Astronomical Engines

Some Background on Reflecting Telescopes

The 200 inch primary mirror for the Hale telescope, ready to be coated with aluminum.

Greetings, everyone.

Today I’m going to talk to you about reflecting telescopes, or telescopes that use a mirror to focus incoming light rather than a series of lenses, such as the refracting telescopes that we’ve already looked at. The reason for this is that I’m going to be showing you some of the great telescopes of our time in later articles, and the bulk of them are the reflector type. If I give you the basic background on those instruments now, then I won’t have to natter on and on in each article, explaining things over and over again, and forcing you to read, heaven forbid. It will also cut down on the amount of writing I’ll have to do, and that way we’ll all be happy. I will anyway, and that’s the important thing.

At this point, I’m going to start quoting liberally from Wikipedia, because it is correct and I can’t really say it any better. So, from here on, blocks of text in italics are direct quotes from Wiki. This article will be rather long, so you might want to bookmark it or something so that you can refer to it later after your eyes glaze over.

Reflecting Telescopes

A reflecting telescope (also called a reflector) is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. Almost all of the major telescopes used in astronomy research are reflectors. Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position.

A curved primary mirror is the reflector telescope’s basic optical element that creates an image at the focal plane. The distance from the mirror to the focal plane is called the focal length. Film or a digital sensor may be located here to record the image, or a secondary mirror may be added near the focus to modify the optical characteristics and/or redirect the light to film, digital sensors, or an eyepiece for visual observation.

The primary mirror in most modern telescopes is composed of a solid glass cylinder whose front surface has been ground to a spherical or parabolic shape. A thin layer of aluminum is vacuum deposited onto the mirror, forming a highly reflective first surface mirror.

At this point I should add that some newer telescopes, such as the Keck telescopes, use segmented mirrors that are a collection of smaller curved mirrors placed edge to edge to make a much larger and lighter primary mirror. These mirror segments are quite thin, and have computer controlled actuators underneath them that deform the overall mirror’s shape to correct for atmospheric distortions. I’ll talk more about this type of active mirror in later articles.

Reflecting Telescope Designs

There are quite a few different designs for reflecting telescopes, but we’re only going to discuss the types that are relevant to the instruments we will be looking at in later articles.


The Gregorian telescope, described by James Gregory in his 1663 book Optica Promota, employs a concave secondary mirror that reflects the image back through a hole in the primary mirror. This produces an upright image, useful for terrestrial observations. Some small spotting scopes are still built this way. There are several large modern telescopes that use a Gregorian configuration such as the Vatican Advanced Technology Telescope, the Magellan telescopes, the Large Binocular Telescope, and the Giant Magellan Telescope.


Light path in a Gregorian telescope.

The Cassegrain design and its variations

The Cassegrain telescope (sometimes called the “Classic Cassegrain”) was first published in an 1672 design attributed to Laurent Cassegrain. It has a parabolic primary mirror, and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. Folding and diverging effect of the secondary creates a telescope with a long focal length while having a short tube length.

Light path in a Cassegrain telescope.


The Ritchey–Chrétien telescope, invented by George Willis Ritchey and Henri Chrétien in the early 1910s, is a specialized Cassegrain reflector which has two hyperbolic mirrors (instead of a parabolic primary). It is free of coma and spherical aberration at a nearly flat focal plane if the primary and secondary curvature are properly figured, making it well suited for wide field and photographic observations. Almost every professional reflector telescope in the world is of the Ritchey–Chrétien design.

Focal Planes

We’re nearing the end. Stay with me now.

Prime focus

The observer/camera is at the focal point (shown as a red cross).

In a prime focus design no secondary optics are used, the image is accessed at the focal point of the primary mirror. At the focal point is some type of structure for holding a film plate or electronic detector. In the past, in very large telescopes, an observer would sit inside the telescope in an “observing cage” to directly view the image or operate a camera. Nowadays CCD cameras allow for remote operation of the telescope from almost anywhere in the world. The space available at prime focus is severely limited by the need to avoid obstructing the incoming light.


A prime focus telescope design.

Nasmyth and coudé focus

The Nasmyth design is similar to the Cassegrain except no hole is drilled in the primary mirror; instead, a third mirror reflects the light to the side.


