Choosing and Using a Schmidt Cassegrain Telescope

by Rod Mollise

Choosing and Using a Schmidt Cassegrain Telescope Author Rod Mollise Isbn 9781852336318 File size 24 3MB Year 2001 Pages 357 Language English File format PDF Category Astronomy Amateur astronomy is becoming increasingly popular mostly because of the availability of relatively low cost astronomical telescopes such as the Schmidt Cassegrain and Maksutovs The author describes what these instruments will do how to use them and which are the best he draws on 25 years of experience with telescopes There are sections on accessories observing

Publisher :

Author : Rod Mollise

ISBN : 9781852336318

Year : 2001

Language: English

File Size : 24.3MB

Category : Astronomy

Other Titles in this Series
The Observational Amateur Astronomer
Patrick Moore (Ed.)
Telescopes and Techniques
C.R. Kitchin
The Art and Science of CCD Astronomy
David Ratledge (Ed.)
The Observer's Year
Patrick Moore
Seeing Stars
Chris Kitchin and Robert W. Forrest
Photo-guide to the Constellations
Chris Kitchin
The Sun in Eclipse
Michael Maunder and Patrick Moore
Software and Data for Practical Astronomers
David Ratledge
Amateur Telescope Making
Stephen F. Tonkin
Observing Meteors, Comets, Supernovae
and other Transient Phenomena
Neil Bone
Astronomical Equipment for Amateurs
Martin Mobberley
Transit: When Planets Cross the Sun
Michael Maunder and Patrick Moore
Practical Astrophotography
Jeffrey R. Charles
Observing the Moon
Peter T. Wlasuk
Deep-Sky Observing
Steven R. Coe
Stephen F. Tonkin
The Deep-Sky Observer's Year
Grant Privett and Paul Parsons
Field Guide to the Deep Sky Objects
Mike Inglis

A Guide to Commercial SCTs
and Maksutovs

Rod Mollise
With 53 Figures



Cover photographs: By courtesy of Meade Instruments Corporation and
Celestron International
British Library Cataloguing in Publication Data
Mollise, Rod
Choosing and using a Schmidt-Cassegrain telescope: a guide
to commercial SCTs and Maksutovs. - (Patrick Moore's
practica! astronomy series)
1. Schmidt telescopes - Handbooks, manuals, etc.
2. Cassegrainian telescopes - Handbooks, manuals, etc.
1. Title
ISBN 978-1-85233-631-8
Library of Congress Cataloging-in-Publication Data
Mollise, Rod, 1953Choosing and using a Schmidt-Cassegrain telescope: a guide to
commercial SCTs and Maksutovs/Rod Mollise.
p. cm. - (Patrick Moore's practical astronomy series,
ISSN 1431-9756)
Includes index.
ISBN 978-1-85233-631-8
ISBN 978-1-4471-0227-4 (eBook)
DOI 10.1007/978-1-4471-0227-4
1. Reflecting telescopes. 2. Catadioptric systems.
1. Practical astronomy.
QB88 .M65 2001
Apart from any fair dealing for the purposes of research or private
study, or criticism or review, as permitted under the Copyright,
Designs and Patents Act 1988, this publication may only be
reproduced, stored or transmitted, in any form or by any means,
with the prior permission in writing of the publishers, or in the case of
reprographic reproduction in accordance with the terms of licences
issued by the Copyright Licensing Agency. Enquiries concerning
reproduction outside those terms should be sent to the publishers.
Patrick Moore's Practical Astronomy Series ISSN 1617-7185
ISBN 978-1-85233-631-8

© Springer-Verlag London 2001

Originally published by Springer-Verlag London Lirnited in 2001
4th printing 2004
The use of registered names, trademarks, etc. in this publication does
not imply, even in the absence of a specific statement, that such names
are exempt from the relevant laws and regulations and therefore free
for general use.
The publisher makes no representation, express or implied, with
regard to the accuracy of the information contained in this book and
cannot accept any legal responsibility or liability for any errors or
omissions that may be made. Observing the Sun, along with a few
other aspects of astronomy, can be dangerous. Neither the publisher
nor the author accepts any legal responsibility or liability for personal
loss or injury caused, or alleged to have been caused, by any
information or recommendation contained in this book.
Typeset by EXPO Holdings, Malaysia
58/3830-543 Printed on acid-free paper SPIN 10983768


