Celestron C90 Mak Review: Recommended Spotting Scope

C90 as a Spotting Scope:

Celestron today markets the C90 as mainly a spotting scope, unlike the case with the Classic C90s that was discontinued in the late 1990’s.

I judge today’s Celestron C90 as an excellent spotting scope thanks to its compactness and its close focus range.

With the C90, I could even focus on objects as close as 15 feet! This makes it very ideal for some sports like target shooting or even hunting, where usually 100–200 yards of distance are being used. None of the refractors I know of can do this without adding extension tubes or having a very long focuser, both of which can vignette the light cone and the views.

C90 as an Astronomical Scope:

Because of the inclusion of Vixen dovetail bar in the Celestron C90, I can easily put it on any astronomical mount that has a Vixen dovetail saddle. I believe an EQ-3 class equatorial mount or something like the Explore Scientific Twilight I or Vixen Porta II would be optimal to be used with the Celestron C90. I see the tripod included with the C90 as being too small to be of any use, but still not terrible for the price.

If I were you, I’d also swap the correct-image diagonal for a 90-degree star diagonal before getting straight to astronomical observation. The included 45-degree diagonal is awkward for astronomy, but still for the price, it’s not a huge deal.

I find the viewfinder of the C90 very easy to align and it also doesn’t move once it’s aligned.

I’ve tested the included 32mm Plossl eyepiece and found it to be good for low power. But you’ll want additional shorter-focal-length eyepieces for higher magnification views, which is what a small Maksutov like the C90 excels at. I can practically take up to 200x magnification with this small wonder if viewing conditions permit me to.

I think the C90 has only two main limitations when it comes to astronomical use:

1. It’s small at just 90 mm of aperture (light-gathering capability). So, we’re limited to the Moon, planets, and only the brightest deep-sky objects.
2. The long focal length of the C90 makes the above point more of a problem. The 1250mm focal length implies a lower field of view with any given eyepiece. As the brightest deep-sky objects almost always tend to be big (the Pleiades, for example), you can’t even fit them fully in your field of view.

The C90’s baffle tube is shiny, which leads to glare and other contrast-reducing reflections (baffle tubes aren’t supposed to be shiny). To solve it, what I do is 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 I can overlook this since it’s only a minor expense.

C90 for Astrophotography:

The C90’s visual back (which holds the diagonal) is T-threaded, so you can put your DSLR on the back with the help of a T-ring ($10 or so). But I’d warn you to tone down your expectations, as the results will never be as good as the Schimdt-Cassegrain (SCT) type of telescope.

Also, the long focal ratio of nearly f/14 makes the scope useless for long exposure astrophotography of deep-sky-objects, so you’re limited to quick snaps of the Moon and planets.

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 causes 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 seal 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 it 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.

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.