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Home > Reviews > Telescopes > Cats > ORION 180mm MAKSUTOV-CASSEGRAINIAN

ORION 180mm MAKSUTOV-CASSEGRAINIAN
By Lawrence Carlino - 3/20/2008

By Lawrence Carlino


Dedicated lunar and planetary observers are a special breed of amateur astronomer. They demand high-quality optics capable of yielding exceptional image fidelity, superb contrast, and unrivaled sharpness. Unfortunately, these strict criteria are difficult to meet and usually result in a considerable expenditure of cash.

The ubiquitous Schmidt-Cassegrainian scopes have their following, though the design has its compromises for optimum lunar and planetary observation as it normally sports a fairly large, contrast-robbing secondary obstruction.

The Maksutov-Cassegrainian, however, enjoys an established reputation as the best of the short-tube catadioptric designs. From the introduction of the superb 3.5” Questar decades ago to the contemporary offerings from the US, Russia, and China, the prospective user has a wide range of choice running the gamut from petite 90mm instruments to giants in the 300mm (or larger) class.

Personally, I’ve owned almost a dozen Mak-Cass scopes over the years and have found their reputation to be justified in most cases. These instruments have ranged from the 90mm Celestron and Meade ETX through Intes and Intes-Micro 5 and 6-inchers to the Meade (and now Orion) 7-inch scopes.

As with any category of telescope – size matters, and I sold the smaller instruments in search of the catadioptric “holy grail”: a reasonably-priced MCT with quality optics, light weight, and sufficient aperture for high-resolution planetary observing. Could the new Orion 180mm possibly be the answer?


PHYSICAL PROPERTIES

The Orion 180mm (7.1”) Mak-Cass is a Chinese Synta product and the big brother of the company’s well-regarded 90, 102, 127, and 150mm MCT’s. In basic design, it’s identical to the Canadian-branded Skywatcher 180mm Mak, but with different tube color and accessory options. Its overall appearance could be described as somewhat underwhelming, the 20-inch long dark metallic gray tube looking a bit like a slightly elongated and smaller version of an “ash can” depth charge from a World War II destroyer. Spartan, but displaying reasonably good fit and finish, the optical tube weighs in at 15.4 pounds and utilizes a 1.25-inch visual back, moving-mirror focusing, and an Orion dovetail for mounting a red-dot or optical finderscope. A CG-style dovetail rail runs a good percentage of the tube length and allows easy balancing and attachment to a variety of standard mounts. Recessed collimation screws are provided in the telescope’s rear cell should adjustment be needed. The scope is offered as an OTA only or with Orion’s SkyView Pro or Sirius GoTo or non-compturized equatorial mounts. I opted for the OTA alone and mounted the scope on a Celestron CG-5 for optical and mechanical evaluation.

The combination proved to be quite steady, yet still light enough to permit a fully assembled, single-trip foray to the observing site. With a dampening time of less than 2 seconds, the mount allowed high-power observation without undue vibration.


THE OPTICS

Removing the rather loose-fitting plastic dust cap, I found the huge meniscus at the scope’s front end to be an impressive sight: absolutely uniform coating and a smaller-than-expected secondary obstruction. The interior of the tube displayed a very dark, non-reflective flat black finish with little evidence of dust, metal shavings, or contamination. The main light baffle, as typical with most CATs, protrudes into the tube’s interior and does a good job of suppressing extraneous light. As this is a Gregory-Maksutov design with the secondary mirror taking the form of an aluminized spot on the corrector lens, a truncated conical baffle is affixed (glued, undoubtedly) to the corrector. Unfortunately, this addition to the scope has the negative effect of growing the fine 23 percent secondary obstruction to something on the order of 29 percent. That’s still comfortably smaller than that found in the typical Schmidt-Cass or short f/ratio Mak-Cass, but it is one design feature the telescope could probably do without.