The modern Schmidt-Cassegrain telescope is an
incredibly capable tool for the amateur astronomer.
But it is becoming a very complex one, with each new
model boasting more features, more gadgets, and,
increasingly, more reliance on computers. The manufacturers do their best to make their telescopes userfriendly and their manuals accessible, but nothing can
substitute for straight talk and experienced tips from
brother and sister amateurs. And that's what this book
is all about. It's a compilation of the SCT wisdom I've
picked up since the exciting evening in 1976 when I
unpacked my first Orange-tube C8.
No work of this kind can possibly be the product of
one person. lowe what I know about astronomy in
general and SCTs in particular to the wonderful
teachers I've had over the years: "teachers" meaning
not just classroom instructors, but also those seasoned
amateur astronomers who took a few moments to help
a confused and anxious beginner sort things out.
There are many people I need to thank, but at the top
of the list are the members of my Internet "set-user"
group, and especially the following friends who went
out of their way to offer their encouragement and
knowledge: Leonard Akers, Dave Bird, Alan Bland, Al
Canarelli, Phil Chambers, Dan Cimbora, Bruce Cloutier,
Steve Coe, Michael Covington, Michael Cunningham,
Paul Goelz, Marcie Greer, Joe Hartley, Phillip Hosey,
Simon Hastie, Mike Hosea, Walter Knapp, Michael
McNeil, Peter Moreton, Joe Morris, Gary Otteson, Jeff
Schaub, Larry Span, Glen Speck, David Tetreault, Greg
Thompson, Rick Thurmond, Bob Van Deusen, and
Russell Whigham.
My most heartfelt thanks, though, go to the two
people without whom this book would honestly not
exist. My best friend and long-time observing companion, Pat Rochford, spent quite a few hours of his
precious spare time checking the manuscript, as did my
wonderful wife Dorothy, the brightest star in this


Choosing and Using a Schmidt-Cassegrain Telescope

astronomer's sky, and my constant inspiration. If you
enjoy what follows, they deserve the largest part of your
Rod Mollise
Selma Street
April 2000


1 Why a CAT? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1
2 What's a CAT?


3 Inside a CAT. . . . . . . . . . . . . . . . . . . . . . . . . . .. 19
4 Which CAT?


5 The SCT Out-of-Box Experience: Initial
Telescope Assembly and Checkout ......... 103
6 Accessories for your CAT ................. 127
7 From That Very First Night: Field Set-up,
Checkout and Observing.................. 163
8 Care and Feeding of a CAT: Alignment,
Testing, Maintenance and Troubleshooting .. 219

9 Keeping your CAT Happy: Hints, Tips and
Projects ................................ 259
10 The Advanced CAT: Imaging and Computers .. 295
11 Afterword: 25 Years with a CAT - How to

Keep Going ............................. 347
Index ..................................... 355


If you have even the slightest interest in the fascinating
world of telescopes and astronomy, you have probably
seen one of those short, stubby and wonderfully hightech looking Schmidt-Cassegrain telescopes (SCTs). If,
as a new amateur astronomer, you've been reading the
astronomy magazines - Astronomy Now, Sky and
Telescope, or Astronomy - you have no doubt been
mightily impressed by the many full-page, full-color
advertisements the SCT manufacturers run in every
issue. Reading the enthusiastic ad copy, you'd think the
Schmidt-Cassegrain is the only telescope worthy of
your consideration. This mayor may not be true, but
putting aside advertising hyperbole, the SCT may well
be the best and most versatile telescope for the average
amateur astronomer. As we'll see, SCTs do have
weaknesses as well as strengths, but the fact that these
portable observatories have claimed a very large share
of the telescope market over the last 25 years makes it
obvious that there's something good going on here.
Advertising copy alone wouldn't account for the
astounding continuing popularity of these instruments.
For the novice, at first glance the SCT or CAT (short
for catadioptric, a type of telescope containing lenses
and mirrors) is both familiar and strange-looking. It
appears to be a little like the telescope we're used to
seeing in the movies and on TV. Its eyepiece is in the
right place at the bottom of the tube, it is perched on a
tripod, and it can be pointed at the sky. But there the
similarity ends. The main impression given to the
prospective SCT buyer is that this thing is an absolute
maze of gears, dials, switches, and lights. Daunting.


