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 it’s the Petzval. Found in some of the
best refractor astrographs from Takahashi, Pentax and Tele Vue, you’d be
forgiven for thinking that it’s 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 it’s pretty
small. So it is, but less so than you might expect. As the picture shows, it’s
quite a bit larger than a TV60; closer to TV76 size.
ZS66
and TV60: both with dew-shields extended.
…
and again with a TV76
It
says ‘ED’, but the ED element is in the Petzval lens; this is not an APO.
Optics
Let’s
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 very slow focal ratio (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).
It
says ‘ED’ on the lens ring. But whilst in a scope like the NP-101 (or FSQ-106)
the front doublet is indeed an ED apochromatic lens, in the WO Petzval and the
original Tele Vue Genesis it’s a regular long-focus achromat. Both claim ED
glass/fluorite, but this refers to the corrector, not to 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.
Tube
Overall
fit and finish is typically polished WO fare with a beautiful black-anodised CNC
tube, like an early AP Traveler, that shows every print. The dew-shield slides
nicely and the internals look excellent too, with one large ridged baffle
rather than the traditional knife-edge ring-baffles.
Focuser
The
super-smooth Crayford focuser features a dual speed pinion and a scale for
imaging.
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 FS-60C which gives 7.4 degree at 355mm FL).
Dual
speed focuser with SCT-fit visual back.
Mounting
When
it comes to mounting the ZS66 it’s again typical small-Chinese refractor fare
with a shoe that doubles up as Vixen-fit dovetail or a tripod mount with a
standard ¼-20 thread on the base. So, the ZS66 goes comfortably on a
lightweight photo tripod or a small Vixen compatible mount.
The
ZS66 mounts fine on a small photo tripod and fluid head.
Accessories
The
ZS66 came in a nice soft case, a typical and appreciated feature of William
Optics scopes (the TV-60 does too, for a substantial extra sum).
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 don’t quite have that ‘wow’ space-walk
effect that you with a TeleVue NP-101 (or even a
Genesis).
The
view of the Moon and planets was serviceable, but not outstanding and overall I found the TV-60 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. But here the secondary lens
sits in the (moving) focuser tube, so it’s possible to see that problems are
more likely to arise than for a Petzval lens fixed in the main tube.
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 isn’t the first WO that I’ve 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.
The
WO ZS66 is really recommended, unless you star test before buying to ensure
it’s a good ‘un. Even so a conventional doublet might be better, if less
exotic.