Last updated on
4/11/
2006

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Cassegrain Telescope Basics
Cassegrain Optics (OTAs) Cassegrain Mounts

[From Seeing in the Dark by Timothy Ferris (excerpted in the September 2002
  issue of Smithsonian magazine), where he is describing professional
  astronomers using what were at that time (the early 20th century), the largest-
  in-the-world California telescopes:]

“... But observing the professional way wasn't usually a lot of fun. To be up there in the cold and the dark, riding in the observer's cage and carefully guiding a long exposure on a big glass photographic plate, with icy stars shining through the dome slit above and starlight puddling below in a mirror the size of a trout pond, was indubitably romantic but also a bit nerve-racking.  Big-telescope observing was like making love to a glamorous movie star: you were alert to the honor of the thing, but aware that plenty of suitors were eager to take over should your performance falter.”

Sadly, things haven’t changed much for professional astronomers since then.  No one ever gets to observe through the big professional scopes with the naked eye, since high-resolution CCD cameras, spectroscopes, and other instruments are far more important to astronomical research during the night hours available on these scopes. Professional astronomers who love observing with their own eyes, purchase and use amateur scopes of their own.

There are a lot of different amateur telescopes available, but a real revolution occurred some years ago when two manufacturers (Celestron and Meade) integrated a microcomputer with a telescope.  Called “GoTo” scopes, these instruments allow an amateur astronomer to select an object from a list in a hand-held control unit, whereupon the scope automatically aims itself at that object. Amateur astronomy then took a giant leap in popularity because many of the most beautiful objects in the sky are fairly dim and very hard to find with the naked eye - the computer really helps.  There are a lot of GoTo telescopes available, and to sort out how they differ and why their prices cover such a large range you need to understand the three primary components of a telescope. 

A scope consists of an Optical Tube Assembly or OTA (the “tube” you see when you look at a scope), a mount that the OTA is bolted to, and an eyepiece at the end of the OTA.  The purpose of the OTA is to collect much, much more light than your eye can collect (on the order of a thousand times more), to reveal dim but beautiful astronomical objects. The purpose of the mount is to facilitate your aiming the OTA at different objects, and to hold them stationary (i.e. counteract the Earth’s rotation) so you can enjoy them for a while.  OTAs and mounts can to some extent be “mixed and matched” with each other, to derive scopes with a variety of purposes (and prices). 

A single eyepiece is provided with a new scope, just as a single lens is typically provided with a new camera.  But a particular eyepiece largely determines the magnification of the object you are observing, and the magnification you really need depends on what you are observing. So you will need several eyepieces for any given scope because you will want to observe objects with a variety of different characteristics. Eyepieces are somewhat (but not completely) unrelated to the scope itself, just as camera lenses are optically somewhat unrelated to the camera body they are mounted on. Additional eyepieces are purchased separately from the scope, so for this reason eyepieces are described in a separate part of this Web site.

OTA Considerations
Going back to scopes themselves, most GoTo scopes use a cassegrain catadioptric optical design for the OTA .  This design combines a primary light-gathering mirror at the rear of the scope with a glass correcting plate at the front, on the back of which is a secondary mirror that reflects light from the primary back to a hole in the center of the primary, through which you observe the image.   This optical design “folds” the light path back on itself and that dramatically shortens the length of the scope and reduces its weight as well, making it a lot more portable. This is a major advantage for most people (i.e. anyone who doesn’t own a permanent observatory), so the type of scope most popular for folks in the beginner-to- intermediate range is a GoTo catadioptric. 

That was a very brief explanation and there are more details you ought to be aware of. For example, among cassegrain designs there is are significant differences between a Schmidt-Cassegrain, Maksutof - Cassegrain, and Schmidt Camera design, and these differences have very important practical implications as to the ultimate limitations of your scope. So click on the Cassegrain Optics link at the top of this page to get more information on the attributes of these different designs. Typically, the most popular smaller-diameter GoTo scopes are Maksutof designs, and the larger-diameter scopes are usually Schmidt-Cassegrain telescope, or SCT designs.

