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A truly tiny scope – the Synta MC90

November 18, 2015

MC 90 set up for birding

I realized in conversations with Doug and Terry recently that I have not blogged about the Synta MC90. I got one of these way back in the spring of 2009, on my quest for the perfect travelscope. I only used it for about a month before putting it on semi-permanent loan to my brother Todd. I did briefly mention the scope and show the photo of it set up on the hood of the car in this post.

In the early 2000s it was offered by a variety of vendors – I’ve seen units online labeled Synta, Orion, and Omcon. There’s a review of the scope on Cloudy Nights here, and the same article appears verbatim here.

MC 90 business end

It’s an odd little thing: a 90mm Maksutov Cassegrain, but operating at a fast f/5.6 (500mm focal length) instead of the more typical f/13.9 (1250mm). The most obvious con is that with such a steep light cone, the secondary mirror has to be quite large, which degrades contrast. Pros are that it can get a much larger true field of view than the ~1.3 degrees that the f/13.9 scopes are limited to – I’ll have to check, but unless it’s somehow vignetted it should be able to do over 3 degrees – and that it is incredibly compact.

MC 90 length

It is the same ‘barrel size’ as the longer 90mm Maks, but only about 2/3 as long. Take off the diagonal, eyepiece, and strap-on finder (in this case, a red-dot gizmo from StellarVue) and the scope almost disappears, which has two nice effects. First, just about any mount will be sufficient. In the photo at top I have it mounted on a Manfrotto MKCOMPACTACN-BK tripod and the whole setup weighs only 5.5 lbs (2.5 kg).

C90 orange tube in cooler

Second, if you need a case for this scope, you probably can’t do better than to get a six-pack cooler from Wal-Mart, which will set you back about six bucks. In this photo the scope in the “case” is my old orange-tube Celestron C90, but the MC90 fits inside just as easily. The black drawstring bag holds the finder.

Given how much I’ve written about them in the past, you may fairly wonder where my love for little Maks has gone. Each one got killed by a different problem.

SW 90 on tripod 1

My 90mm SkyWatcher Mak got out-competed by other scopes. The Apex 127 is no more trouble to set up but performs significantly better on just about everything, and if I’m in the mood to roll with a smaller scope, the C80ED has better contrast (as demonstrated in this post), takes magnification about as well, and is more versatile. I still have the little SkyWatcher Mak, mostly because I haven’t gotten around to selling it, but I haven’t used it in well over a year and I don’t imagine that I will miss it.

Happiness is--

From left to right: XT12, XT10, AstroScan, C90, XT6, SV50, 5″ SkyWatcher Newt on homemade dob base. Out of all of these scopes, the only ones I still have are the XT10 and SV50.

I sold the orange-tube C90 a long time ago. I really, really wanted to love that scope. I like the idea of that scope. It’s built like a tank, and since it focuses by rotating the barrel like a giant camera lens, there’s almost nothing that can go wrong with it. If that focus action ever does get sticky, there are instructions online somewhere showing you how to disassemble and regrease it. Basically, as long as our civilization can produce grease, and the scope doesn’t get dropped or left out in the rain, it should never, ever wear out.

BUT that rugged, rotating-tube focuser is also the problem. With such a long focal length, you start at medium powers and go up to high power pretty rapidly. That’s just the natural métier of a long focal length Mak. My problem was that it was almost impossible to focus the scope at high powers without nudging it off-target, and in fact the shakes that were induced by having my hand on the OTA were usually enough to make precise focus a guessing game anyway. It wasn’t a mount problem, it was me needing to paw at the scope to get it to do anything. Don’t know how many other people had the same problem, but I note that for its current, popular, and inexpensive incarnation (still around $160), Celestron has gone to the same rear-mounted focus knob as everyone else.

Synta MC90

Image borrowed from Cloudy Nights.

Finally there’s this MC90. I never really gave it a fair shake back in the day. I only had it about a month before loaning it to my brother, and I’ve only used it on one or two evenings since, neither of them in the last five years. Also, my observing interests have changed. Back in 2009 I did a lot more lunar and planetary observing, and now I’m more interested in DSOs and low-power, widefield scanning, where the MC90 might do better. I’m going to borrow it back this Christmas and give it a serious workout. I’ll let you know how that goes.