Choosing and Using a Schmidt-Cassegrain Telescope

Even frightening! But nothing could really be farther
from the truth. The SCT is one of the most user-friendly
and simple to operate telescopes on the market today.
And capable? There is nothing out there that can match
the SCT for versatility.
Why do amateur astronomers love the SchmidtCassegrain telescope? Simply because it is capable of
doing many things well. Other telescope designs may
have advantages for certain applications, but the key to
the CAT's amazing popularity is its adaptability. An
SCT is a "system." Numerous accessories and add-ons
are available for these telescopes because SchmidtCassegrain manufacturers, unlike their counterparts in
the computer industry, have willingly standardized
many of the design features. For example, a camera
adapter designed by Meade (one of the two big SCT
manufacturers) will fit and work just as well on a
telescope made by their chief competitor, Celestron.
This makes things easy for the equipment purchaser
and also encourages third party development of
accessories and equipment for these telescopes. Some
of the best add-ons for SCTs are made not by Meade or
Celestron, but by members of a growing number of SCT
accessory makers. If you can possibly imagine it focusing motors, computers, camera adapters, tracking
motors, and much, much more - it is probably available
for your CAT. And you can be assured it will fit and
operate as designed. This wealth of accessories also
ensures that your telescope will grow with you in your
new hobby. You may start out with nothing more in
mind than getting a good look at the Moon. But, if
amateur astronomy really "takes" with you, as it does
with many, you may find yourself taking astrophotos a
few years hence. And the wonderful thing about the
SCT is that it is just as capable of doing complex tasks
like photography as it is of giving you a pretty image of
that good, old Moon!

"Jack of all trades, master of none." An old aphorism,
and sometimes true. But not true in the case of the SCT.
While nothing in the design of the SCT is astoundingly
innovative, the basic layout of this scope is extremely
sound and allows these instruments to achieve their
renowned versatility. The SCT doesn't just do a lot of

Why a CAT?

things - it does a lot of things very well. Here are some
of the things you can do with your CAT:

General Visual Observing
Whether your interest is viewing the Moon and planets
or cruising the dark depths of space in search of faint
fuzzies - star clusters, galaxies and nebulae - the CAT is
very well suited for the visual observer. Part of the
reason for this is its generous aperture, the large size of
its mirror in relative terms. Quality is important in
telescopes, but the sort of image you get in your
telescope, whether of a planet or of an unimaginably
distant galaxy, is more than anything else dependent on
the size of your telescope's main lens or mirror. Small
refractors, the traditional lens-type telescopes, can often
provide beautiful views. But for most tasks your SCT
will beat refractors every time, even very sophisticated
refractors costing many times more than your humble
SCT. Why? Because even the largest refractors are
usually limited to an aperture size of 6 inches
(150 millimeters). The average SCT, on the other hand,
features an aperture of 8 inches (200 millimeters). A
6 inch refractor can provide incredibly wonderful
images. But most of the time you'll see more detail
with your 8 inch SCT! This is not surprising. An 8 inch
telescope delivers almost twice as much light to your
eye as a 6 inch.
And it's not just the optics which make an SCT a
wonderful tool for the visual observer. Almost all
Schmidt-Cassegrains are equipped with clock drives.
These are motors that allow the telescope to track the
stars. A big reflecting telescope, a Dobsonian, with its
enormous mirror can provide wonderful views. But the
SCT owner with her 8 inch is often able to detect more
details than the Dob owner. Why? Because the image
can be viewed in comfort. The scope keeps that planet
or star cluster centered in the eyepiece and all you have
to do is look. No nudging the telescope - and possibly
losing that much-searched-for galaxy in the process.

One glance at the beautiful color pictures in the
astronomy magazines and a novice astronomer wants


