Celestron C90 Mak Review: Recommended Spotting Scope

The Majestic History

The C90 is a Maksutov-Cassegrain, an optical design invented by a young Soviet engineer named Dmitry Dmitrievich Maksutov. Maksutov sought to invent a compact, easy to manufacture telescope suitable for use in schools and the military.

The Maksutov-Cassegrain uses a very fast spherical primary mirror with a corrector lens at the front. If this sounds familiar, it’s because the Schmidt-Cassegrain works in exactly the same manner. The difference is the shape of the corrector.

While a Schmidt uses a nearly flat fourth-order curve on a thin piece of glass, the Maksutov corrector is a highly curved, thick meniscus lens. This makes it far easier to manufacture with conventional optical facilities (a Schmidt requires special tools unique to it), and also far more shatterproof.

The thickness of the corrector makes it difficult and expensive to construct a good large Maksutov. The cost skyrockets at large apertures and the large, thick mass at the front of the tube cause thermal problems – I regularly use one of the largest Maksutovs in the world and it has severe thermal issues, mainly due to the thickness of the corrector.

The original Mak-Cass used a separate secondary mirror attached to the corrector, just like a Schmidt-Cassegrain.

However, the American optical designer John Gregory realized that with some slight modifications the secondary could be nothing more than an aluminized spot on the back of the corrector. This allows for minimal collimation, if any is needed at all, and furthermore makes the telescope far easier to be sealed airtight. Gregory’s idea was genius and within a few years, the Questar corporation began marketing 3.5”, 7”, and 12” Gregory-Maksutovs.

The Questar 3.5 is a beautiful instrument, but is extraordinarily expensive. Until the late 1970s, it was the only choice if you wanted an extremely portable telescope with no collimation required and without inconveniences such as an open tube. Enter the original, orange C90.

With no significant difference specs-wise, besides a slightly shorter focal length (1000mm for the original C90 vs 1250mm for the Questar and modern C90), and going for a fraction of the price of the Questar, many who couldn’t afford the Questar flocked to the C90 as an answer.

However, the original C90 had a couple of problems. The main one was its focusing system. The original C90’s focusing mechanism is basically like a telephoto lens. The entire upper section of the tube twists. This works fine when the C90 is on the rather rare Celestron-made fork mount, but on a more standard mount or a simple photo tripod, it can lead to vibrations, making it hard to focus and use at high magnification.

The orange C90 faded away in the 1980s, though you could still buy a black-tubed, later rubber-armored version (usually with mediocre to poor optics) through the late 1990s. Then the C90 completely disappeared.

The New C90 Mak Getting Reviewed

In the mid-2000s, Celestron remade and re-released the C90 in the form available today.

With a 1250mm focal length, conventional focusing system (the primary mirror slides back and forth on a rod), and better quality control, the modern C90 more or less duplicates the Questar at around 1/20th of the price! Also, its modern multi-coatings make images quite a bit brighter than in the original C90.

Celestron markets the C90 as mainly a spotting scope, but with a Vixen dovetail, one can easily put it on an astronomical mount (an EQ-3 class equatorial mount or something like the Explore Scientific Twilight I or Vixen Porta II would be optimal), swap the correct-image diagonal for a 90-degree star diagonal, and get straight to observing. I find the viewfinder very easy to align and it wouldn’t move once it’s aligned.

The included 32mm Plossl eyepiece is good for low power, but you’ll want additional eyepieces of shorter focal length for higher magnification views, which is what a small Mak like C90 excels at. You can take up to 200x magnification with this small wonder.

The C90 has only two main limitations when it comes to astronomical use:

1. It’s small. You’re limited to the Moon, planets, and bright deep-sky objects.
2. The long focal length makes #1 more of a problem, as the brightest deep-sky objects tend to be big (the Pleiades, for example), but the C90’s 1250mm focal length won’t allow you to fit them in the field of view.

The C90’s baffle tube is shiny (baffle tubes aren’t supposed to be shiny) and leads to glare and other contrast-reducing reflections. To solve it, you can buy some adhesive flocking paper or black velvet and insert it into the baffle tube. It’s annoying that one has to do this, but it’s a minor expense.

The C90 is an excellent spotting scope thanks to its compactness and also its close focus range. It can focus on objects as close as 15 feet! A refractor simply can’t do this without adding extension tubes or having a very long focuser, both of which can vignette the light cone.

The C90’s visual back (which holds the diagonal) is T-threaded, so you can put your DSLR on the back with a T-ring ($10 or so).  But be sure to tone down your expectation as it will never be as good as SCTs.

For terrestrial use, this makes it awesome as a super-long focal length telephoto lens. The close focus is at 15 feet making it very ideal for some sports like target shooting or even hunting where usually 100-200 yards distance are being used.

For astronomical viewing, the C90’s sharp optics make it a great choice for looking at the Moon, planets, and double stars. However, the small aperture means it is limited for deep-sky work.

For astrophotography, the long focal ratio (nearly f/14) makes it useless for long exposures, so you’re limited to the Moon and planets.

The C90 comes with a soft carrying case, so it can be your ideal travel buddy. For airline travel, it’s small enough to be a carry-on (checking it will lead to its destruction) – your mount/tripod can go in luggage.

The tripod included with the C90 is too small to be of any use with it, and the included 45-degree diagonal is awkward for astronomy, but for the price, it’s not a huge deal.