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” apochromat for planets, but 4” apochromats
aren’t really portable, at least not in the way an 80mm apochromat 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, they did …
At
A Glance
|
Telescope |
Orion
Optics OMC140 |
|
Aperture |
140mm |
|
Focal
Length |
F14 |
|
Focal
Ratio |
2000mm |
|
Central
Obstruction (incl. holder/baffle) |
30% |
|
Length |
~460mm
(18”) |
|
Weight |
3.5Kg |
Data from Me.
Design and Build
Build quality on the little Mak’ is
generally very good, better than some of the rather agricultural mechanically
(if excellent optically) that Orion used to make.
The only demerits are the awful finder bracket supplied, on
which the screws jut out at crooked angles, and the dew shield, which is
functional but basic and made of rolled metal.
The OMC 140 is a conventional Maksutov
with silvered-spot-secondary, but aspherised to
deliver a flat-field.
Optics
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 smallish 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.
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.
One other thing to mention about this premium example of the
OMC140 is that it has Orion’s multi-layer mirror coatings that they call
‘Hilux’.
Tube
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.
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 OMC140 certainly fits the requirement for compactness. It
measures 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.
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.
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.
Accessories
Orion offer a soft case for the
OMC140. 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.
The OMC 140 is carry-on portable in its soft case.
In
Use – Daytime
Although it’s
probably a bit heavy for field use, the OMC140 delivers astoundingly good
high-power daytime views and would be an amazing tool for long-distance birding
(nesting raptors, for example).
You can tell
the quality of the daytime view by the result when it’s used as a 2000mm
telephoto (Pap heaven!), see (un-retouched) image below.
View with 50mm lens.
Same view with OMC140 – note extreme sharpness, high resolution
and contrast, right across the image.
In
Use – The Night Sky
General
Observing Notes
Once it’s up and running, the little Mak’
isn’t terribly impressive at first if you are used to apochromats. 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°. 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-apochromat view, you realise
it’s rather good; very good, actually.
There are a few other 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.
Cool
Down
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.
The Moon
Turn it to the Moon and the results are 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 apochromat, 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.
Planets
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” apochromat. 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 – those 1/10th PV
optics at work.
Planetary and Lunar performance is very high, once it’s
properly cooled.
Deep Sky
Deep sky objects are excellent for a mere 5.5” telescope and
the field is commendably flat, as promised.
For double stars, the fine optics and good baffling again
mean high performance. Rigel B is very easy to pick out of the glare. The
carefully-baffled OMC140 doesn’t suffer from the stray light issues I’ve noted
on some Maksutovs.
Overall, performance on deep sky is first rate, once you get
over the fact that it doesn’t give the “jewels on velvet” view of a refractor.
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 apochromat.