With a focal length of 2700mm and an f/ratio of 15, this scope is optimized for high magnification viewing, any aspiration for wide-field vistas being quickly eliminated by the narrow passage (only 24 mm wide) through the back of the optical tube. A thread-on 1.25” visual back precludes the use of 2-inch accessories without investing in aftermarket adapters. A partial solution can be found, however, in adding the inexpensive SCT thread adapter ring available from ScopeStuff. This accessory screws onto the non-standard original threads of the OTA and converts them to accommodate the ubiquitous Celestron/Meade SCT visual back. This allows the use of a 2-inch star diagonal and an f/6.3 focal reducer that shortens the scope’s efl to about 1700mm. Observations using the focal reducer did show a gain in maximum available field-of-view, but at the expense of some vignetting.

In conducting tests to determine the widest field-of-view available, I found that a combination of a Celestron f/6.3 focal reducer and 26mm Plossl eyepiece maxed out at about a .8 degree actual f.o.v. (65x) with negligible light loss at the extremities. A 30mm eyepiece showed significant vignetting, and a 32mm Plossl was even worse. Though I didn’t try any 2-inch items in the optical train, it’s obvious that they would be unable to do better, the narrow “mouth” of the Orion’s back plate limiting the overall size of the light cone.

Throughout these tests, it became apparent that the telescope showed a slight image shift as the focusing knob was run its full travel. Though motion was quite smooth and linear, a migration amounting to about 7 or 8 arc-seconds was apparent. This small wandering became mildly annoying only at powers above 250x or so – not really a major factor at lower magnifications, and better than that of most mid-aperture SCT’s.


OBSERVATIONS

Limited field-of-view and imperfect focuser aside, the Orion 180 Mak immediately impressed me with its high-quality optics. Classic out-of-focus star tests revealed a well-corrected system with nicely defined Fresnel rings, a smooth optical figure, and no hint of spherical aberration or astigmatism. Collimation of the scope was perfect – a rare treat.

For comparative purposes, I had access to several other telescopes: These included 6-inch and 8-inch Celestron SCT’s, a superb Takahashi FS-128 fluorite refractor, a 130mm APM f/6 triplet APO, and (the King) a Tak FS-152 fluorite doublet. Over a period of some three months, I was able to get a good idea of their relative performance. I could also draw upon my recent experience and detailed performance notes of the Meade LX-50 Mak-Cass and Russian-made 5, 6, and 7-inch Maks.


THE PLANETS

Mars, with its ochre disk shrinking to 12 arc-seconds, made for a small but demanding target. Under relatively steady skies on two frigid late-January evenings, the Orion Mak showed its prowess by serving up a sharp, well-detailed image of the Red Planet. At 300x, using a 9mm TeleVue Type 6 Nagler, the scope easily defined the shape of Syrtis Major, Sinus Meridiani, Sinus Sabaeus, and the bright Hellas Basin. Limb brightenings were visible as well as the north polar shroud. Smaller markings such as Nodus Alcyonius and the Hyblaeus Extension were also glimpsed in moments of steady seeing. The sharp cut-off of the Martian disc with the surrounding sky background revealed almost no “spillover,” something that can be an annoyance with less-than-excellent optics. Detail was slightly more difficult to discern with the APM 130, f/6 APO at comparable magnification as excess red softened the view. The Takahashi FS-128, however, displayed better contrast than the Orion Mak, though it showed no additional detail.

Saturn, glorious in almost any decent telescope, provided quite a treat in the Mak 180. With the rings tilted only 8 degrees, the planet nevertheless generated a “wow” with a sharply defined image that captured the delicate colors and textures of the ringed world. At a paltry 90x, using a 30mm Celestron Ultima eyepiece, the Cassini Division was visible, and the South Equatorial Belt complex showed its characteristic caramel hue. With the power bumped up to 203x (TeleVue 13mm Plossl), additional detail could be glimpsed in the southern polar regions, and the “C” or Crepe Ring was fairly easy to pick up. In comparison, both of the 5 and 6-inch Takahashi’s displayed slightly better contrast and sharpness, as one might expect given their superb optics and pedigree – but the difference was not huge.

Both the Celestron 6 and 8-inchers also delivered fine views, but with a touch less sharpness and contrast than that of the Orion Maksutov. The larger aperture and excellent light throughput of the C 8, however, did show Saturn’s inner satellites more easily, and allowed higher magnifications without dimming the image excessively.

