Orion OMC-140 Maksutov-Cassegrain Telescope
Introduction
The Maksutov-Cassegrain telescope is similar in design to the more familiar
Schmidt-Cassegrain telescope (SCT) except they use a deeply curved corrector
plate and often have an aluminised, or silvered, "spot" directly applied
to the corrector plate, rather than using a central hub with the secondary
mirror mounted in an adjustable mounting. There appears to be no valid reason
why a "Mak" should perform any better than a similar sized SCT, yet many
of them are reported to be superior to their SCT cousins. The central obstruction
is often quoted as being the reason why refractors produce higher contrast
images than an equivalent SCT. The central obstruction of the OMC-140 is
approximately 36% of the area of the corrector plate, so is no "better" in
that respect an a typical SCT.
The focal length of the OMC-140 is 2000 mm, with a corrector plate 140mm
in diameter, giving a focal ratio of f14. The back plate is threaded
to take standard Meade and Celestron accessories. Note the central baffle
tube is quite narrow, the longer variety of Barlow lenses foul the baffle
tube - even when used with the 2" adapter tube. The modern trend of shorter
(or "shorty") Barlow lenses should be OK in this respect.
The OMC-140 can be purchased in standard form with 1/4 wave optics or for
approximately 100 UK pounds more at a certified 1/6 wave. The optical tube
comes complete with a Vixen GP compatible dovetail plate, this plate has
several 1/4" Whitworth threaded holes (photographic tripod thread), 6 x 30
finder, 1 1/4" right angle adapter and 25mm Plossl eyepiece. A dew shield
and 2" adapter are available as options.
Operation
The OMC-140 was purchased second hand from a friend who had already found
a means of mounting it alongside his refractor. This mounting was achieved
with a pair of tube clamps from an old Vixen 6" refractor that had been drilled
and tapped to take three Allen headed bolts. On the inside of the tube clamps
a length of plastic electrical conduit cover had been formed into a ring
to protect the tube from damage by the Allen bolts. By bolting these tube
clamps to a standard Losmandy dovetail plate, it was easy to install the
OMC-140 side by side with my existing Meade 10" SCT on a Losmandy G-11 mount.
The side by side mounting plate is Losmandy part number DSBS. Balancing the
two telescopes was easy as the DSBS plate can be moved from side to side
and each telescope's dovetail plate can be moved forwards and backwards for
balance.
Having added the telescope to the existing mount, the first job was to
re-calibrate the Losmandy Gemini "goto" system. In the past, using the rather
loose fitting Meade SCT mirror, the alignment was prone to errors of up to
half a degree. The OMC-140 appears to have no noticeable mirror shift when
focussing, by using the OMC-140 for mount alignment, rather than the Meade,
seems to have produced a much better alignment.
Focussing on bright stars as part of the mount alignment, gave nice dark
backgrounds to the sky with crisp star images. The "in and out" off-focus
images appear very similar and symmetrical indicating good optics and colimation.
There are three plugged holes on the back plate to give access to colimation
screws should it be necessary.
The telescope was purchased for planetary viewing and needless to say Mars
is currently the favourite (August 2003). Despite the present low altitude
of Mars from the UK, a surprising amount of surface detail could be seen
without difficulty. This in part may be due to the relatively slow focal
ratio of the optics, certainly Mars is dazzling in the Meade 10" f6.3 SCT
and is too bright to see many surface details without filters.
Focussing a Meade or Celestron SCT can be troublesome due to an often large
amount of image shift when focussing, this is especially difficult when using
a CCD camera to take images of the planets. With the Meade 10" SCT the image
of a planet will often move right out of the field of view when focussing,
on the OMC-140 there is no discernible image shift and the focus knob turns
with a silky smoothness.
There are a couple of minor points to note, using one of the longer "traditional"
barlow lenses with the 1.25" adapter doesn't allow the barlow to fit inside
the baffle tube within the telescope. It almost fits with the 2" (optional)
adapter. Using a Televue x2.5 barlow wasn't a problem as the barlow is quite
short. Also, when using the telescope without a right angle adapter (with
the 1.25" adapter), there wasn't sufficient focus travel to allow some of
my eyepieces to focus. Again this isn't a problem with the longer 2" adapter.
I suppose the bottom line is the 2" adapter is needed for many
applications!
The previous owner of the OMC-140 had it mounted alongside an Astrophysics
6" refractor, it performed almost as well as the very expensive AP refractor.
Compared to my Meade 10" SCT it certainly outperforms it visually on Mars
and produces pin sharp star views.
The OMC-140 is available directly from Orion Optics (not to be confused with
the USA Orion) or from many dealers. The telescope seems to be produced to
order and may take a couple of months to be deliverd. The current cost of
the optical tube assembly is approximately 600 UK pounds (August 2003). The
Orion web site is at
www.orionoptics.co.uk
Since originally writing this review, the focus assembly became loose with
no obvious means of re-fitting it. After a short telephone call to Orion,
the method of re-fitting it was established. The rear cell of the telescope
is removed after taking out the three Allen headed bolts, and the two Pozidrive
bolts used to fix the finder scope. The rear cell cover was a very tight
fit and not easy to pull off. With the cover removed, re-fitting the "micrometer"
focus assembly took only a minute or two. The image, shown above right, was
taken prior to re-fitting the focus assembly.
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