Hello everyone, I’m Darrell Heath with the UALR College of Arts, Letters, and Sciences;
welcome to The Night Sky.
If you are considering buying a telescope for the first time my advice to you is: don’t.
At least not until you’ve done your homework and then carefully considered all of your
options.
I would even go so far as to say that before you start to research telescopes you should
first consider purchasing a pair of binoculars.
Binoculars are relatively cheap, have a wide field of view, and will allow you to see a
variety of celestial objects.
For example: our moon as well as those that orbit around Jupiter are all within easy range
as are a great many deep sky objects such as the Orion Nebula and the Andromeda Galaxy.
Many double stars and star clusters are easily accessed with binoculars and by sweeping them
along the Milky Way you’ll see a great many spectacular sights.
Along with the pair of binoculars I’d recommend a good field guide so that you can begin to
know the sky and how it moves as well as how to identify various constellations and bright
stars.
This background knowledge will serve you well when you finally do purchase a telescope and,
should you decide that amateur astronomy is not right for you, then you are not out a
lot of money and you can then use those binoculars for nature watching or for use at concerts
and sporting events.
Plus, even if you do go on to buy that telescope you will still find that a pair of binoculars
are a very useful bit of supplemental gear.
I always keep a pair handy with me when I am out observing and use them on a regular
basis.
Assuming that you’ve followed my advice and are now ready for a telescope, what should
you buy?
Astronomers get asked this question a lot and a friend of mine often says that the question
is kind of like asking someone: what kind of car should I drive?
There are a lot of things you need to take into consideration.
Things like: what do you intend to do with the scope?
Are you just interested in viewing the moon and planets or do you also want to observe
deep-sky objects?
What kind of budget do you have?
Do you want something that’s easy to use or are you comfortable working with complicated
electronics?
And there are questions that you probably haven’t even considered yet, like portability.
Is the scope you want to buy one that you can easily pick up and carry outside of, if
you have to drive to a good observing location, will it fit inside your vehicle?
In this episode I’ll introduce you to some telescope basics and hopefully it’ll be
of use to you when making the decision as to what kind to buy.
So, what’s the most important factor you need to consider when looking for a telescope?
If your first thought is “magnification” then, sorry, that is not the correct answer.
You need to steer well clear of any telescope being sold based upon its magnification virtues
in particular.
One of the most important features of any telescope is that of aperture and the larger
the aperture the more light gathering power your telescope will have.
What does this mean for you?
Well, with more light gathering power the better celestial objects will appear through
your telescope.
A bigger aperture means that objects will appear brighter and clearer than they will
in many smaller aperture scopes.
But don’t succumb to aperture fever.
Remember what I said about portability earlier?
This is where you need to stop and consider if the scope you buy is one that you can carry
around or transport to another site.
And, with an increase in aperture there’s a corresponding increase in price so your
budget will also be a big factor.
As to what aperture is optimal for you, the answer will depend on the type of telescope
you wish to buy.
Telescopes come in three basic types and each uses a different optical design for collecting
light and then making that light available to your eye as a clear and distinct image
The first kind of telescope we’ll consider, and indeed, the very first kind of telescope
ever built back in 1608 by the Dutch spectacle maker, Hans Lippershey, is called a refractor.
This is the kind of scope most people think of when they hear the word “telescope”.
At their most basic, refractors are a simple, closed tube with a lens at one end for collecting
light (this is called, “the objective”) and then focusing that light down at the other
end, where the eyepiece goes, and then into a magnified and viewable image.
Using glass for the objective has distinct advantages and disadvantages (by the way,
never buy a telescope that uses plastic parts, especially for the optics).
The advantages of a refractor made with quality components are that they are low maintenance
and generally provide you with sharp, clear images.
These scopes are ideal for looking at the moon, planets, and some of the brighter deep-sky
objects.
The disadvantages come from a problem that refractors have called “chromatic aberration”.