Choosing and Using a Schmidt-Cassegrain Telescope

to start taking astrophotos. Astrophotography is a
difficult, maddening and rewarding pursuit. Just about
any scope can be adapted to take photos of the deep sky
or the Moon or the planets. But most telescopes will
require fairly extensive modifications before they're
really up to the task. Refractors must be provided with
guide telescopes and often need better mountings and
clock drives. Newtonian reflectors may require that
their mirrors be repositioned in the telescope tube
before a camera can even be brought into focus. In the
past, making a telescope ready for photography often
meant the amateur had to rebuild it from the ground
up. But not the SCT. These telescopes were designed to
be able to take sky pictures easily. With the addition of
a few simple accessories, all fork-mounted SCTs can be
used without modification for astrophotography. Take
a mid-range, or even a bargain SCT model; add a
35 millimeter camera body, an off-axis guider (a special
type of camera adapter), and a few other items, and
even a relative novice can, almost from the beginning,
take photos which will compare quite favorably with the
pictures the astronomy magazines print.
This is not theorizing, nor is it manufacturers'
rhetoric. I proved this to myself I hadn't tried
astrophotography in 20 years; I had found the process
just too frustrating. I'd spent weeks fine tuning myoId
Cave Newtonian reflector, and suffered many hours of
shivering Ozark Mountain cold. And for what? A few
blurred black and white pictures of the Orion Nebula (I
knew it was the Orion Nebula, but none of my friends
could tell from my photos). Even after buying my first
SCT I never returned to photography. Just too much
work for no results. But a few years ago I bought my
current CAT, a Celestron Ultima C8, a telescope
designed from top to bottom with the photographer
in mind. Seemed like a shame not to put a camera on it.
OK, I'd give it a try.
I purchased a very inexpensive off-axis guider, set the
telescope up in a friend's light-polluted backyard,
pointed the scope at Orion, squinted through the
guiding eyepiece and exposed away. Seemed pretty
easy. Much easier than taking pictures with myoId
Newtonian. In fact, it really seemed too easy. I just
wasn't convinced that fumble fingered me could take
deep space photos. The next day I made a quick trip to
the photo finisher and handed over my roll to the lab
technician. I waited in agony for the mini-lab to spit out
my pictures - I didn't expect much, but I just couldn't


Why a CAT?

help hoping. When the technician brought my prints
over I could hardly believe it! There was the Great
Nebula, big and beautiful. The wondrous colors,
brilliant reds and blues, were simply unbelievable. No,
it wasn't perfect, but it was pretty darned good. I could
hardly wait to share my images with my astronomy
friends and my wife. Let me emphasize that this doesn't
say much about my still lacking skills as an astrophotographer. What it says worlds about is the ease of
astronomical picture taking offered by the SCT.

Advanced Applications
But the answers to the "what is it good for?" of a CAT
don't stop with visual observing and astrophotography
by any means. Since these scopes are so versatile, they
can be used in some pretty serious scientific endeavors,
and are being used for these tasks by amateur and
professional astronomers every day. Want to hunt for
asteroids? SCTs operated by amateurs using sensitive
electronic CCD cameras are finding new ones every
single night. Interested in double stars? An SCT, with its
generous focal length, makes measuring these stellar
systems' separations and position angles very easy.
Variable stars? Hang a CCD camera or a photometer off
the back of your CAT and have a field day checking the
brightness of the distant stars. Want to do these things,
but don't like going out into the cold dark? There are
several modern SCTs that can be remote controlled
from the warmth and comfort of your computer room.

I'm enthusiastic about my CATs, and I don't mind
saying so. But after using these scopes for 25 years and
having owned a number of them, I can admit that
they're not perfect. What is? The SCT design, like the
design of any telescope, is a compromise. I don't think
these minuses do much to reduce the telescopes' overall
strengths, but you should be aware of them. Expect to
be made aware of them at any gathering of amateurs.
There is a faction within the amateur community who
seem to delight in SCT-bashing. There may be some
truth to some of these claims, but on closer examina-


Choosing and Using a Schmidt-Cassegrain Telescope

tion, most prove to be based more upon supposition
than actual telescope use in the field.

Contrast Problems
A Schmidt-Cassegrain is an obstructed telescope. This
means that it has a secondary mirror - an obstruction
- placed in its light path. According to optical theory,
this may cause degradation in contrast, something
which is particularly troubling for the planetary
observer. Most reflecting type telescopes do have this
central obstruction due to the placement of their
secondary mirrors. But some critics will tell you that
the SCT is the prime offender in this area because its
secondary mirror, due to the basic design of the
telescope, must be relatively large, often comprising
30% of the total aperture of the telescope.
The simple fact of the matter is that any obstruction
placed in the light path, no matter how small, will
damage contrast to some extent. Even a custom
Newtonian reflector with a very small secondary mirror
has lost out compared to the unobstructed refractor.
The question is, "Does the larger secondary mirror of
the SCT make things much worse?" Based upon my 25
years of experience the answer is: "no," or at least "not
much." Listen to some of the "experts," and you'll start
believing that because of this supposed problem the
average SCT must deliver an image about as sharp as
that offered by a 60 millimeter department store
refractor. But then go look through an SCT and prepare
to be amazed. Despite talk about a large secondary
robbing light, sharpness and contrast, the job these
telescopes can do on the planets is simply amazing. I've
had refractor fans stare in amazement at the image of
Saturn offered by my Ultima 8 SCT. An image that is
sharp, contrasty and beautifully detailed even at 500x.

