it achieves higher magnifications using more comfortable, longer focal length eyepieces without the need for a Barlow. In terms of light grasp and resolving power it exceeds the capabilities of an 80mm refractor on both scores (the effective clear-aperture light grasp of a C90 once you account for transmission losses and obstruction, is 85mm). Most importantly, at under $200, it's cheaper than a refractor.
First, Jim, thanks for this post. I'm seriously considering getting one of these for travel purposes (and public star parties under a moon-blighted sky when there are no conjunctions that might require a 1.5 degree plus field). You've clarified this post already, in pointing out that the C90 is about like your AT72ED minus the gigantic field, which I would agree completely. I would also agree that, for Deep Sky, this little Mak will indeed perform like an 85mm clear aperture scope.
But still, I think for full disclosure to the masses, one should consider a few characteristics of the C90. First, it does NOT have an oversized primary, as Meade touted about its Maks. I realize that Ed Ting (and many others) still rate the Synta Maks as superior to their Meade counterparts, but just because Meade has put out, perhaps, an inferior product doesn't invalidate the claim their making regarding primary mirror size.
It's important to consider the aspherical corrector plate as demonstrated here
. The drawing is exaggerated for effect, but the defocusing effect of the corrector plate means the primary mirror, in order to operate at the size of the corrector plate optically, must be oversized relative to the corrector plate. The Synta's are not, as Gary Seronik wrote about in his S&T review of the Orion Mak 127 soon after the Apex line came out (2001?) Seronik wrote that the 127's actual functioning aperture was really 121mm, and not 127mm, due to this lack of an oversized primary. Proportionally scaling his figures to the 90mm model, one gets 85.75mm as the functioning full aperture of the Synta 90mm. When one subtracts the Pi-R-Squared of this 85.75mm and the Pi-R-Squared of the 31mm painted secondary, one ends up with the photon equivalency of an 80.7mm unobstructed aperture instrument. Of course, on planets, it can't possibly perform at this level, since it's an obstructed view, so the 72mm ED or long tube 60mm achromat scope sounds about right. OTOH, for DSOs, in my experience at least, obstructed aperture instruments tend to produce results of the effective aperture of the primary, almost regardless of the CO size (at least when it's below 40%, which it is well below on all the longer focal ratio Synta Maksutovs).
And speaking of focal ratio, using my calculations above, I put the C90 as operating at F/14.58, so pretty slow indeed. But I also concur with your findings, Jim, that the maximum TFOV you'll get out of one of these is about 1.3 degrees. That won't frame the Pleiades, and can't help with conjunctions or when a planet cruises near the Pleiades, Beehive, etc. But of course, it will easily put any planet well inside the field, including every moon you might be able to see for Jupiter and Saturn. But the TFOV issue does make me wonder about this
. Something else for less than nothing! Decisions, decisions.
And kudos to 314Sprout finding Enceladus. Man, you and Jon Isaacs sure have it good in San Diego! You've GOT to live in the best place in North America for clarity of skies. Mine aren't bad, and I'm not complaining at all here in West Texas when I consider my eastern origins (Alabama and Iowa), but South California sounds like the best -- and the climate ain't half bad, neither! Since we're in the ice box that's gripped the central US, you indeed have it good.
Don't underestimate the 90mm Mak