The most basic type of refractor is called an “achromatic” and it uses a two lens
element for its objective.
This is all well and good but when you start to increase the aperture above 100 mm that
two lens element system begins to fail at being able to focus all of the incoming light
into a single point.
This results in the chromatic aberration I just mentioned, where objects like the moon
and planets will have a very distinct and very annoying blue or purple halo around them.
Fortunately, this problem is solved with an “apochromatic” configuration that uses
a special kind of glass with 3 elements rather than two and some very special lens coatings.
Unfortunately, this means that the costs for a good apochromatic refractor goes up tremendously.
Whereas a decent achromatic may cost you a few hundred dollars a good apochromatic will
likely run you around a couple of thousand dollars or more.
And that’s just for the tube assembly, it doesn’t include the mount (which we are
going to look at in just a moment).
A good, bare minimum aperture for a refractor will be about 60 or 70 mm.
The next type of telescope is called a “reflector” and, as the name suggests, it does not use
glass lenses for the objective, it uses mirrors.
These types of telescopes are often called Newtonians in honor of Sir Isaac Newton who
invented them back in 1668.
The main light collecting mirror is called the “primary”, is concave in shape, and
located at one end of an open tube.
The primary mirror gathers light and then reflects it back up the tube to a secondary,
smaller mirror suspended on thin veins known as “the spider”.
The secondary mirror then bounces that light off at a right angle to the tube and then
into the eyepiece.
As with any telescope there are advantages and disadvantages to this design.
The advantages are that the image is not prone to chromatic aberration, because mirrors replace
glass lenses you can build much bigger telescopes, and, for anyone on a very strict budget- reflectors
can be made very cheaply and you therefore get more bang for your buck with a reflector
than you will from any other telescope type.
In fact, you can even build a good Newtonian on your very own but be warned, the primary
mirror has to be curved to a very specific degree and that can be both labor intensive
and very tricky to do.
The disadvantages are that reflectors tend to be larger and bulkier than a refractor
and are not always easy to transport.
They can also be somewhat high maintenance, especially with large apertures, as the mirrors
will have to be realigned on occasion.
Another disadvantage with a reflector is that you will have to let the mirror adjust to
the outside air temperature before you can begin observing.
This is a process that can take 20 minutes or more depending on the aperture of the scope.
And, finally, images will never be as clear and crisp as they are in a refractor.
For a good Newtonian I’d recommend an aperture of at least 6 inches.
The cost for a basic 6” reflector will usually run you about $250 or $300.
The third type of telescope is called a “catadioptric, or “Cassegrain” and it’s a hybrid of
the other two so it uses both lenses and mirrors to form an image.
Like the reflector it uses a curved mirror at one end and then a lens at the top known
at the “corrector plate” which corrects for any optical abnormalities.
The tube system is all closed, unlike a reflector, and it is all very compact.
This is a general all-purpose type of telescope and the complex light pathway inside the tube
means that it can be made much more compact than a reflector with similar aperture size.
The downside is that this folded pathway for the light results in some of the light being
lost before it gets to your eye and the image is much dimmer than in either a refractor
or a reflector.
Also, these kinds of telescopes are more expensive than a good Newtonian but not nearly as much
as they would be for a large refractor.
A good, basic catadioptric starts out at around $600 or $700 and then goes up from there.
Just as important as the telescope itself is the mount that it will rest upon.
You need a mount that is going to provide stability and not be shaky as well as one
that will allow you to move the scope with little effort while finding and then tracking
objects upon the sky.
Briefly, telescope mounts come in two basic designs: the Alt-Az mount and the Equatorial
Mount.
Alt-Az stands for altitude and azimuth and these are perhaps the most common type of
mounts for backyard telescopes.
These mounts allow you to move the telescope up and down (the altitude) and from side to
side (the azimuth).
If you’ve ever used a camera tripod, then you already have a good idea as to how the
alt-az mount works.
