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.
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.
What I ended up with was a simple, but well made (and long!) Newtonian with a tiny (18% diameter) central obstruction that falls below the 20% limit generally thought to be noticeable. The mirror sits in a nicely made cell with big brass collimation knobs. A vixen 1.25” rack and pinion focuser is basic, but works perfectly. The secondary sits on a thin-vane spider and the tube internals are all nicely painted flat-black. Don’t expect Takahashi quality here – the tube is rolled aluminium with a big joint running down it – but it’s plenty good enough and light weight as well.
At first I 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, so I converted it to sit in Orion’s own all-aluminium Dobsonian mounting, which worked much better.
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.
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.
The star test is as near perfect as I have ever seen. 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, but the problem is the Dob’ mounting, which at those magnifications needs nudging every few seconds to keep the object in view.
I recall two particularly memorable viewing sessions with this telescope. 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.
The other session I remember was one morning, just before dawn and lasted rather longer than that brief epiphany on Mars. 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 are just incredible, like Lunar Orbiter photos with far more detail than you’ll find in an atlas.
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. I set this 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?”
Although it was optimised for planetary and lunar use at high powers, at medium 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. Which brings me to the fundamental flaw in this amazing telescope.
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.
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 big equatorial for 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!