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Takahashi Mewlon 300 Clone (by Orion Optics) Review

I had long wanted to try a big reflector: specifically, a Takahashi Mewlon 300 (ever since I had seen one at APM telescopes on a colossal EM500 mount). What a magnificent beast! And the word on the forums was that the big Mewlon was the ultimate ‘planet killer’.

Unfortunately, there was a problem or three with my dreams of a Mewlon 300. First was the cost – truly astronomical. Then there was its vast size and weight (back then I had no dome). Third was that I had no mount near big enough.

I had been mightily impressed with the planetary and lunar views through an 8” 1/10th wave Newtonian from Orion Optics – an English manufacturer of high quality mirrors. Their little OMC140 Mak’ had been optically excellent as well.

So I approached Orion to see if they could build me a lighter, cheaper Mewlon 300 clone. The upshot was that Barry Pemberton at Orion Optics reckoned he could make a near-identical scope at half the weight and a quarter the cost, 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 it took a while for them to build it … really quite a while.

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 Mewlon 300 clone: big and white and shiny and sitting on a Losmandy G11 in Orion’s cluttered factory. Excited? Oh yes.

I was generally very pleased with the result. The workmanship looked good.

Thick-vane spider and 37% secondary baffle: not ideal for a planetary reflector.

Optics – Not exactly what I’d asked for

The Takahashi Mewlon series are a type of Cassegrain, a pure reflector, called a Dall-Kirkham. 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 having steep curves at F3 or F4).

By comparison, a Dall-Kirkham 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. But for an equatorially-mounted planetary scope that hardly matters, which is why Takahashi chose the DK for their Mewlons.

The original Mewlons were designed as Lunar and planetary scopes, so they have a central obstruction of ~30%, smaller than a typical SCT’s at ~35%. This is to maximise contrast. Meanwhile, I had specified an even smaller secondary - a maximum of 25%. Having a small secondary might make the scope less ideal for deep sky, but this was a dedicated planetary scope for me after all.

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 wanted the scope for!

A more welcome innovation was the primary mirror - conical to save weight, centre-supported and Zygoed at a spectacular 1/8th wave, the latter a remarkable achievement for a 12” F3.

Tube

The tube was Orion’s usual seamed sheet-metal, inherited from their Newtonians (they mainly seem to use carbon fibre now), but it had kept the weight to under 15kg as promised.

At the front end, Orion had fabricated a CNC spider with thick integrated vanes and mirror support for collimation stability. Thick vanes are good for imaging because they mean shorter spikes, but less good for planets because they can reduce contrast – more evidence this wasn’t the scope I’d asked for at all!

At the other end, their CNC mill had hewn out a nifty primary cell with opening shutters for fast cool-down, an excellent feature.

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

TS Crayford focuser and mandatory extension tube.

Focuser

The focuser was another (smaller) niggle. 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.

The TS focuser 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.

Mounting

Trying to get the big DK stably mounted is a salutary tale – one worth relating.

As I said, this was intended as a big-league planetary scope that would mount 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 in the end I admitted defeat, 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.

Even after the upgrade, the DK’s sheer size caused me trouble, making it tricky to manoeuvre in my little Pod observatory. What’s more, mounted via a Losmandy dovetail, vibes are a bigger problem than with my big refractor which bolts to a fixed AP plate.

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

The Big DK overloaded my poor EM200.

Accessories

The scope came with rings – again typical of their Newtonian rings at that time - that were a bit thin and flexible and not up to the fine finish of some of the CNC’d parts. I eventually had some 3rd party rings made that worked much better.

In Use – The Night Sky

General Observing Notes

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!).

The Moon

On very, very rare (around here, anyway) nights, you could see the potential. It may not have the crispness of an APO, but showed loads of Lunar detail using just 20mm and 15mm Plossls – no fancy eyepieces required.

At its very best on The Moon (and it certainly would be, with no seeing problems), the detail available can be 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.

Often, though, the Moon was just a haze of vague crater-shapes.

Planets

The long focal length gives big bright, planetary views. Even on a good night, though, you get more light scatter and annoying spider-spikes than in a refractor. Most of the time, though, planets are just big mushy bright coloured balls, like Christmas decorations. Why? After all, I bought this scope for planets.

Is poor optical quality the 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.

Theory suggests that the large central obstruction and thick-vane spider are a factor, worsening sensitivity to poor seeing; certainly 37% is more than anyone would recommend for a planetary reflector.

Would the big D.K. be better with a smaller obstruction? I believe so – 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.

A theoretical side-by-side comparison between the DK and my 7” apochromat (not possible – I don’t have a second large mount to hand), would have the mirror-scope going deeper but delivering inferior views in all other respects – just like the comparison between a C8 and a 4” apochromat I did years ago.

In fact, I could probably have had similar views from a big SCT with a more convenient focuser (and lower outlay).

Summary

The big DK is both good and bad. On one hand it cools down fast, holds collimation well and very occasionally delivered the best (and my friend, 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 planetary 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.

Update

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 really put me off reflectors.

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.

Interestingly, Takahashi have pivoted the two larger Mewlons – the 250 and 300 – into astrographs by fitting them with triplet correctors. Planetary scopes no longer. Orion’s production DKs are catadioptric astrographs too.

Orion Optics’s take on a big Dall-Kirkham is not recommended, especially not for locations with mediocre seeing. Whether the real thing – a Takahashi Mewlon 300 – would have similar limitations I can’t say, but experience with a smaller Mewlon suggests it might.