Orion OMC-140 Manual

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.