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Triple play: the ZenithStar 66 refractors: portability, performance, and pleasure--these refractors from William Optics offer all three. But which one is right for you?

Three new 66-millimeter ZenithStar refractors from William Optics offer a level of craftsmanship and optical performance that far exceeds most people's expectations for instruments so modestly priced. All hardware photographs are by Craig Michael Utter for Sky & Telescope.

As this issue was going to press WO announced that its dual-speed focuser will now be standard on the Petzval and apo scopes. The Petzval price remains the same, but the apo increases to $548.

Zenithstar 66 SD


Excellent mechanical craftsmanship

Excellent optical quality


Requires optional accessories to reach focus

Longer close-focusing distance than SD and apo models

Zenithstar 66 SD


Excellent mechanical craftsmanship

Excellent optical quality


Requires optional accessories to reach focus

Requires optional accessories to reach focus

Zenithstar 66 SD


Excellent mechanical craftsmanship

Near-apo optical performance

Dual-speed focuser ZenithStar 66 SD

WHAT WE DON'T LIKE: Requires optional accessories to reach focus

is it just me, or has anyone else noticed that every decade seems to spawn a telescope that piques the imagination of amateurs everywhere and inspires many to open their wallets? During the 1970s, Schmidt--Cassegrains were all the rage. The '80s saw the meteoric rise of Dobsonian reflectors. In the '90s refractors--especially premium quality instruments--were the scopes everyone dreamed about.

And that brings us to the present and the question of what will reign as the signature telescope of the 21st-century's opening decade. Based on recent trends I think it will be high-end, short-focus refractors. Short-tubes, as they are collectively known, have been around for a while, but the last few years have given rise to a spate of really top-notch models that everyone seems to be talking about. For proof you need look no further than to the trio of 66-millimeter ZenithStar refractors recently introduced by Taiwan-based William Optics (WO).

Someone new to the world of telescopes might wonder why one manufacturer would offer three remarkably similar models. Those more experienced, however, will recognize that the Petzval, SD, and apochromat terms worked into the respective Zenith Star monikers (not to mention the prices) suggest different levels of optical performance. But the names alone don't tell us how much of a difference. To find out, I put all three scopes (on loan from WO) through field and bench tests.

I used them visually and photographically for astronomical observations at night and nature studies by day. In the end I found that each scope has its own personality and features that would likely influence a potential purchaser. While one emerged as my favorite, it was not the scope I was initially drawn to; you'll have to read on to find out which one it was.

Why, They're Beautiful!

Almost without exception, the thing everyone mentions when first setting eyes on this trio is how attractive they are. The elegant mechanical design, superb fit and finish, and high-gloss black-anodized surfaces with gold and silver accents make them works of art. Simply put, each scope's solid heft and good looks suggest it would cost much more than it really does. Indeed, show them to someone who appreciates, say, quality camera gear, and you'll likely find the person amazed to learn how little they actually cost. If there's any downside to this beauty, it's that the shiny black finish readily shows fingerprints.

The base price of each scope includes the optical tube fitted with a 11/4-inch eyepiece holder, retractable dew shield, metal lens cap, and a plastic dust plug for the eyepiece holder. The apo and Petzval models come with very durable, foam-lined soft cases, while the SD has a nice foam-lined hard case that looks like textured aluminum but is really plastic.

Sometimes it's the little things that delight or annoy. For me, one of the little delights involved the oversize thumbscrews on the focuser and eyepiece holder, which make them easy to operate while you are wearing winter gloves. Annoyances, on the other hand, were relatively minor, such as the slightly loose-fitting lens cap on the Petzval scope that inevitably slips off whenever the tube is pointed downward.

Each scope has a solid mounting foot fixed to the tube near the focuser. The foot has two 1/4-20 threaded holes, making it compatible with standard camera tripods, and a pair of inset cork strips that give added grip to prevent the scope from twisting around on its mount. The foot's tapered sides fit into the standard Vixen-style dovetail brackets found on many popular equatorial mounts. This versatile foot always made it easy for me to attach the scopes to the camera tripods and astronomical mounts I wanted to use.

The three scopes have the same basic Crayfordstyle focuser, which can rotate a full 360[degrees] compose photographs and position the focusing knobs at a comfortable angle even when the scope is on a German equatorial mount. A single thumbscrew locks the focuser's position. But beware: if you excessively tighten the thumbscrew, it will form a small detent in the rotation mechanism. This can add a rough feel to its movement and prevent the focuser from locking in positions just to either side of the detent (the thumbscrew tends to slip back into the dimple). Furthermore, the thumbscrew hits the mounting foot on the SD scope, preventing full rotation of the focuser. This happens only with the SD because its foot is reversed relative to that on the other scopes to allow the dewcap to fully retract (the SD has the shortest tube). It's easy to turn the foot around and restore the focuser's full rotation, but then the dewcap only partially retracts, adding about an inch to the overall length of the stowed scope. It will, however, still easily fit into its carrying case.

