Orion Optics OMC 140 Review (1/10th PV)

 

A few years ago, Maksutovs were all the rage. Everyone wanted one and Intes Micro couldn’t make enough of them. Even Astro Physics jumped on the bandwagon and in typical fashion produced the best and most sought after example ever made by anyone. The idea that got around is that Maksutovs give you the same compactness for their aperture as SCTs, but with a better level of optical quality and performance, especially for planets.

I am really what Ed Ting calls a “Refractor Guy”, but I realise that lens scopes have their limitations. One particularly huge limitation is a nasty little rule that basic physics seems to have foisted on us Refractor Guys: You need at least a 4 inch APO for planets, but 4” APOs aren’t really portable, at least not in the way an 80mm APO can be. So I reckoned a first class little Maksutov might fit the bill for a small scope I could take to places with better seeing for planetary oppositions (especially Mars). I scoured around and dithered because I knew I would need excellent optics for my plan to work, much better optics than the bog-stock run of Mak’s provide.

In the end I turned to those old acquaintances, Orion Optics in Crewe and asked if they could make a tenth wave version of their long-running and popular little 140mm F15 Maksutov, the OMC140; they could.

Design and Build

The OMC 140 is a classical Gregory Maksutov, like a Questar, with the secondary as a silvered spot on the inside of the corrector plate (rather than a Rumak which has a separate mirror in a cell, like an SCT). Gregorian Maksutovs tend to have a lot of off axis coma (i.e. stars look distorted near the edge of the field), but this can be fixed by aspherising one surface and indeed the OMC140 has apparently been designed this way by a famous optical expert. In other ways the specification - an F14 with a 30% central obstruction– looks conventional enough. A long baffle tube protrudes up through the main mirror to prevent stray light, but this doesn’t appear to add to the obstruction. You could fit a 2” visual back, but I am told the internal dimensions make for serious vignetting, so this is really a 1.25” only telescope. For my purposes, that’s absolutely fine and at this aperture fixing it would mean a larger central obstruction which is bad news for planetary contrast.

The OMC 140 is a conventional Maksutov with silvered-spot-secondary, but aspherised to deliver a flat-field.

If you want really good performance then optical quality matters more for obstructed optics than for refractors. As I have said elsewhere, I really like Orion’s approach of offering a variety of optical quality levels in the same telescope. Not only is this approach honest and transparent (in my experience of three high quality optics from them, if you pay for excellence, you get it), it allows you to choose the level you need and are willing to fund. Did I really need 1/10th wave PV? Probably not and I neither know nor care what the exact figure is or at what wavelength it was measured (I didn’t bother to pay for the test report). The point is that if you pay for the best then at least you should get a really decent set of optics and that is what matters.

The OMC 140 is carry-on portable in its bag.

The OMC140 certainly fits the requirement for compactness. It measure just 18 inches (46 cm) in length and 6.25 inches (16 cm) in diameter and weighs about 3.5 kg, so it will go on a Vixen Porta mount, or a small equatorial.

The bag is the same length at 20 inches as the case for a TV76 and should be fine as hand luggage on virtually any airline.  

As long as you mount it on a Vixen or Synta mount, you don’t need rings because a Vixen dovetail runs along the bottom of the tube, adding to the light weight and compactness of this telescope: it’s no use having a compact scope if you need to carry a big mount to put it on!

 

The OMC 140 is compact (and collimatable via three ports in the backplate). But it’s longer than a 5” SCT.

Build quality on the little Mak’ is generally very good. The tube itself is thick seamless aluminium (unlike some of their other scopes which use thin rolled steel), although I believe they now use carbon fibre. The tube itself is nicely finished in white and has a very neatly engineered back plate in satin black and a port for standard SCT visual backs. Two finder dovetails are provided. The only demerits in build quality terms are the awful finder bracket supplied, on which the screws jut out at crooked angles, and the dew shield, which is functional but agricultural and made of rolled metal.

Focusing is by moving mirror and the focus knob is from a micrometer, a nifty touch. It’s only single speed but very smooth and precise and free of image shift. Unusually for a Mak’, three openings in the back plate reveal recessed collimation hex screws.

