Improved Reflecting Telescopes


Can you tell the difference between these two telescopes?

I’ll give you a hint. They are both reflectors. I know I wrote before that you’ll normally find the eyepiece (the little bit tacked onto the telescope tube) on the side with reflectors, but as you can see here, this isn’t always the case.

Here’s another hint. The mounting setup isn’t the difference I’m talking about. I realize the most obvious difference is probably that one is on a “fork mount” (right) and the other is on an equatorial mount (left), but I’m thinking of something related to the optics.

Don’t worry, we’ll talk about these two mounting systems in a later post.

So, can anyone venture a guess and tell me what’s different about these two telescopes?

Well, you know how I said I’d give you a hint? That bit about the eyepiece is part of it. The placement of the eyepiece is the only external feature that can clue you in to the difference I’m talking about.

Why? Because the difference between these two telescopes has to do with the path they reflect light along. And where we put the eyepiece depends on where that path leads.

Let me be a little clearer.


First, here’s a quick review of how a reflecting telescope works. It uses mirrors to direct light to the eyepiece. Along the way, it makes all the light rays from an object cross—that is, come to a focus. That’s the point f.

Now take a close look at the diagram I just showed you. Which telescope above does it look more like?

If you answered the one on the left, you’re right. This is a Newtonian reflector, named creatively after Issac Newton because it was his first reflecting telescope. Yeah, that’s the guy who came up with gravity.

A Newtonian focus is common mostly in small telescopes for one reason—make it too high off the ground, and the eyepiece gets hard to reach.

25-inch reflector.jpg

See what happens once your Newtonian gets to be about 25-inches in diameter? That black dot way up at the top is the eyepiece. This astronomer needs a ladder to reach it.

Imagine using this design for one of the major observatories, where the telescope is much bigger. And when I say big, I mean big. I’ll show you some examples of very large telescopes coming up.

Suffice to say, Newtonians don’t work too well for large telescopes. Which is why the Cassegrain focus came about.

I’m guessing that’s some guy’s last name. But what’s important is how the Cassegrain design focuses light…

Cassegrain focus

The Cassegrain focus is a lot like the Newtonian focus. With both, light comes in, hits a concave primary mirror, bounces off at an angle, and hits the secondary mirror. The secondary mirror then aims the light at the eyepiece.

But on a Newtonian, the secondary mirror is at an angle, and aims the light out the side of the telescope. On a Cassegrain, the secondary mirror is actually a convex mirror (just a mirror that bends outward instead of inward, like the primary mirror does).

And this secondary mirror doesn’t aim the light out the side of the telescope. It aims it through a hole in the primary mirror, and out the telescope’s back end.

You can see how this would be a lot more convenient for observatories…

large cassegrain.jpg

This is just a tiny personal observatory. Professional observatories are built much bigger than this, which I’m guessing is just about a person’s height. And already, I really don’t want to squeeze a ladder in here just to get up to a Newtonian eyepiece.

Good thing this telescope uses a Cassegrain focus! That puts the eyepiece at the bottom end, so it’s easy to reach.

Okay…why not just use a refractor, then?

It’s a good question. Refracting telescopes, a different kind of telescope entirely (as in, not a reflector) have the eyepiece at the bottom end, too. So what’s the difference?

Well…the difference lies in the refractor’s basic definition. It refracts (bends) light in order to make originally parallel light rays cross (focus). But bending light comes hand in hand with prisms, and we get something called chromatic aberration

Which produces rainbows on the moon.

moon chromatic aberration

A Cassegrain focus telescope has all the advantages of a reflector—reflecting light rather than bending it—all the while putting the eyepiece conveniently at the bottom end for astronomers to use.

Smart, right?

You may have heard the term “Cassegrain” before, but I’ll bet you almost anything you heard it right along with “Schmidt,” as in “Schmidt-Cassegrain.” Why would that be?

Because a lot of Cassegrain telescopes these days use a thin correcting plate to improve the image you see. This plate is a lens and is perched right atop the secondary mirror…


Hey, this guy looks a lot like my 11-inch Schmidt-Cassegrain. (Of course it does, they’re both Celestrons and I can’t tell the size of this one.)

By the way, Celestron means nothing. It’s just a brand name, like Orion or Meade. Orion sells some of the best telescopes, in my personal opinion, but you can’t go wrong with Celestrons either.

Anyway, I digress. As you see up above, a Schmidt-Cassegrain telescope is exactly the same as a Cassegrain except for one key difference: that correcting lens on the end where light enters.

Wait a second…but I thought lenses produced chromatic aberration? Isn’t the whole point of Cassegrains to do away with that (and other drawbacks of a refracting telescope)?

Well…good news for us, this little correcting plate doesn’t have enough of a curve to bend light that much. Whatever chromatic aberration it produces isn’t that serious. Trust me, I have a Schmidt-Cassegrain, and it’s barely noticeable.

Now, you might think we’ve covered all the varieties of telescopes we possibly can. Truth be told, we haven’t even begun.

But there’s one more little quirk of new-generation telescopes I want to share with you…

Kitt Peak Mayall Telescope

This telescope is pretty darn cool. It’s a 4-meter Schmidt-Cassegrain (4 meters in diameter) and is the Mayall Telescope at Kitt Peak National Observatory. Hey, it’s even in Arizona, where I’m going to college.

By the way, just to be clear, the 4-m diameter is the telescope part. Not the big blue donut-looking thing.

Since this is a Schmidt-Cassegrain, we know how it focuses light, right? Just like a reflector, except the eyepiece is down by the blue ladder.

Wait a sec…why’s there a thing called the “prime focus cage” up there?

That’s because—surprise! —this telescope can do what’s called a prime focus. Instead of the light bouncing off the secondary mirror, that mirror gets taken out entirely.

An astronomer can sit up there at the top of the telescope, where light comes to a focus just past where the secondary mirror would normally be.

Yes, seriously. This lucky astronomer gets to ride around in a telescope as it observes the sky. Kind of like the teaspoons in an amusement park. Not quite enough of a rollercoaster, but still a fun ride.

Well…that’s it on telescopes for now. Tomorrow, let’s take a look at a few other modern improvements to our eyes on the sky…


Questions? Or just want to talk?

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