Orion Optics 12” Dall Kirkham Review

The ‘Big DK’ with its CNC’d thick-vane spider and large secondary baffle.

 

This was my third and final custom-build adventure with Orion... I had long wanted to try a big reflector, having been mightily impressed with the views through their 8” 1/10th wave Newtonian. I started off by discussing a premium, long-focus 12” Newtonian, but soon realised I had no mount for it (or location for that matter – it would hardly be portable).

So I broached the possibility of a big Cassegrain. I wanted a classical Cassegrain, but Barry at Orion persuaded me that the optics are just too expensive to make and that for my needs a Dall Kirkham would be better. Now if you’ve never heard of a Dall-Kirkham, you obviously aren’t aware of Takahashi’s Mewlon range, which are all DKs.

I won’t bore you with too many details, but it all comes down to curves. In a Classical Cassegrain, the primary is a parabola, the secondary a hyperbola: both quite tricky to make well (the primary being F3 or F4). By comparison, a DK has a spherical primary and an elliptical secondary, both easier to make. The catch? Both types are very sharp on axis, but the DK suffers from much worse off-axis coma. For an equatorially-mounted planetary scope that hardly matters.

Now a 12” TakMewlon had long been a dream scope, but was (and is) very expensive and heavy. Barry reckoned he could make a near-identical scope at half the cost and weight, one that would fit on the mount I had at that time – a Tak’ EM200. I ordered on the basis of a basic optical design to my specs and then moved abroad and promptly forgot about it ... probably a good thing, given how long it took them to build it.

Design and Build

Over two years later, back in England, I got a surprise call from Orion. So I drove down the motorway to Crewe one lunchtime and there was my new DK: big and white and shiny and sitting on a Losmandy G11 in Orion’s cluttered factory.

Vented rear cell – a clever innovation that means remarkably quick cool-down for a big Cassegrain.

I was generally very pleased with the result. The workmanship looked good. Orion had made a CNC spider with integrated vanes and mirror support for collimation stability. At the other end, their CNC mill had hewn out a nifty primary cell with opening shutters for fast cool-down.  Another innovation was the mirror itself - conical to save weight, centre-supported and zygoed at a spectacular 1/8th wave, the latter a remarkable achievement for a 12” F3.

True, the tube was their usual thin, seamed sheet-metal, but it had kept the weight to under 15kg as promised. True also that the rings were a bit industrial and not up to the fine finish of some of the CNC’d parts, but overall build quality seemed excellent (though not, of course, like a real Mewlon).

Not what I’d asked for

One small niggle from the start was the focuser. I had originally asked for a Feathertouch, but had been persuaded to go for Orion’s own (then new) CNC Crayford. Unfortunately Orion’s focuser was still under development, so I they had fitted a fast-food Chinese Crayford from Teleskop Service instead. This would have been fine, but in order to get the tube length down and provide lots of back-focus, the scope had been designed to use an extension tube for most eyepieces (with a 3.6m focal length, long FL eyepieces are the only ones that are useful).  Trouble is the extension places more leverage and weight on the focuser and it struggles, often racking out of its own accord.

TS focuser with mandatory extension tube.

A much bigger niggle is the central obstruction. I had specified a small secondary - a maximum of 25% - to ensure good contrast for planetary use. In the event, the secondary was indeed 25%, but behind it was a conical baffle extending out to 37% (bigger than most SCTs’!) Ouch! Unfortunately I didn’t notice this for some time after I had taken delivery of the big DK; Barry from Orion explained that it was to make the scope good for full-field astrophotography, which it does, but that’s not what I’d asked for!

Mount Problems

It’s a salutary tale – one worth relating. As I said, one of the aims of this scope was to allow me to mount a big-league planetary scope on my much-loved EM200. The EM200 is conservatively rated at 18kg, which given the 12kg weight of the Dall Kirkham should have been fine ... Except that add tube rings, a diagonal, eyepiece, dovetail-plate, finder (and at 3.6m F.L., you need a finder!) and it pushes 20kg, making the poor EM200 groan. So this scope was the main reason why I sold the EM200 and had to upgrade. But upgrade to what? A problem with this hobby is the mere handful of mounts that take 20kg. Consequently, the DK eventually led me to buy an AP1200, a major expense I hadn’t really wanted. The moral of the tale? Be realistic when it comes to what you can expect your mount to hold.

It’s a big scope!

In Use

Televue’s NP127 is all about the view, ahhhh that view... diamond point stars, sharp and detailed planets, nebulae full of contrast and subtle shading. And in a way this scope is about the view as well, because it’s just not like that at all.

One basic problem is the field of view. The lowest power you can get is 65x with a 55mm Plossl and trying to photograph the Orion Nebula just yields the central section. Field curvature and off-axis coma isn’t the problem I’d been led to expect, though, and for smaller DSO’s it makes a good astrograph (just a pity that’s definitely not what I bought it for!).

