I have a Pro-Optic (sold by Adorama) 90mm f/5.6 Maksutov, which as far as I can tell was the same 'scope as the Orion MAK-500 90mm, in a different color (white), for less money than Orion wanted. These scopes were also sold (in white) under the name Synta.
At right: the Mak being used with a solar filter for the 2012 Venus transit.
(I sold it at a swap meet because it wasn't a very useful astro scope, started regretting it several years later because it was really quite a good spotting scope/camera lens, and discovered they had vanished from the face of the earth and nobody sold them any more (not even ebay). But then a nice person who had one for sale contacted me, and now I have one again. We now return you to the review I wrote based on the first one.)
Good points: I like the faster f-ratio compared to the more expensive 90mm Maks (ETX, C-90, Questar) -- images are brighter, the 'scope is easier to point, and at f/5.6 it's much more usable as a camera lens for terrestrial photography than the f/11-12 Maks. It's extremely light and portable, and the case that comes with it is compact yet still has plenty of room to carry accessories (many eyepieces, or a camera body and several rolls of film). It does have mirror collimation screws, but none for the corrector plate. It has a built-in T thread, so attaching a camera is painless. The 26mm Plossl that Adorama supplies with the "spotting scope adaptor kit" seems to be quite good (I use it frequently in my larger 'scopes), basically the same as Orion's Sirius Plossl except that the slightly more expensive Sirius has an eyecup.
Bad points are mechanical: the mechanics of the focusing mechanism don't feel solid. Focusing effort changes unpredictably, and I worry that this means that the mechanism may not last for long. This is true of mine and also of the Orion model I played with in their store, so I'm guessing it's true in general and not just a defect in mine. (On the other hand, it's much easier to use than the C90's focuser, where you have to turn the whole barrel of the 'scope, which is much bigger than my hand and difficult to turn accurately.) It's also hard to lock down the mounting ring so that it doesn't turn, which is somewhat annoying for photographic use but isn't a major problem when using it as a telescope.
As an astronomical telescope, it can give you nice wide-field views of all the usual fairly bright nebulae -- views that the folks with the long focal length 'scopes have trouble getting, like Jupiter and M22 in the same field, or M8 and M20 in the same field, or extended objects like M31. Then crank up the power and you can see a couple of bands on Jupiter, or small bright planetaries like the Ring nebula. It's better than a typical 60mm refractor, but not as good as a 4.5" reflector or an 80mm refractor.
As a terrestrial camera lens, it's reasonable for the money; the usual doughnut highlights on out-of-focus background objects that you see with any mirror lens (caused by the shadow of the secondary mirror), it takes forever to focus so it's difficult to follow wildlife, and it's not extremely sharp -- I don't think the folks at Nikon are too worried about being put out of business any time soon, but I'm happy with it as an addition to my lens collection.
|Squirrel in a tree. This slide was underexposed, which accounts for the poor color here; that's not the fault of the Mak. Note the doughnut highlights in the background, which are caused by the central obstruction and will be visible in daylight shots with any catadioptric lens.|
|The lens does much better in daylight with a solid background.|
|Lunar eclipse. Here's another of totality, or see my eclipse page for the whole set (warning, the scans on the non-totality images are poorly done; I haven't gotten around to re-scanning them).|
It's a compromise: does lots of things reasonably well, does nothing spectacularly well, but for the size and price, it's a good deal.
I had a chance last year to compare it side-by-side with a friend's C90. Our primary test was the Orion Nebula (M42). The two 90mm Maksutovs showed similar amounts of nebulosity, but the view through the C90 was decidedly prettier because the star images were cleaner. The f/5.6's collimation was somewhat off, so the star images showed coma compared to the images in the longer-focus C90. We wanted to compare views of Mars, but unfortunately clouds rolled in before we had a chance.
I've tried to adjust the collimation of the mirror, but haven't gotten it good enough yet to improve the star images significantly.
Update: I sold the Mak in '98 because I decided I didn't need as many 'scopes as I have, and I wanted something which would function better as a portable astronomical scope at the cost of perhaps not being as good a camera lens. I considered buying an Orion Short-Tube 80 refractor, but then saw an 80mm f/7 homebuilt scope (Vixen optics) advertised on Astromart, and bought it. It's a much better astronomical 'scope, especially on planets and double stars, than either the ShortTube refractor or the little Mak, but the downside is that it's much larger, somewhat slower (as a camera lens) and weighs 5 lbs.
But shortly after selling the Mak I started regretting it. It's far more portable than any 80mm refractor. But they're no longer available anywhere, not even on ebay! But someone who read this web page review offered one for sale, and I happily bought it, and now have a Mak 500 again. It's still not very good for astronomy, but it's fine for bird walks, bird photography and lunar photography.