Adding further optics to a Nasmyth-style telescope to deliver the light (usually through the declination axis) to a fixed focus point that does not move as the telescope is reoriented gives a coudé focus (from the French word for elbow).This design has often been used on large observatory telescopes, as it allows heavy observation equipment, such as spectrographs, to be more easily used.


Nasmyth/coudé light path.

And that’s all for today.  As always, you can ask questions in the comments.


  • OA5599

    What are the shapes below the surface in the lead photo–those things that look like flashlight batteries and flower petals? Is it negative space formed by removing material to keep the mirror's weight down, or something to perhaps keep the shape stable?

  • The Professor

    Those are voids created during the casting process to keep the mirror's weight down. The weight of the mirror had to be reduced, as a monolithic block of glass of that size would easily deform under its own weight. Corning glass came up with the idea (I think. I'll have to try and check) of the honeycomb void design that would reduce the weight significantly while maintaining rigidity. Corning had a tough time making it work, however. The shaped blocks that they used in the mold wouldn't stay in place once the glass had been poured, and would float to the surface of the glass. I'm sure that much Shouting and Stomping was done. It took them 2 or 3 tries before they figured out how to make it work.

    • FЯeeMan

      Wow! I thought they were foam blocks of some sort to hold the glass up. I didn't realize that they were voids in the glass itself.

      The more you know…

  • BlackIce_GTS

    Is constructing a primary mirror by vacuum depositing aluminum a newer technique then using a spinning dish of mercury? It sounds like it would produce a less geometrically ideal mirror, while being less fiddly to implement.

    • The Professor

      Vacuum coating the mirror with aluminum vapor is a much superior method. Spinning a dish of mercury will give you a spherical profile, but the mirrors on professional telescopes are very carefully ground parabolas, in general, in order to correct the various optical aberrations that appear in large optical instruments. The extremely thin coating of aluminum follows the curvature of the glass without distorting the shape. The tolerances are unbelievably fine.
      Does that answer your question? Or did I miss the point?

    • The Professor

      I need to make a follow up comment here. I was in error when I said that the only figure you get with a spinning mercury mirror is spherical. If I had thought about it for longer than 10 seconds I would have realized that, but there you are. Anyway, you can spin a parabola with a mercury mirror, but I haven't yet found how you would correct for aberrations and coma. I'll blunder across it, eventually.

      • BlackIce_GTS

        Yes that answers it.
        I always thought mercury spinning was a really neat idea, although it would seem like the nature of such a process would be… volatile? fickle? erratic? What's the word I'm looking for…

        • The Professor

          Unstable. Any small vibration, like from the motor or a bearing, would distort the mirror surface.

          • BlackIce_GTS
          • The Professor

            Hmph. You need to remember that unlike you and Alff, I don't suffer from the need to make a bad pun out of nearly anything I read. No, I have the habit of making bad puns out of peoples names, which is worse.

  • This is an appropriately timed article from my point of view, as I spent part of today going back and forth to the roof of the adjacent building where we had set up a Meade 8" LX10 (Schmidt-Cassegrain optics) so the students in one of my department's courses could view sunspots. (Yes, of course, with a filter.)

    We didn't use its tracking system, however, as it is much more impressive for the students to discover just how quickly the Sun moves out of the field of view. Nearly constant manual adjustments are required to keep up with it, which never fails to surprise them.

    • The Professor

      How fun! That's high on my list of things to do with a telescope. Some of the filters show incredible aspects of the sun, and I want to play with all of them. Meade telescopes are very good, so you have a nice instrument there. I went to the Meade website the other day to check out their current product line. And to drool a bit. They have a 20" Ritchey-Chrétien reflector that is to die for, but it costs as much as a new car, unfortunately.
      The closest thing to a telescope that I've looked through was a spyglass-on-a-tripod thing that my folks bought from Radio Shack for some reason (they had never bothered to use it). I set it up for them so we could look at the moon, and it really is surprising how fast the image moves across your field of view.
      Good stuff! It sounds like your classes are fun.

    • That reminded me of a photograph of the ISS transiting the sun from a while back…it must have been very hard to catch an image…
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