Overall, a comparison of the Orion MCT with a host of past and current scopes showed that only 2 of similar aperture yielded a significantly better and more detailed image: the Tak FS-152 and a 7-inch Intes-Micro Maksutov-Newtonian that I owned a couple of years ago. Performance, for all practical purposes, was almost identical to that of the (now defunct) Meade LX-50 Mak-Cass.


THE MOON

With 180mm of aperture and its long f/ratio, the Orion 180 delivered some outstanding images of the lunar surface. Observing our celestial neighbor in most phases from 2-day-old crescent through waning gibbous, I found that the scope produced views of stark contrast, high resolution, and absolutely no spurious color. At 90x, using a Celestron 30mm Ultima eyepiece, the first quarter demi-globe displayed a wealth of sharp detail including the glorious sight of the just-emerging Plato, Alpine Valley, and northern mare sprinkled with minute detail. Earthshine could be detected on the moon’s unilluminated half, but stray light was visible with the lunar disk offset in the field-of-view. Baffling for stray light might be termed adequate, but not exceptional. A few days later, the waxing gibbous moon and fairly steady atmosphere combined to create some very impressive views. The delicate Rima Birt near the Straight Wall stood out nicely for its full extent at 260x (TeleVue 10.4mm Plossl), and the intricate chains of tiny craterlets near Copernicus were cleanly defined. This is where the telescope itself seemed to “disappear,” leaving the impression of observation from an orbiting spacecraft.

Nevertheless, the Takahashi 5 and 6-inchers provided an even better experience with their dead-sharp images and superior contrast. The effect is difficult to describe objectively, but the complete lack of central obstruction in a high-quality APO refractor always gives it a bit of an edge; yet it is amazing that the Orion Mak was able to offer some strong competition.


DOUBLE STARS

In an arena where many Schmidt-Cassegrainian scopes are less than optimal, the Orion MCT really shined. Good double star resolution requires a tight stellar Airy disc with as little light possible spilling into the diffraction rings or beyond. While not quite the equal of the Tak APO refractors, the Mak performed nicely on a variety of classic resolution tests.

Rigel, for example, displayed its close and faint companion easily at 90x, the secondary star defined as a sharp pinpoint separated from the brilliant primary by plenty of dark sky. Doubling the magnification to 180x kept the view sharp and free of false color. Nearby, the much closer and more difficult Alnitak was cleanly resolved at powers of 150x and above, with tightly focused Airy disks and moderately prominent first diffraction rings. This was a sight that reminded me of the image generated by the Meade 7-inch Maksutov, but the Orion’s smaller secondary obstruction gave it an edge.

Castor, Algeiba, and the neat triple Iota Cass were also strikingly well resolved with the 180. Again, the clean images approached refractor tightness, with the Mak losing the comparison by a bit because of the larger diffraction rings generated by its central obstruction. Castor, when observed at 208x with a 13mm TeleVue Plossl, was beautifully resolved into two well-separated, blazing white suns with a wide gap of darkness between. Very nice. Try as I might, however, the elusive companion to Sirius couldn’t be seen, but the lack of steady atmosphere also rendered it invisible in the Tak refractors and an excellent Celestron 14.


DEEP-SKY OBSERVING

Obviously, the long focal length of the Orion Mak-Cass reduces its effectiveness as a deep-sky instrument. The limited field-of-view makes it impossible to squeeze in the full extent of the Pleiades or Andromeda Galaxy. However, the scope generates a very dark sky background that provides fine contrast.

Using a Celestron f/6.3 focal reducer with ScopeStuff adapter ring and standard SCT visual back, I did manage to get some very pleasing views of some of the winter’s best deep-sky showpieces. With this combination and a generic 25mm Plossl yielding 68x, the Great Orion Nebula was awash in delicate tendrils of sweeping nebulosity set against a dark background. The Trapezium stood out in sharp pinpoint relief, with the “e” star being just visible. With the power pumped to 120x, the more difficult “f” star popped into view. As compared to the image provided by a Celestron 8 SCT, the Mak lacked brightness, but partially compensated with tighter star images and slightly better contrast. When compared to a 6-inch Celestron SCT, however, the Mak did better in both areas, as one might expect. Judging by the relative brightness of the images in the three scopes, it appears that the Orion Mak uses mirror coatings of standard reflectivity (perhaps 88-89 percent), so its light throughput efficiency is lower than that of the Celestrons with their XLT coatings.

