Descontructed 90mm Maksutov

What’s this weird looking thing?


Every now and again, we get waifs and strays in the shop. The other day this one came in. Once upon a time it was a 90mm Maksutov Cassegrain spotting scope. 

I find this sort of thing fascinating. When they’re pulled apart, you can see how the focus mechanism on a Cassegrain works. When you rotate the focus knob, the whole primary mirror moves away from or closer to the eyepiece. Here's a second photo of the mirror extended as far as it goes.

With Cassegrains, it’s very important, when the focuser moves, that the mirror stays perpendicular to the light rays, or the image gets distorted. This is generally known a “mirror flop”. I’ve had a look at this one, and the mirror seems very stable. I haven’t been able to wobble it, even though it’s on a single shaft. I don’t want to mangle the scope any further, but I probably will later on.

Larger Cassegrain telescopes often have locking mechanisms on the primary mirrors to stop the mirror moving under gravity as the mount changes direction.

The other interesting thing about the scope is the baffles. In this next photo I’ve included some light rays.

You can see the scope protects itself from spurious light by using the black cone-shaped plastic to block any light from coming in from the side. Stray light can be a real pain for astrophotography.

There’s even a baffle on the corrector plate. In fact, when I put the corrector plate upside-down on my desk, it looked exactly like a glass pot lid.

Is it just me, or does anyone else like pulling apart old equipment?