A Schmidt-Cassegrain telescope can only perform well
if it is collimated, that is, if its two mirrors are properly
aligned. SCTs are particularly sensitive to misalignment
of their mirrors. The slightest maladjustment of the SCT
optical train can completely destroy the quality of
planetary images. The good news is that SCT alignment


Why a CAn

is easy, and once collimated, an SCT may hold this
alignment for many months. My personal SCT, for
example, was still in perfect adjustment after traveling
hundreds of miles over pothole-laden US interstate
highways in the back of a rental truck on the way to the
Texas Star Party.

Small Aperture
If you're a novice amateur, the 8 inch mirror in the
average SCT may seem big, especially if you've been
using a 2 or 3 inch refractor previously. But there is no
denying that in the amateur astronomy world of today
an 8 inch mirror is on the small side. This will hit home
at your first big star party when somebody pulls up next
to you and sets up his 30 inch Dobsonian telescope.
There is no way that your 8 inch can compete with a 30
inch on the deep sky. But an 8 inch aperture telescope is
plenty big enough to keep you going and interested for
a lifetime. It will show you literally thousands of deep
sky objects - probably more than you'll ever get around
to observing. Many of the brighter objects like those
that comprise the Messier catalog will show considerable detail, too. Galaxy M51 will show off its spiral
arms, the Great Orion Nebula will seem to flow across
the sky and into infinity, and Globular Cluster M13 will
easily break down into countless diamond-dust stars.
Remember, too, your CAT is capable of doing many
things. That big Dob is really only suitable for visual

Are SeTs really Portable?
I used to think they were. They are certainly a lot easier
to load, transport, set up and break down than my 60s70s style Newtonian reflectors were. SCTs are promoted
as being very portable by their manufacturers, but I
would change this to "very transportable." Except for
the lightest bargain-model CATs, most of these scopes
are not exactly lightweights, with a weight of 50 pounds
(20 kilograms) for the combination of telescope and
tripod being common. They also require quite a bit of
setup compared to some other scope types. In my
experience, the usual star partystar party routine goes
something like this:


Choosing and Using a Schmidt-Cassegrain Telescope

Drive up. Unload tripod and place it so that it is
oriented to the north. Dig out and use a compass if it's
too light to see Polaris, the North Star. Unload the
scope in its case from the trunk. Next to me the owner
of a 12 inch aperture Dobsonian has placed his rocker
box (mount/telescope base) on the ground, gotten the
tube out of the back seat and placed it in the rocker.
He's ready to go. I grit my teeth. Opening the case, I
retrieve the wedge attachment bolts and insert one into
the scope base. I lift my heavy Ultima SCT onto its
wedge and adjust it so that I can insert two more wedge
bolts. I tighten these down. I find the star diagonal and
visual back and mount them on the scope rear cell.
Next I search through my accessory box for my dew
shield, and, if necessary, my dew heaters. I attach these
and start setting up my external battery power supply if
I need one on this night. As it's now dark, I start the
polar alignment process. My neighbor has already
viewed several objects. After a few hours, the sky clouds
over. Time to pack it in. I reverse the setup process. As
I'm loosening the first wedge bolt I look over at the Dob
owner. His scope is packed away and he's drinking a
cup of coffee while grinning at my plight!
All of this is quite true. But it is also true that when
you do have your SCT set up, you have at your disposal
a virtual portable observatory, ready to take on any
task. A CAT does take some time to assemble, but it can
be packed easily into even the smallest vehicle. Unlike
the Big Dob owner, your choice of telescope doesn't
necessarily dictate your choice of vehicle.

Is a CAT for Me?
A Schmidt-Cassegrain telescope is a wonderfully
capable telescope. But is it the right telescope? For
you? Only you can answer this question, but the
following should help.