Great on The Moon

On very, very rare (around here, anyway) nights, you can see the potential. It may not have the crispness of the APO, but the long focal length gives big bright detailed planetary views and loads of Lunar detail using just 20mm and 15mm Plossls – no fancy eyepieces required. Even on a good night, though, you get more light scatter and annoying spider-spikes than in a refractor, but that affects planets, not The Moon.

At its best on The Moon (and it certainly would be, with no atmosphere to screw things up!), the detail available is staggering. I recall a night when I was able to explore the Hadley Rille region at very high power in a way I hadn’t before or since, with the Rille looking more like Schroter’s valley than the usual hairline you have to strain to pick out.

Otherwise mostly disappointing

Most of the time, though, planets are just big mushy bright coloured balls, like Christmas decorations; the Moon is just a haze of vague crater-shapes. Why?

Theory suggests that much of the problem has nothing to do with the scope at all, but is down to the size of the convecting air cells which are the cause of bad seeing. A small telescope (say under 8”) sees through a single cell making the image wobble but not break up, whilst bigger ones mix light coming through several cells into a mush. Certainly that agrees with my experience. On a night when an APO delivers a rippling, but usable view, the big DK just delivers a mess. Theory suggests that larger central obstructions are also a factor here, worsening sensitivity to poor seeing. Would the big D.K. be better with a smaller obstruction? I would say that, perhaps only marginally, it would – which adds to my frustration. After all here was a (far from cheap) scope I’d had custom built and waited years to get ... But what I ended up with was something optimised as an astrograph when I’d asked for a specialised planetary scope.

‘Big’ Problems

Is optical quality a problem? Emphatically not: the star test is superb and focus quite snappy given the long focal length. Is it cool-down? No, the vented back takes good care of that. Collimation? Again no, the stiff spider and general ruggdness means collimation has held up perfectly. Instead, I suspect, that most of the difficulties stem from the big aperture and obstruction. Then again, the thick CNC spider vanes are a good innovation for an astrograph, shortening diffraction spikes and improving collimation stability, but probably robbing contrast for the planets.

Were I to do a similar side-by-side comparison between the DK and the 7” TMB (not possible – I don’t have a second large mount to hand), the result would likely be just the same as the comparison I did years ago between a 4” TMB and a C8, with the mirror-scope going deeper but delivering inferior views in all other respects. I could probably have had similar views from a big SCT with a more convenient focuser (and lower outlay).

Size? Well in theory the Dall Kirkham should fit into my POD a little more easily than the 7” refractor; in practice the sheer bulk of this big scope make it just as tricky to manoeuvre. What’s more, mounted via a Losmandy dovetail, vibes are a bigger problem than with the refractor which bolts to a fixed AP plate.

It just clears the POD ... just, but careful manual slewing is the only way.

Summary

The big DK is both good and bad. On one hand it cools down fast, holds collimation well and has delivered the best (and my friend Ian, a real Lunar buff, agrees) high-power views of Luna ever. For its size it is light weight compared to the “real thing” (a Mewlon 300) and is of mostly high optical and mechanical quality.

On the other hand, some of the build quality (mainly the tube itself) is not the best and it deserves a better focuser (like the Feathertouch I originally asked for).

The real problem, though, has little to do with Orion’s workmanship. The problem is that Orion built a DK astrograph, when I had wanted a planetary scope. Just as theory predicts, large Cassegrains with big obstructions don’t offer great views unless the seeing is near perfect and where I live that’s very rare. Perhaps the most telling thing is that when I haven’t used it for a while, I mount the 12” up in my observatory thinking I must just have had a run of bad nights, thinking surely I don’t need my expensive 7” APO when I’ve got a 12” DK with 1/8th wave optics sitting around gathering dust. Then, a few weeks later, the 7” APO goes back on and stays there for the rest of the season, whilst the 12” DK goes back to the floor of my study.

Addendum

In the end I sold it ... as did the guy I sold it too ... and the guy he sold it to. I believe it has now gone to the U.S., where steadier skies may well let it deliver to its potential. Would the Big DK have been better if they’d built the specialist planetary scope I’d asked for, rather than the kind of general-purpose astrograph they specialise in? I’ll never know. Unfortunately, rightly or wrongly, it has finally put me off reflectors for good.

In a way it all comes down to the very reasons I set up this website. On paper this scope looked like a winner. If I lived in New Mexico this could probably have been my only scope; all those tiddler APOs could have gone on Astromart. Trouble is, I don’t live in New Mexico and in rainy Northern Europe where I do live the crummy seeing often means big reflectors with big obstructions deliver very mediocre views.

Not really recommended.