Assuming good quality of the primary mirror and correcting lens (which is usually a good assumption if you purchase one made by a reputable manufacturer), the most important attribute of an OTA is the diameter of the primary mirror.  A larger mirror will gather a lot more light and thus will reveal dimmer objects, or more detail even in brighter objects.  As you may remember from high-school geometry, the area of a circle (like a mirror) increases as the square of its radius; doubling the radius of the primary mirror quadruples its light-gathering power. So why not purchase an OTA with a huge mirror? There are two impediments to this: weight and price, and of these two, weight may be the larger issue although price quickly becomes a barrier as well. A larger mirror/correcting lens combination increases its weight roughly as the square of the radius in the same way it increases its light-gathering power. Unless you have the physical attributes and youth of a pro football lineman you may well have a hard time carrying and lifting onto its mount, any fork-mounted OTA larger than one with an 11” diameter by yourself (and an 11” OTA may be beyond your ability).  And if you can’t easily set your scope up, you won’t do it often and you will find it gathering dust, wasting your investment (this happens a lot with folks that have a lot of disposable income who “gotta have the best” and start amateur astronomy by purchasing large scopes).  Unfortunately, the weight/cost issue is to some extent related to demographics - the folks who have the disposable income to purchase a larger scope, tend not to be young and thus they have the most problem with the scope’s weight. (Yep, it’s just like high-performance sports cars - the majority of the people who would enjoy them the most, are too young to be able to afford them.) So while it is true that larger OTAs also cost a lot more, even if you have unlimited funds you may not find that a large OTA is a good investment especially when you are just starting out.  (You can overcome the weight problem to some extent, by using a German Equatorial Mount but this is only because that mount breaks down into more pieces than a fork mount so each piece is lighter.  You still need to carry and set up all of those pieces so setup is more of a hassle, plus GEM mounts are less convenient to use than are fork mounts.) In summary, when you are starting out you should be careful in considering anything larger than an 8” to 10” or 11” cassegrain scope.  These scopes are large enough to reveal deep-sky objects but light enough that you won’t mind setting one up and using it.

After you have used your scope for a while, you may have the opportunity to view deep-sky objects in a larger scope at a star party, at an astronomy club, or using a friend’s scope. The view in a large -aperture scope is wonderful, but you may then find yourself afflicted with a disease called “aperture fever”.  The only cure for aperture fever is an injection of a large amount of money <grin>. One way to avoid this disease is, as Steve Coe eloquently puts it: “Never observe with more aperture than you can afford”. After many years of observing, Steve Coe and other members of the Saguaro Astronomy Club have also concluded that you need to increase your scope’s aperture (mirror diameter) by 4” to see a definite difference in the deep-sky objects you observe; a 2” aperture difference isn’t all that noticeable. (Personally I have found that the 3” increase in aperture diameter between my older 8” SCT and my newer 11” is very noticeable.) This contributes to the deleterious financial effects of aperture fever.

All that said, it is definitely true that more light gathering power does reveal more detail in dim deep-sky objects. Relative to an 8” mirror, here is the approximate light gathering power of larger-aperture SCT scopes (including the deduction required due to the larger secondary mirror): 9.25” = 1.35X; 10” = 1.6X; 11” = 1.9X; 12” = 2.3X; 14” = 3X; and 16” = 4.2X.  Since an 11” mirror approximately doubles the light gathering power of an 8” mirror and may still be manageable in terms of weight (depending on the strength of the owner), 11” SCTs are becoming quite popular.

Note, however, that there is one more disadvantage to larger scopes (other than their weight and cost).  Because of their longer focal length, they will of necessity provide a more narrow field of view regardless of which eyepiece you use. So larger scopes cannot display some of the largest star clusters or nebulae. To overcome this limitation, their owners typically mount a Rich-Field Scope atop the SCT’s OTA, so this limitation isn’t particularly serious.

Mount Considerations
Given the choice of the diameter of the OTA, your next choice is the quality of its mount. Roughly speaking there are two choices of mounts: lighter-weight ones designed for visual observing only, and heavier-duty mounts that allow you to move into astrophotography.  Again this is a bit of an over-simplification of a very, very important issue; click on the Cassegrain Mounts link at the top of this page for more details on mounts.

Summary
For visual observing only, in the 8” size Celestron has the NexStar 8 and Meade has the LX-90, both of which which have received good reviews.  If you believe you want to progress into astrophotography the most popular mid-priced scope/mount combinations have in the recent past been the Celestron Ultima 2000 (U2K) and the Meade LX-200 series.  (Click here for a discussion as to why many amateur astronomers - myself included - had chosen the 8” U2K over the 8” LX200). 

Celestron replaced the U2K with the NexStar 8 GPS (and introduced the Nexstar 11 GPS), and Meade has introduced a line of LX-200 GPS scopes.  Celestron also introduced the CGE line of scopes - these are German Equatorial Mount (GEM) scopes that make the weight problem of lifting large (11” or 14”) OTAs onto a tripod much more manageable than for fork mounts.  Again, click on the Cassegrain Mounts link at the top of this page for more details on the differences between the previous GoTo mounts and these newer mounts.
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As you become more familiar with telescopes you may enjoy reading Unusual Telescopes by Peter Manly, Cambridge University Press, 1995. It’s a fascinating account, written with a dry sense of humor, of telescope designs that are out-of-the-ordinary either out of necessity, for special purposes, or just because of the creativity of the builder. Many of we amateur astronomers are fascinated by astronomical equipment almost as much as by stargazing itself. If you are one of us, read this book.


 

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