had just gotten a used Celestron C8, an 8 inch SCT, to hang on the other side of my WO 80mm APO refractor. (Thank you again Bill!)

Because the WO EZtouch mount is an Alt-Az, it doesn’t track, nor does it have slow motion controls. I thought; wouldn’t it be nice to use a zoom eyepiece in the C8, with its 2032mm focal length, F/10 optics, so that when I found an object to look at, I could increase magnification without having to change out eyepieces and risk losing the object in the field of view. Having no prior experience with the C8, I have since tried it with all of the eyepieces I own, but my guess was a good one. The WO Zoom pretty much stays in the C8 the entire night while observing a variety of objects.

The WO Zoom II is definitely a keeper for me, and the remainder of this review will explain why, including my test results using the eyepiece in the C8, the Megrez 80FD, and the Stellarvue 50W finderscope, viewing at night and during the day.

What is a WO Zoom II eyepiece?
It has the typical 1.25” WO barrel, meaning it has a tapered all black barrel, that doesn’t catch on the retaining ring of my diagonals. Why everyone hasn’t gone to this type of smooth, no hook-up design is beyond me. According to the specifications on the WO website, the Zoom II is described as follows: Focal Length 7.5-22.5mm, Lens Structure (#Elements/#Groups) 8/5, Field of View 42-66 deg, Eye Relief 18-20 mm, Weight 260g, Internally baffled, All lenses' edges and internal spacers blackened, Fully multicoated. As far as I can tell, the specs are accurate, but I am not an optical eyepiece expert, just an average observer with about 5 years experience in the astronomy hobby.

How does it perform overall?
Excellently! To sum it up, there is nothing inferior or subpar about the optical performance of this zoom eyepiece. In every test I threw at it, the eyepiece impressed me with its edge to edge sharpness, lack of reflections, and lack of distortions. The performance was very good whether I tested it in the C8, the WO-M80FD, or the SV-50W. In all cases day or night, I couldn’t find any significant aberrations or distortions. On Jupiter, I could sometimes find some reflections, when viewing off axis, but they weren’t that bad and viewing planets off-axis is not something one generally does for any length of time. It allows for 3X range of zoom from end to end of its focal lengths. It provides magnifications that range from 90X to 271X in the C8, which on most nights exceeds my skies seeing conditions. The zoom functionality allows me to smoothly dial it into the magic magnification so the best views can be achieved regardless of the seeing conditions. This proves to be a perfect range of magnificaitons in my C8 for planetary viewing giving me the ability to match the best magnification to the seeing conditions, which usually is in the 200X-250X area for Jupiter and Saturn.

The Urban Astronomer’s Best Friend – The magic magnification defined
On DSO’s like globular clusters, and planetary nebulae, the ability to zoom provides an exceptional way to increase the details without sacrificing the chance of moving the scope off target while switching eyepieces. In my light polluted skies, galaxies are pretty much limited to the brightest ones, and diffuse nebula are tough on all but the darkest nights, but the Zoom II eyepiece performed well on those as well, allowing me to ease into the best magnification for detecting and viewing these object. The ability to zoom smoothly, into and out of the magnification range on DSOs allows for one way to combat light pollution, and I call that the “magic magnification”. Take an object like a globular cluster, we can use the Hercules Cluster as one example. In any telescope, increasing the magnification darkens the background, and makes the object appear brighter, but only to a point. There is essentially an ideal magnification, or magic magnification for each combination of scope-object-sky that brings out the most details and makes for the best viewing of that object. The Wild Duck Cluster, M11, is another of the best examples. With the 80mm refractor, this object is dim, a worthless soft fuzzball, no fun to look at it’s location in my skies due in part to light pollution, and increasing magnification beyond about 100X doesn’t really help with only 80mm of aperture, With the C8 this object really comes alive somewhere in the 150X to 200X range. As you zoom into the object, more and more individual stars become resolvable until you reach the magic point where you see the darkest sky, the most stars, and the best resolution of this small tight open cluster. That magnification point can also depend on the position of the object in the sky, and the seeing conditions, especially if there is heavy light pollution lower to the horizon. The zoom eyepiece provides the ideal way to reach the magic magnification needed for getting the most out of each DSO being observed. It really does make sense when you think about it, but you really don’t have to think very hard since with the zoom eyepiece, you just zoom in and out until it looks best to you. That point is not likely to be an exact match with a fixed eyepiece unless you have the Kings collection of all sizes, and have the time to keep switching eyepieces until you find that best one.