In Use

I think I know why Mak’s are no longer popular: cool-down.  The combination of a large enclosed airmass between two lumps of glass (the front one very thick, unlike the thin corrector in an SCT) seems to make for a lot of problems in this area. Leave it open with the focuser up for less than an hour and it’s still pouring out warm air. So unless you allow the OMC 140 an hour to cool, tube currents spoil the view. In terms of size this is a grab-and-go ‘scope, but the time taken to reach operating temperature mean long waits unless you live in a freezer.

Once it’s up and running, the little Mak’ isn’t terribly impressive at first if you are used to APOs. The field of view is narrow because of the long focal length and you can’t really get low powers: a 32mm plossl gives 60x and just 0.79 degrees. What’s more, stars lack the brilliance they do in a good refractor. This feels at first like an 8” SCT, albeit a dim one; it reminds me of why I’m a refractor guy.

For this reason, I didn’t bother exploring the capabilities of the little Maksutov for a long time after buying it. In fact, once you get over the it’s-not-an-APO view, you realise it’s rather good; very good, actually. Deep sky objects are excellent for a mere 5.5” telescope and the field is commendably flat, as promised. But of course, this telescope is all about high powers.

When you do start to up the magnification on a night of good seeing, the little OMC comes into its own. Magnifications of 250x and above are easily obtainable, thanks to the long focal length, and the good optics take them very well. Even though the focuser is precise, a 1/10th inner knob would be useful at these powers. The Mak’ snaps to focus and at high powers a lot of detail is available, even on Mars at just 12” disk size. Make no mistake, the high optical quality shines through on this telescope.

On one recent night, I started off using the OMC140 to view Mars, before moving on to my 7” APO. The little Maksutov gave similar images, though at a smaller scale; mostly the same albedo detail was visible in both telescopes. This telescope will give views of Mars at least as good as a fine 4” refractor, so that all the major features are easily visible and not only at the closest oppositions. Planetary contrast is very good, surprisingly so for an obstructed telescope.

Turn it to the Moon and the results are equally impressive, with very high resolution for the aperture. No, the view doesn’t have that “wow” factor you can get with a wide-field high-power eyepiece in an APO, but actual resolution of fine detail is excellent. Fine rilles and craterlets jump out at you in a way they don’t with smaller apertures.

For double stars, the fine optics and good baffling again mean high performance; for example, Rigel B is very easy to pick out of the glare.

Overall, performance on the sky is first rate, once you get over the fact that it doesn’t give the “jewels on velvet” view of a refractor. What’s more, I discovered, almost by accident, that the OMC 140 makes a stunning 2000mm telephoto lens: a Paparazzo’s dream. Just screw on a 1.25” adapter and plug your DSLR in the back.

View with 50mm lens.

Same view with OMC140 – note extreme sharpness for this image scale.

There are a few niggles, though. Probably the biggest inconvenience after the slow cool-down is the visual back: it’s set so close to the back plate of the telescope that it’s hard to get a comfortable position at the eyepiece without banging your head. Worse, the short visual back means that the eyepiece sometimes gets in the way of the focuser knob.

 

Summary

Name me another telescope that is an airline carry-on, weighs less than 4 kilos and will show significant detail on Mars. No, I thought you couldn’t. Even if you can, add a new price under a thousand pounds and you’ll really struggle.

The OMC is not a refractor and it’s not as versatile as a refractor, but that said it will give you detail on Solar system objects that few carry-on portable telescopes come close to matching. If you think of this as a plain-looking 5.5” premium Questar OTA, it isn’t expensive either.

Maksutovs are out of fashion now and quality ones like this don’t sell used. That’s a shame because a scope like this has some real advantages for Planetary and Lunar use. Cool down is a bit slow as for any Mak’, but for critical planetary resolution it is unmatched in such a small package: no compact refractor I know of can touch it on Mars.

Highly recommended, as long as you accept that it cools slowly and is less versatile than an APO.