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5 comments

  1. Matt, what a cool little scope. I now understand your infatuation with it. Essentially you get decent wide field capability AND high magnification competence in a single and uber portable ‘scope. Plus, it’s damn cute.

    And, of course, now I want one.

    Doug


  2. Matt, I’d be curious to see a shootout between this MC90 and your ED80. I’ll bet the ED80 comes out ahead, not just on contrast, but on light gathering ability, because that central obstruction on the little Mak is huge! Usually these Maks come with undersized mirrors (the meniscus may be 90mm, but they spread out the light, and the mirror is still only 90mm), so you start off operating at something like 85mm or less. Add the central obstruction, and I’d bet you’re now under 80mm. Not trying to knock it; I love me some Maks. Just wondering how it fares is all.


  3. Doug wrote:

    Essentially you get decent wide field capability AND high magnification competence in a single and uber portable ‘scope.

    Widefield, yes. Uber portable, definitely. Magnification capacity is yet to be determined. I never pushed it very hard back in the day, and at f/5.6 it is going to take some work to get to high powers – even a 5mm EP will only give 100x. So that’s something for me to test next month.

    Jon wrote:

    Matt, I’d be curious to see a shootout between this MC90 and your ED80. I’ll bet the ED80 comes out ahead, not just on contrast, but on light gathering ability, because that central obstruction on the little Mak is huge!

    That is a very good idea, and I will do it. Should be easy enough, since the MC90 can ride on my DwarfStar and the C80ED can go on the AZ-4, so I can have them both set up at the same time. (One reason I’ve never gotten around to the C102/Apex 127 shootout is that I only have one serious mount so I can’t set up both scopes at once.)

    I’m very curious about the light-gathering of the little Mak myself. I would not be at all surprised if the C80ED yielded a brighter image. It has very, very nice optics. And I’ve always found Maks a bit dim for their stated aperture. I know that’s partly because almost none of them actually work at the stated aperture, as you pointed out. I suspect that the number of reflections and air/glass surfaces don’t help.

    It would also be informative to compare the MC90 to my 90mm f/13.9 SkyWatcher Mak. It will take some doing to get them both in the same magnification range, but it would be interesting to compare image brightness and contrast. I know from previous adventures that the SkyWatcher has contrast problems – if the MC90 beats it, that’s a big credit in its favor, and if it can’t, it’s truly dire.

    Finally, my son and I have a range of smaller refractors: the Celestron TravelScope 70, a long f/l 60mm Meade, the f/10 50mm GalileoScope, and the f/4 50mm SV50. Will be interesting to see how they stack up against the MC90. Should be fun, too.


  4. I got curious so I ran some quick and dirty numbers. The photo looking down the barrel of the MC90 is the highest resolution version I have. Using GIMP I measured the diameter of the meniscus as 187 pixels and the diameter of the secondary at 83 pixels. That works out to a central obstruction of 44% linearly and 20% by area.

    Since we just need comparative values, not absolute, we can cheat the math a little bit. The light-gathering of an unobstructed 90mm scope would be proportional to 9×9=81. If we subtract 20% for the area blocked by the secondary, that’s 64.8, which is pretty close to 8×8. So the MC90 could only gather as much light as the C80ED even if it was working at a full 90mm aperture, which it probably isn’t.

    I believe there are ways to determine the actual working aperture of the scope by shining lights through and using rulers and such – there are threads of this stuff on CN that I will read up on. I’m trying to think through whether a faster Mak should have a more or less curved meniscus. I would think that a less curved meniscus would spread out the light less, thus bringing the effective aperture of the scope closer to the stated aperture. From my photos I *think* the MC90 has a less curved meniscus than the 90mm SkyWatcher Mak, but that’s kind of a hairy guess and not even really an anecdote, let alone a fact.

    For comparison, for the 90mm SkyWatcher Mak I get a central obstruction of 32% linearly and 10% by area, and that’s assuming that the scope actually works at an effective aperture of 90mm, which it doesn’t.

    Now I am really interested in testing these scopes against smaller refractors on faint targets.


  5. The brightness of my Orion 90mm Mak appeared about equal to an AT72ED in side-by-side viewing. I’ve also compared the Mak with my ES 80ED and while both are very sharp (going up to 200x on the Moon shows no loss of sharpness) the 80ED gives a brighter image with more contrast.

    Of course, a 90mm Mak is significantly less expensive any 80ED (even when you could get one from AWB).



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