William Optics ZS66 Petzval Review

If there is one design of telescope, apart perhaps from the Ritchey Chretien, which has created a buzz in recent years its the Petzval. Found in some of the best refractor astrographs from Takahashi, Pentax and TeleVue, youd be forgiven for thinking that its a high end only design. Certainly, the four element configuration allows great freedom in correcting all the aberrations of a refractor objective to leave the sort of big-chip covering flat field in a short focal length that sees imagers paying thousands for the privilege.

But in fact a budget Petzval was available for a while, costing hundreds rather than thousands of pounds the William Optics ZS66.

Design and Build

Given that the ZS66 has (wait for it) a 66mm aperture, you might imagine that its pretty small. So it is, but less so than you might expect. As the picture shows, its quite a bit larger than a TV60; closer to TV76 size.

ZS66 and TV60

and again with a TV76

Overall fit and finish is typically polished WO fare with a beautiful black-anodised CNC tube and a super-smooth dual speed Crayford focuser. The dew-shield slides nicely and the internals look excellent too, with one large ridged baffle rather than the traditional knife-edge ring-baffles.

It says ED, but the ED element is in the Petzval lens; this is not an APO.

One thing I really like with most WOs (and others sporting the Chinese Crayford focuser) is the way the focuser rotates: loosen that set-screw above the swan and you can change the eyepiece position without risking dropping your diagonal or changing focus. All focusers should have this feature.

As you can see, the focuser has a 1.25 visual back, but the thread is standard SCT, so a 2 visual back (or direct fit 2 diagonal) could easily be fitted. Combined with the 400mm focal length, this gives the potential for one of the widest true fields of view available - 6.6 degrees (just a bit less than the Tak FS60C which gives 7.4 degree at 355mm FL).

 

Dual speed focuser with SCT-fit visual back.

Lets look at the heart of this telescope the Petzval lens system in more detail.

Petzvals consist of two doublets: one at the front like usual and another at the back, often in the focuser drawtube. The trick is that the front doublet actually has a long focal length (in this case perhaps F10-12) so reducing chromatic aberration. Meanwhile the rear lens serves as a kind of reducer-flattener, taking the focal ratio down to F6 (400mm).

In a scope like the NP101 (or FSQ106) the front doublet is an ED APO lens, but in the WO Petzval and the original TeleVue Genesis its a regular long-focus achromat. Both claim ED glass/fluorite, but this refers to the corrector, not the objective. So the ZS66 (like the TV Genesis) is not really an APO, it is effectively a long focal length achromat. So the WO ZS66 is probably going to perform as, for want of a less oxymoronic term, a semi-APO.

The Petzval lens is in the end of the focuser drawtube.

 

When it comes to mounting the ZS66 its again typical small-Chinese refractor fare with a shoe that doubles up as Vixen-fit dovetail or a tripod mount with a standard -20 thread. So the ZS66 goes comfortably on a lightweight photo tripod:

It mounts fine on a small photo tripod.

In Use

The design and finish of the little ZS66 impressed me greatly packing quality and clever features. In use the news was less good.

The field is certainly wide and reasonably flat, but not as perfectly corrected as a premium Petzval, with some drop-off near the edges. So extended clusters and star fields are nice in the ZS66, but dont quite have that wow space-walk effect that you with a TeleVue NP101 (or even a Genesis).

The view of the Moon and planets was serviceable, but not outstanding and overall I found the TV60 more capable, despite having less aperture.

Chromatic aberration was, if anything, a little worse than I had expected with some dull purple fringing to the Moon and bright planets a similar level to the Genesis, but that scope is a 100mm F5, a format which places much more demands on the optics than a 66mm F6. Having said that, the purple is a muted shade and better controlled than in a short-tube achromat.

However, if the CA correction is better than a standard ST80, the sharpness was, I thought, a bit worse. A star test showed why. The ZS66 was out of collimation not badly so, but the diffraction pattern had that tell-tale skewing to one side. Unfortunately (just like other Petzvals it must be said) there is no adjustment possible.

Now this raises an interesting question. One of the objections to Petzvals has been collimation sensitivity, but Roland Christen has said, in a forum post, that the design is no more prone to miscollimation than any other. However, given that in this case the secondary lens sits in the (moving) focuser tube, its possible to see that problems are more likely to arise than for a fixed objective.

Summary

The ZS66 is a beautiful looking little refractor that has been well designed with excellent fit and finish, like most WOs. Sadly the optical performance fell well short of the expectation set by appearances: in use this scope performs little better than a good 60mm short tube achromat (like a Borg mini-60).

Worryingly, the ZS66 isnt the first WO that Ive come across with optical issues. It is interesting that Sky-Watcher (another Chinese brand) scopes are generally less shiny, but I have yet to find the view through one anything less than excellent.

Not really recommended, unless you star test before buying to ensure its a good un. Even so a conventional doublet might be better, if less exotic.