The SeT May Be Your Scope If
You haven't specialized in a particular area of
astronomy, and probably don't intend to. An astro
dilettante like the author will go from visual planetary
observing today, to galaxy hunting tomorrow and

Why a CAT?

scientific data taking next week. You need a scope you
won't outgrow, one that can change as your interests
You want to take pictures. But you don't want to
spend a lot of money. Tens of thousands of dollars can
easily be spent in the quest of the perfect astrophotography setup. A relatively inexpensive SCT can take
marvelous photos of the sky, and is probably capable of
better photographic performance than most astrophotographers will ever need.
You want to be able to do a little observing and
photography from your light-polluted backyard and
travel to dark sites for real picture taking and deep sky
observing. The virtue of the SCT here is that while it
performs amazingly well from dark sites and is
transportable enough to make trips pleasurable affairs,
it can also do surprisingly well from the backyard. Its
long focal length is a big help in taking pictures from
the average suburb (a short focal length wide-field
telescope may have its film fogged from sky glow before
much of an exposure can be accumulated). Long focal
length can also be a big help for that staple of the urban
observer, planet watching. The fllO optical system
means that high magnifications appropriate for producing large image scales on the planets can be achieved
without Barlow lenses or uncomfortably short focal
length eyepieces.
You're a "techno geek." You like gadgets and
electronics and wouldn't dream of owning a noncomputerized telescope. If computers and electronics are your
thing, you've come to the right place! There are SCTs that
guide you to the object you're interested in, SCTs that
move themselves from object to object and even SCTs
that do the work themselves, taking images of selected
objects while you doze in a warm bed miles away.
You're physically challenged. Even a 6 inch Dobsonian
telescope is impossible for you to handle. SCTs win
hands down here. Their short tubes make them naturals
for use by physically challenged, slightly built or young
people. An 8 inch SCT may be too much for some of
these folks, but SCTs are also available in small, ultra
portable sizes: 5 inch, 4 inch and 3.5 inch SCTs and
similar scopes are a staple of the market. These small
CATs still offer that portable observatory experience
with exquisite optics, built-in drives, computers and an
array of accessories similar to that offered for their big
sisters (often SCT accessories used on the larger scopes
will work on the smaller ones as well).


Choosing and Using a Schmidt-Cassegrain Telescope

An SeT May Not Be For You If
You just want to look, and mainly at deep sky objects.
You don't care about taking pictures - you'll buy a
book or view them on the Internet. You want to see
galaxies and nebulae and you want to see them live,
looking as much as possible like photographs. Think
"20 inch Dobsonian" if this describes you, not "SCT."
You're an advanced photographer or CCD imager,
and you're particularly interested in wide-field shots.
You want perfection - and you have the money to pay
for it! You could still be happy with a top-of-the-line
SCT, but you may be happier with a big apochromatic
(color free) refractor.
You don't like gadgets of any kind, and they don't
like you. The thought of hauling a battery pack, much
less a computer, into a damp field gives you pause.
Your motto is "the simpler, the better." Look for a
medium-sized (6-10 inch) Newtonian telescope, particularly a Dobsonian. It'll keep your blood pressure
down and your hairline intact.
Still can't decide? I urge prospective SCT owners to
actually try one of these telescopes before getting their
hearts set on one. Most cities and towns of any size in
the U.S. and Europe have active astronomy clubs.
Members of most societies will be only too happy to
help you with your decision. Quite a few amateurs will
consider it their personal mission to help you select the
right scope (that's the nature of the people in this
hobby). You'll no doubt run into quite a few SCT users
who'll be only too happy to let you be "copilot" on their
scope at the next star party.
No club? No SCT owners you can find locally? I urge
you then to at least try to seek out a shop selling these
telescopes; even if you have to travel to another city. It
is very important that you at least get an idea of how
big these scopes are (again, much bigger than they look
in the advertisements). It may be that you need a
smaller SCT. Or possibly you'll find that you can easily
handle a 10 or 11 inch or larger CAT. In any case, you
do have this book. If you stick with it, by the end you'll
be as competent as can be without hands on experience,
in not only choosing, but setting up, using and enjoying
a Schmidt-Cassegrain telescope.