The Double Cluster in Perseus also put on a fine show with the Orion 180. At 90x, using the Celestron 30mm Ultima (focal reducer removed), the stellar groupings were razor sharp with colors of the brightest members rendered pure and very striking. The stygian sky background provided a view that approached that of an APO refractor.

A quick look at galaxies M 81 and M 82 also confirmed the scope’s deep-sky abilities: an image not terribly bright, but wonderfully satisfying because of its excellent contrast.


COOLING DOWN

Maks have been tagged with a bad rap over the years because of their difficulty in achieving thermal equilibrium. Taking a Mak from a warm house into sub-freezing weather can be an exercise in patience and frustration. Several years ago, I agonized over the full one-hour cool-down time for my miniscule Meade ETX 90. I fully expected the 180mm to be worse.

Fortunately, with the instrument ensconced in an unheated garage and a small temperature differential with the outside, it came to thermal stability almost instantly, only losing its edge briefly under rapidly cooling night-time air. No problem here.

However, when I attempted to create a “worst case” scenario, the scope didn’t fare as well. Transporting the 180 from a 65 degree F. indoor location to a 12 degree F. outdoor observing site on a February evening, I watched carefully for the first signs of acceptable image quality. With no cooling fans or other means of accelerating the process, the big Mak was left at the mercy of the assorted laws of thermodynamics, and it took a lengthy 2 hours and 10 minutes to achieve equilibrium. With the outdoor temperature falling another 4 degrees during the process, the scope’s image was truly ugly after a full hour outside: sweeping arcs and aurora-like curtains substituted for star images, and Saturn couldn’t be brought close to a sharp focus.

The first sign of decent image quality began at about 85 minutes into the test, with fairly rapid improvement following. I suspect, that in warmer climates or with the addition of a drawtube mounted cooling fan, this time could be reduced considerably.

The good news is, with proper foresight and planning, the Orion Mak WILL cool down under the very worst conditions, and it will produce a fine image when it does.


IN CONCLUSION

Overall, the Orion 180mm Maksutov-Cassegrainian turns out to be a very pleasant surprise. Its impressive optical quality renders it a good choice for the dedicated lunar, planetary, and double star observer who can’t buy into the expensive APO refractor game. It produces perfectly apochromatic images of high contrast and excellent sharpness. When compared to the no-longer-produced Meade 7-inch Mak-Cass, it ranks as its equal with the added advantage of having a smaller secondary obstruction and much lighter overall weight. The Orion scope may fall just a bit behind the Intes-Micro MCT of similar aperture, and it does lag noticeably behind a high-quality 7-inch Maksutov-Newtonian or 6-inch APO refractor – but it does cost a whole lot less. Compared to a good 8-inch Schmidt-Cass, the 180 delivers an image of slightly better contrast, but at the expense of image brightness.

Mechanically and aesthetically, the Orion ranks as “adequate,” but I do wish that they would paint the tube a smooth gloss white and open the narrow rear aperture to allow a wider field-of-view.

The real question here is in the consistency of the telescope’s optical quality. I certainly received a good one, and most comments on the various astronomy forums seem to indicate that is the general rule. There have been reports, however, of misaligned optics and paint debris within the tube assembly for the Orion scope or its Skywatcher clone. Fortunately, Orion’s excellent reputation for customer service and liberal return policy should ensure getting a quality instrument in the end, and most probably the first time.

If the prospective buyer can plan around the long cool-down times and accept the high-power-only nature of the gray beast, he or she will be getting a real bargain. At less than $1200 for the optical tube, it’s a lot of very capable telescope for the money.


Clear and steady skies!

Larry Carlino

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