Orion OMC-140 User manual

Orion OMC-140 Maksutov-Cassegrain Telescope
Introduction
The Maksutov-Cassegrain telescope is similar in design to the more familiar Schmidt-
Cassegrain telescope (SCT) e cept 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 appro imately
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
appro imately 100 UK pounds more at a certified 1/6 wave. The optical tube comes
complete with a Vi en GP compatible dovetail plate, this plate has several 1/4" Whitworth
threaded holes (photographic tripod thread), 6 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 Vi en 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 e isting
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 e isting 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 focusing, by using the OMC-140 for mount
alignment, rather than the Meade, seems to have produced a much better alignment.
Focusing 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 collimation. There are three plugged holes on the
back plate to give access to collimation 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.
Focusing a Meade or Celestron SCT can be troublesome due to an often large amount of
image shift when focusing, 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 focusing, 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 2.5 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 e pensive 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 delivered. The current cost of the optical tube assembly is
appro imately 600 UK pounds (August 2003).
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 fi 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.
© 2005 Dave's Astronomy Magazine www.astromag.co.uk