The focuser's drawtube has a 43-mm clear aperture, and the business end has the same threads found on the rear cells of Celestron and Meade 8-inch Schmidt-Cassegrain telescopes (SCTs). In fact, the 11/4-inch eyepiece holder supplied with the refractors is very much like the SCTs' 11/4-inch visual back. Any accessory that can be threaded onto the SCTs can also be attached to the refractors. But this doesn't mean that they'll work properly, since unlike the SCTs, the refractors have fixed focal planes and the Crayford focusers have only 65 mm of travel.

The SD scope has two features not found on the other models. The nicest is a dual-speed focusing system (a $148 option on the Petzval and apo models). The main focus knobs on the SD provide the same motion found on the other two scopes, but an additional knob offers about a 10:1 gear reduction. This is especially helpful for high-magnification observing and photography. The other feature on the SD is a millimeter focusing scale silk-screened onto the drawtube.

Focusing Matters

Given the wide range of accessories that can be attached to the drawtube's SCT threads, and the versatility of these scopes for observing things near and far, there are several caveats that purchasers should consider. Chief among them is that these scopes will not reach focus with a 11/4-inch eyepiece simply inserted into the eyepiece holder. At a bare minimum you'll need an extension tube or a star diagonal to get sufficient back distance. Furthermore, the Petzval does not reach focus even with the optional 11/4-inch diagonal sold by WO--you still need a small extension tube of some sort.

Explaining all the setups that will and won't reach focus with these scopes isn't practical, so I'll simply offer some suggestions based on my testing experience. Visual observers should order these scopes with one of the optional WO 2-inch star diagonals that have SCT mounting threads. I never encountered a 11/4- or 2-inch eyepiece from WO, Celestron, Meade, or Tele Vue that couldn't reach infinity focus with this type of diagonal.

Photographers working with any film or digital SLR camera should order the WO 2-inch Adapter (which screws onto the focuser's SCT threads) and the 2-inch Photo Adapter, which is basically a 45-mm extension tube that slips into the 2-inch Adapter and is threaded for a standard camera T ring and 2-inch filters.

With the 2-inch Adapter attached to the focuser you can fit conventional 2-inch accessories to the telescope, but with only rare exceptions you can't reach infinity focus using standard 2-inch star diagonals, even the models from WO. Another accessory that won't reach infinity focus in any configuration is the WO Binoviewer.

The focus table at lower left gives the distance that each telescope's infinity focal point falls beyond the focuser's drawtube when it is fully racked in. It also gives the minimum distances (useful for nature studies) for which the scopes will focus using WO 2-inch SCT and 11/4-inch diagonals and a WO 20-mm wide-angle eyepiece. This eyepiece has about the same focus position as the Meade 26-mm Super Plossl, which is quite possibly the most ubiquitous eyepiece on the planet!

Under the Stars

Based on the look and feel of these instruments, I had high expectations when I turned them skyward. For the most part I wasn't disappointed. Although the Petzval carries the name and basic 4-element optical configuration of the highly regarded Tele Vue-NP101 refractor (reviewed in S&T: May 2002, page 44), I anticipated some limitations because it is the lowest priced of the three WO scopes. At low and moderate magnifications the images are crisp and easy to focus. But bright stars and the limb of the Moon are tinged with a blue halo, and there is a thin green hairline along the Moon's bright limb.

I didn't find this false color troublesome, but some observers are bothered by chromatic aberration more than others.

A critical star test revealed the Petzval to be free of astigmatism, but it was easy to see a modest amount of spherical aberration. It wasn't enough to affect low- and moderate-power views, but it was enough to take the "edge" off high-power images of the Moon and planets. All three scopes have excellent internal baffles, and there was no sign of significant glare or scattered light even with the full Moon centered in a low-power field.

Deciding to jump to the most expensive member of the trio, I next tested the apo, which features a three-element objective. True to its apochromatic heritage, the images are beautifully free of false color, showing only the smallest touch of blue halo at the Moon's limb. Night or day, the apo images have the rich color fidelity that one expects from an apo. This scope also scored a near-perfect star test. Clearly it was in the running to be my top pick.

Based on its price, I expected the SD to fall somewhere between the Petzval and apo. And indeed it did. It shows a small amount of spherical aberration, but not enough to affect even the highest-magnification views most observers are likely to use. What surprised me, however, was the color correction--it was nearly as good as that of the apo. This is especially remarkable for an f/6 instrument (the apo is f/7) with a two-element objective. Short of doing critical side-by-side tests, I perceived the SD and apo scopes as having essentially the same level of color correction.

While the optical performance of the three scopes ranked in the same order as their costs, it is noteworthy that there is so little difference between the SD and the apo. Furthermore, you shouldn't take this ranking as meaning that the Petzval is a mediocre scope - by itself it is a very fine visual performer that exceeded my expectations. It's only when compared with the SD and apo that its shortcomings are emphasized.

Another optical consideration for any short-focus refractor is the quality of the eyepiece. To level the playing field as much as possible, I tested the wide-field performance of each scope with a Tele Vue 35-mm Panoptic, an eyepiece known for delivering superb star images to the edge of its field. This eyepiece also delivers 85 percent of the maximum true field obtainable with any 2-inch eyepiece. The apo and SD give very good star images across nearly three-quarters of the Panoptic's field, while the Petzval covers about two-thirds of the field well.

