Orion Optics 8” 1/10th Wave PV Planetary Dobsonian
Review
APOs are great for planetary and lunar viewing; they offer superb,
pin sharp images with none of the softness you often get with SCTs. Trouble is
they are expensive and usually of limited aperture.
If you have been dreaming of the day when the Chinese are able to
make a portable 8 inch APO for under a thousand quid, keep dreaming! Seriously,
this is not going to happen any time soon because APO-quality 8” ED
blanks from the major suppliers (Ohara and Schott) don’t exist and if
they did would be hideously expensive. Even if the glass was available, big APO
lenses will always need a lot of highly skilled hand finishing along with very
careful assembly in a sophisticated cell. The only major suppliers of big APOs today
– TEC and APM/LZOS – charge the price of a new BMW for an 8”
and if that situation changes it will be because they start charging even more!
Then there’s always the Takahashi FCT-200 with a list price of $125,000
(but at least it includes the mount).
On the other hand, if you dream of getting similar performance to an 8” APO for under
a thousand pounds, then dream no longer. Long focus Newtonians have always been
simple to make well and with the advent of interferometer testing they can be
made to an exceptional level of optical quality. Add in the possibility of a
very small central obstruction, easy collimation and just two light scattering
surfaces and a long-focus Newtonian has the potential to perform closer to a
big APO than almost any other design. Well anyway, that’s what I
reckoned. So back before I could afford a big APO, I sought out an 8” F8
Newtonian optimised for planetary viewing.
At A Glance
|
Telescope |
Orion
Optics 1/10th PV 200mm F8 Newtonian |
|
Aperture |
200mm |
|
Focal
Length |
1600mm |
|
Focal
Ratio |
F8 |
|
Central
Obstruction (incl. holder/baffle) |
18% |
|
Length |
~1500mm |
|
Weight |
~7Kg |
Data from Me.
Design and Build
Orion Optics, based in Crewe (England – don’t confuse
with Orion USA), always had a reputation for Newtonians with fine optics, but
“agricultural” mechanicals. Then a few years ago, so the story
goes, they took some of their mirrors to the National Physical Laboratory for
testing and were appalled to find them rather less perfect than their simple ronchi
tests had suggested. So Orion bought a Zygo interferometer (a tool for
precisely testing optical surfaces) and today make a large range of telescopes
with various available levels of optical quality, from basic ¼ wave PV
up to 1/10th wave as a special order. This is a great system –
you only pay for the optical quality you need for your application.
I asked Orion to build me an 8” F8 Newtonian before they
were offering a standard scope with this spec’. It has the highest,
zygo-tested optical quality they can manage. Why did I specify such a high
level of optical perfection? Because I reckoned that would virtually guarantee
a good result, regardless.
The F8 200mm
Newtonian has a tiny (18% diameter) secondary obstruction for APO-like
contrast.
Mirror-cell
with handy collimation knobs.
Optics
What I ended up with was a simple, but well made (and long!) 200mm
F8 Newtonian with a tiny (18% diameter) central obstruction that falls below
the 20% limit generally thought to be noticeable.
The Pyrex (I think I had the option to specify Zerodur, but it was
very expensive) mirror sits in a nicely made cell with big brass collimation
knobs. The secondary is supported by a thin-vane spider.
Orion’s premium mirrors like this one come with their
‘Hilux’ multi-layer coatings.
Ordering the mirror, I could have paid for a Zygo test report, but
I was happy to trust Orion that it was up to the 1/10th PV
spec’ I’d asked for.
Tube
The tube is traditional Newtonian fare. Don’t expect
Takahashi quality here – the tube is rolled steel with a big joint
running down it – but it’s plenty good enough and quite light
weight as well. The tube internals are all nicely painted flat-black
Mounting
I originally intended to mount the OTA on my Vixen GP (stop
laughing), as it only weighed 7kg. Unfortunately, the length of the tube meant
vibrations that never stopped and I thought the poor little GP would break, literally.
So I converted it to sit in one of Orion’s own all-aluminium
Dobsonian mountings, which worked better. However, such a long OTA in a
Dobsonian mount with smallish bearings, means you need to get the balance
right. Also, heavy eyepieces cause ‘sag’ – the scope
gradually sinks groundwards.
For equatorial mounting, you would need at least an EQ6-sized
mount for this OTA and the OTA length would require a very short tripod and
probably rotating tube rings to keep the eyepiece position sensible.
200mm F8
Newtonian weighs just 7 Kg, but is very long and unwieldy.
Close-up of
Orion’s Dob’ bearings.