What makes an SCT work? Optics are the heart of any
telescope. To a very great extent everything else about
an instrument, its mounting, its capabilities, even its
price, is determined by the telescope's optical design.
So, let's begin our examination of the inner workings of
these cosmic voyagers with a look at telescope optics.
The Schmidt-Cassegrain is an optical hybrid. It
combines some of the best characteristics of two
different types of telescope, the refractor and the
reflector. The secret to understanding the optics of
our CATs is an understanding of the way these two
simple telescope designs work.

The simplest telescope of all is the refractor, the lenstype telescope. It was a refractor that Galileo Galilei first
turned to the heavens on that wonderful Italian evening
in 1609. Galileo didn't invent the telescope, and may
not even have been the first person to use one for
astronomy, but he brought the idea of using a telescope
to observe the Moon, planets and stars to the attention
of the Renaissance's growing scientific community. We
can only wonder what took man so long to come up
with this idea. The basic telescope is, after all, a very
simple affair.
The secret of the telescope is found at the end of the
refractor's tube. In this design a large lens is used to
gather light (see Figure 2.1). It may be made from a

Choosing and Using a Schmidt-Cassegrain Telescope


Refractor (lens type) telescope


Focal poon'

:J\ J.[~ Obie;c"ve~(len=s) ==::::===+::;:~==1


Focal length

Newtonian reflector (mirror type) telescope


Secondary mirror

Primary mirror (objectivel-



Focal pointEyepiece


Focal length
lplus distance from centre of secondary mirror to focal pointl

single piece of glass, or it may be composed of several
elements, but its only purpose is to collect light, much
more light than the tiny lens of the human eye. This
light is formed into an image a distance from this
objective lens. At this time the image of the heavenly
body in the field of view of the telescope is still small,
though it is now very bright. A magnifying lens is
placed just beyond the telescope's focus point to
enlarge the image and reveal fine details. This lens,
the eyepiece, works like a household magnifying glass to
enlarge the small image delivered by the objective lens.
At the most basic level, that's all there is to a
telescope: a lens, the objective, to gather lots of light,
and a magnifying glass, the eyepiece, to make this
image bigger. Astronomers in the seventeenth and
eighteenth centuries used this simple refractor to open
up the cosmos to human view for the first time.
Unfortunately, the Galilean refractors, and even the
improved models that came after Galileo's day, suffered
from some severe problems. The most devastating of
these is chromatic aberration. A simple lens does not

figure 2.1. Basic
refracting and reflecting

What's a CAT?


bring all the rays of light to focus at exactly the same
point. This is a result of the refraction caused by light
passing through the glass. Eventually a refractor would
be made with much reduced chromatic aberration, but
until that happened, refractor-using astronomers had to
put up with an image that was blurred and ringed with
spurious color.
Then a genius, perhaps the greatest genius the
human race has yet produced, Isaac Newton, turned
his mind to the problem and came up with a solution.
Why use a lens? A lens isn't the only thing that can
collect light. Why wouldn't a concave mirror do as well?
Isaac Newton's telescope design replaces the refractor's
convex lens with a concave mirror (see Figure 2.1).
Light from celestial objects is collected by this mirror
and reflected up the tube to a small flat secondary
mirror that diverts the image out the side of the tube for
easy viewing. Here, the image can be magnified by an
eyepiece just as in the refractor. Using a mirror in this
fashion completely eliminates the color problem.
The refractor and the Newtonian reflector seem quite
different, but some of their characteristics are measured
in the same ways. The diameter of a telescope's main
lens or mirror is its aperture. The exact spot where the
rays of light from the objective come to focus is its focal
point. The distance from the lens or mirror to the focal
point is the telescope's focal length. The ratio of the
telescope's focal length to the diameter of its objective
lens or mirror is its focal ratio. A 6 inch diameter
mirror or lens with a focal length of 48 inches has a
focal ratio of 8 (48/6). This is usually written as f/8 (or
whatever the ratio is).

We've been talking about Newtonians and reflectors
synonymously. But not all reflecting telescopes, even the
very earliest ones, have followed Isaac Newton's original
and simple design. Throughout the history of these
telescopes, opticians, both amateur and professional,
have been experimenting with a wide variety of designs.
Often the only thing two types of reflecting telescope
have in common is that both use mirrors rather than
lenses as their light-gathering elements. Two of these
alternate designs, the Cassegrain and the Schmidt
camera are the direct ancestors of our SCTs.

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