The photographs of the Orion Nebula on page 79 made through the scopes speak for themselves. All three instruments showed some image degradation at the edge of the field covered by the APS-size CCD in my Nikon D70 digital SLR. Without a field flattener, which isn't yet offered, these scopes can't be expected to provide sharp detail across the full dimensions of a 35-mm camera frame. But this is a significant handicap only for astronomical photography. In nature shooting, for which all the scopes work well, most scenes easily tolerate some degree of edge softness because of a telescope's inherently shallow depth of field.

Color aberration is the Petzval's limiting factor for astronomical photography. This scope shows even moderately bright stars surrounded by strong blue halos, and the brightest ones have pronounced red and purple fringes. The SD and apo scopes, on the other hand, have nearly identical color performance. If I were making a choice between them for astrophotography, I'd decide based on their focal lengths, since the apo's image scale is 16 percent larger than that of the SD.

While on the subject of focal length, I did find slight differences between my measured focal lengths for the SD and Petzval scopes and those given in the WO specifications. The discrepancy for either scope, taken by itself, isn't a big deal. However, the specs tabulated on page 78 might seem confusing to sharp-eyed readers since they look reversed - my measured value for the SD appears close to the WO value for the Petzval and vice versa. The values in the table are correct.

Which One to Pick

Many factors affect someone's choice among a selection of telescopes, but in the case of these WO refractors, aperture is not one of them. For visual work the Petzval is a very nice little telescope, especially for anyone who wants a quality short tube refractor for a very attractive price.

Human nature usually makes us covet the best. The apo, with its excellent color performance and first-rate star test, certainly fills that bill, and I was pretty sure it would be my favorite scope after my first night with it under the stars. But as the weeks wore on, I found myself leaning more to the lower-cost SD model. Its compact size, dual-speed focuser, excellent photographic performance, and close-focusing distances (both photographically and visually) made it the scope I most often selected when I was observing for my own enjoyment. While the "SD" in the name refers to the "super-low dispersion" glass used in the objective, I began thinking of it as meaning "sweet deal." Regardless, anyone who picks one of these WO refractors will certainly find that it delivers excellent performance for the money. And in that sense, they're all sweet deals.
A Family of Three

ZenithStar 66-millimeter refractor
with Crayford-style focuser, 11/4-inch
eyepiece holder, mounting foot, lens
cap, and carrying case.

US street price:

Petzval model       $348
SD model            $399
Apo model           $498

William Optics USA

11155 Knott Ave., G & H

Cypress, CA 90630


Bottom-line summary: Beautifully made and highly versatile trio of portable, short-tube refractors that serve visual and photographic needs both day and night. While they can be used with regular camera tripods, they are better suited to heavy duty models with fluid or geared heads and dedicated astronomical mounts.

While definitely not a "refractor nut," senior editor Dennis di Cicco admits to liking these high-quality instruments a lot.

                      Petzval             SD

Effective aperture    63 mm               66 mm
Focal length          388 mm              397mm
Minimum tube length   12 1/2 in (32 cm)   10 3/4 in (27 cm)
Tube weight           3.52 lb (1.60 kg)   3.54 lb (1.61 kg)
Maximum true field
1 1/4-inch eyepiece   4.0[degrees]        3.9 [degrees]
2-inch eyepiece       6.8[degrees]        6.6 [degrees]


Effective aperture    66 mm
Focal length          461mm
Minimum tube length   13 in (33 cm)
Tube weight           3.66 lb (1.66 kg)
Maximum true field
1 1/4-inch eyepiece   3.4[degrees]
2-inch eyepiece       5.7[degrees]

* All values measured by Sky & Telescope


                                Close-focusing distance

            Focal plane *       1 1/4-inch diagonal

Petzval     168 mm                    --
SD          141 mm               6.1 m (20 ft)
Apo         146 mm              11.5 m (38 ft)

            Close-focusing distance

            2-inch diagonal     SLR camera

Petzval     11.8 m (39 ft)      6.5 m (21 ft)
SD           3.5 m (11 ft)      3.0 m (10 ft)
Apo          5.4 m (18 ft)      4.4 m (14 ft)

* Distance of focal plane for infinity beyond racked-in focuser


William Optics ZenithStar 66-mm Refractors

            Petzval   SD      Apo

Optics      ** 1/2    ****    **** 1/2
Mechanics   ****      ****    ****
Overall     *** 1/2   ****    **** 1/2

*****       Sensibly perfect. No meaningful improvements possible.
****        Any shortcomings will go unnoticed in normal use.
***         Problems noticeable but do not seriously affect
**          Problems noticeable during normal use--performance
*           Problems so severe that the equipment is virtually

Ratings are intended to convey performance compared with equivalent
equipment and should not be used to predict the relative performance of
instruments having markedly different apertures or optical designs.
COPYRIGHT 2006 All rights reserved. This copyrighted material is duplicated by arrangement with Gale and may not be redistributed in any form without written permission from Sky & Telescope Media, LLC.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2006 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:di Cicco, Dennis
Publication:Sky & Telescope
Article Type:Product/service evaluation
Date:May 1, 2006
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