What’s with the Field of View changing?
Well, this is a bit of an interesting specification with zoom eyepieces like this. Why? It’s quite simple actually. As the eyepiece zooms to decrease focal length (thereby increasing magnification) the relative field of view increases. Shouldn’t it be the other way around? Well, no, it doesn’t work like a zoom lens on a camera, because in this case, the eyepiece itself does not change length, and only some of the internal parts move, but the field stop does not change size. What happens is that as you zoom the eyepiece in (increase magnification) the lens elements move internally, and the true field of view gets smaller, but at a slower ratio than the increase in magnification. This means that at 22.5mm, the eyepiece has a relative FOV of 42 degrees, but at 7.5mm the eyepiece has a relative FOV of 66 degrees. I tested this on my charts in the basement, and my measurements confirmed that this was true, and in fact slightly more than what WO has published these FOV specifications to be. I know it seems counter intuitive at first, but if you are confused by this, just give it some thought, and you will understand it. Even if you still don’t get it, just take my word for it, since it is pretty easy to see this effect when you zoom in on a chart or large open cluster of stars. Because of this, I find that I generally don’t observe much at 22.5mm, and instead use it only to locate the object. Well, let me rephrase that by saying “locate the object in the C8” since in most cases I have already located the object using the combination of the 50mm SV finder and the 80mm WO Refractor, which are nearly exactly aligned on the EZTouch mount. So basically, I use both scopes, and pick and choose which is better to observe with depending on the skies, and the object. It makes for a great combination while observing.

Optical Performance
I’ve used the Zoom II in the WO 80FD as well, and it performs equally as well, but it honestly is better suited to my SCT C8 for most of my viewing because I have the UWANs for the widefield views that I tend to use in the refractor. It’s much harder to loose the object in the refractor while switching between the 16mm, 7mm, and 4mm ultra widefield UWANs, because it has a much shorter focal length, hence a much lower magnification with each eyepiece. I also have tried the Zoom II in the SV50W finderscope, both at night and during the day. It performs well and does come to focus, and makes for a surprisingly good and small spotting scope. The only issue is that the screw that tightens to hold the eyepiece in the prism of the 50W does not allow the eyepiece to drop all the way into the prism diagonal, so it only fits in part way. This is because the eyepiece protrudes out at 90 degrees to a significantly wider width, immediately at the end of the 1.25” barrel.

On my indoor test charts the Zoom II performed well in all three scopes in the daylight. It shows a very flat field of view, no pincushion or barrel distortions, and is sharp out to the edges in both the C8 and the 80FD. In the SV50 finderscope, it does pretty well, in spite of its F/4 200mm focal length, showing a crisp field of view out to about 75% to the edge. I wouldn’t recommend buying the $119 50mm SV Finderscope and this $289 eyepiece as a great combination for a spotting scope, but it does work quite well in this combination. On the other hand, a high quality refractor, 66mm or higher, along with a good quality prism diagonal, and this Zoom II eyepiece would likely yield a killer combination as a daytime spotting scope.

The only optical anomaly I could notice is that during the daylight, the field stop is not crisply focused in the middle of the zoom range, while it is near the ends of the range. At night I didn’t notice this at all.

Mechanical Quality / Fit and Finish / Ergonomics?
The eyepiece is very well made, essentially of the same build quality as the UWAN eyepieces. The eyecup is wide, and twists up and down like the ones on the UWANs work, and is very similar in its appearance and quality. The eye relief is very good, and the ability to position the eyecup, combined with the large front optical element makes this an easy eyepiece to view through. Someone with glasses would have no problem using this eyepiece, and I confirmed this by trying it with reading glasses on. The zooming mechanics are driven by twisting the marked barrel that is encased in a knurled rubber coated assembly, and works smoothly, but stiffens up somewhat in very cold weather (which I try to avoid, but in Winter, you live with it now and then). There are no click stops at the marked focal numbers, so without looking, there is no way to know in advance what the eyepiece is set to. I prefer this, since the variability is not limited to stopping at a click, and not knowing where you are at is not a problem for me. I would call it almost parfocal, but not quite, as the eyepiece does not stay in perfect focus while zooming. It doesn’t change by much, and it’s easy to refocus slightly after changing focal lengths. I have been tempted to take it apart to see how it looks inside, but this is one I have been afraid to tinker with. The coatings look excellent, and every part that I can see from the outside appears properly blackened, and antireflective.

What’s not to like?
It’s a large eyepiece, and heavy so beware of balance issues if you aren’t used to eyepiece of this size. The field of view being narrow at the lowest magnification, is also a limitation that one has to live with in all zoom eyepieces, and I wish it could be different, but I find that I am generally using it in more of the 10mm-20mm range where the field of view is typical of non-widefield eyepieces anyway in my SCT.

My overall conclusion is that I am very satisfied with this eyepiece, and it does exactly what I hoped it would do. It allows me to observe objects at the magic magnification for the best views possible in my SCT C8, and change magnification easily without losing the object from the field of view.