Focuser
The focuser is a vixen 1.25” rack and pinion focuser. It is
basic, but works perfectly. Nowadays Orion offer a range of CNC focusers with
the usual refinements such as fine focus.
In Use – The Night Sky
General Observing Notes
This is an amazing telescope. It takes high magnification like
nothing else I’ve used and has provided the sharpest, most detailed lunar
and planetary views of any scope I’ve owned. It really is
“refractor like” and completely outclassed my Takahashi FS128
(itself a superb instrument) and in many ways equals my more recent 7 inch TMB
APO.
In terms of magnification, you can push this all the way to 60x
per inch and beyond with a 3mm eyepiece, like a Nagler or Radian. On nights of
good seeing the resolution available is then just spectacular
Overall, the Dob mounting works well at low to moderate powers and
is compact and well made, but the eyepiece is still often in an uncomfortable
position between sitting and standing and objects disappear so fast from the
field of view at high magnification (even using Naglers), that you are forever
nudging the tube. What’s more, the see-saw effect of the long tube makes
balancing between different weight eyepieces a pain as well: often, the tube
swings up when you swap eyepieces and you lose your target.
It would be hard to recommend a Dobsonian like this for high
powers, because it is so inconvenient to use. Yet high powers are just the
thing this telescope was built for and excels at.
Dob’
fans will shoot me for saying so, but in my view Dobsonians are far from ideal
for high powers and the very long tube on this one makes things worse. An
equatorial platform would improve things … a bit.
Cool Down
Cool down, if you’re used to SCTs and (even
worse) Maksutovs, is quite reasonable – less than an hour from a warm
house.
Star Test
The star test is as near perfect as I have seen.
The Moon
I remember one morning, just before dawn, an extraordinary view of
the Moon. A quick peek with a four inch refractor had confirmed remarkably good
seeing. The moon was well into last quarter and the 8” Dob’
revealed staggering detail.
I recall exploring inside
the crater Kepler; I spent ages looking for craterlets on the slumped wall of
Gassendi A. Overall, on steady nights, views of the Moon in this thing were
just incredible, like Lunar Orbiter photos with far more detail than
you’ll find in an atlas.
Mars
I recall two particularly memorable viewing sessions with this
telescope on Mars. The first was during the Mars opposition of 2005. I had it
set up with a 3.5mm Nagler giving 457x and was going through the usual routine
of: find Mars with low power, then swap to the Nagler, view for a few seconds,
nudge the scope, lose Mars, swap back to the low power and so on ...
Then on one cycle, for just a few magical seconds, the seeing
froze and Mars looked like you see it in those Hubble photos, with masses of
detail, more than I have ever seen before or since. Then it drifted out of view
and by the time I had it back, the seeing was back to the usual mediocre.
Saturn
I had the OO Dob’ set up alongside a friend’s 8”
Meade one night and compared them on Saturn. The SCT was collimated and cooled
and not a bad example, but my mate just kept asking “Why is the view in
the dob’ so much better?”
Make no mistake, performance here is in a different league from an
8 inch SCT and more comparable to 6 inch+ premium APOs costing ten times the
money.
Deep Sky
Although it was optimised for planetary and lunar use at high
powers, this scope offers lovely views of tight clusters (particularly
globulars) and small DSOs like M57, but of course you don’t need the 1/10th
wave optics for that.
Summary
So this is “The Answer”, right? The “cheap as an
ETX but performs like a big APO” that we all dream of on drizzly winter
nights? Well not really ... You see the compromise is convenience.
At over five feet long with the eyepiece on the side at the top
(it’s a Newtonian, remember), mounting it on a Vixen GP was hopeless,
laughable: terrible vibrations and impossible eyepiece positions. You would
need a massive equatorial to handle this telescope and you’d need
rotating rings to keep the eyepiece position sensible.
The Dob’ mount works better, but at high powers it mainly
offers mucho frustration interspersed with occasional awesome views. All these
drawbacks are inherent to long-focus Newtonians, but the high magnifications on
offer by this instrument enhance the frustration factor. If you go this route,
you really need an equatorial platform to exploit the performance and that is
going to temper the price advantage a bit.
All-in-all, a fantastic planetary performance bargain and very
tempting, but so inconvenient in pure Dob’ form you might not use it that
much unless you get a tracking platform. The shorter F6 would probably be a
better compromise for most as a simple Dobsonian.
Still, if you
want to stand a chance of glimpsing Nix Olympica with your own eyes and
you’re on a budget, this must be an option: larger scopes won’t do
the job under most seeing conditions and smaller or poorer quality ones
won’t resolve sufficient detail.
Alternatively, keep including a cheap, big APO on your list to
Santa.
Very highly recommended for planetary and Lunar observers, but with reservations!