More sophisticated variations of this type of mount has the telescope supported on a
fork mount which allows the telescope to pivot around and up and down between the arms of
the fork, which, in turn, rotates around a central axis.
Another, and very common, variation of the alt-az mount is the Dobsonian mount.
One of the most common telescopes in amateur astronomy are reflectors and back in the 1950’s
amateur astronomer John Dobson was looking for a way to mount large reflector telescope
tubes so that they could be made cheaply and were easy to use.
He came up with a design that allows the telescope tube to fit inside a cradle that’s in turn
mounted on a spinning, wooden disc.
Think of it as a lazy susan for telescopes.
Moving a telescope up and down and from left to right is all very easy but the stars in
our night sky don’t move in that way.
Most stars appear to rise in the east and then make long arcs across the sky before
setting in the west, while stars towards the celestial poles all appear to move in tight
circles around a central pivot point (in our hemisphere that happens to be Polaris, the
North Star).
The alt-az design makes tracking the motions of celestial objects across the sky somewhat
difficult.
This problem is solved by the second kind of mount: the equatorial mount.
This system is a bit more complicated to use than the alt-az mount and can be a bit intimidating
until you get the hang of it.
As with the alt-az mount you are moving the telescope up and down and from left to right,
but with a difference.
With the equatorial mount the telescope is tilted backwards so that one of the axes of
the scope’s motion is aligned with the star Polaris (which is going to be equal to your
observing latitude in degrees above the northern horizon).
This is called “polar alignment”.
This configuration then allows the scope to track the apparent motions of the stars across
the sky.
It also means, that once you’ve aligned the scope you can then find objects in the
night sky accurately by using a system of celestial coordinates.
As I said, this mount is more complicated to use but once you’ve done it a few times
it’ll all feel perfectly natural to you.
If you have a generous budget and the knack for working with electronics, then you should
know that all of the above telescope designs and mounts can be purchased with computer
driven motors.
The computers contain databases for the locations of thousands of objects in the night sky and,
once you’ve given the computer information as to the date, time, and your location, it
will direct the scope to where that object is and then track it across the sky.
These systems are not recommended for the beginner, especially if you are not gadget
oriented.
Finally, a word about magnification.
The magnification power comes from the eyepiece that you use, and all telescopes allow you
use eyepieces of different sorts interchangeably.
A standard-sized eyepiece is about 25 mm (and remember, you should always begin observing
with your lowest power eyepiece) and the magnification that it, or any other eyepiece will yield
is determined by dividing the focal length of the telescope by the focal length of the
eyepiece.
Focal length is just the distance between your mirror or lens and the point where the
image is brought into focus.
Each telescope will have the focal length clearly marked on the tube somewhere.
So, if you have a 25 mm eyepiece and your telescope tube has a focal length of 1000
mm then that eyepiece will magnify an object 40 times.
But keep in mind, your telescope’s aperture is going to firmly set how much light you
can gather and no amount of magnification is going to change that.
This means that as you increase the magnification you are just spreading that light out over
a larger area and the object’s brightness will decrease with an increase in magnification.
Well, there you have it: some of the fundamentals you need to know before purchasing a telescope.
I hope that it helps and just remember this old maxim in amateur astronomy: the best telescope
is the one that you will use the most often.
So, weigh your options and then choose the scope that you think that you will be most
comfortable to use.
Also, don’t be afraid to attend star parties and monthly club meetings put on by the Central
Arkansas Astronomical Society in order to look over various kinds of telescopes and
to ask questions about them.
Believe me, amateur astronomers love to talk about their telescope gear and will be more
than happy to field any questions that you might have.
Before we go I want to suggest two new books for your reading pleasure.
The first book is The Scientific Secrets of Doctor Who by Simon Guerrier and astronomer
Dr. Marek Kukula (as well as an assortment of various authors of Doctor Who fiction).
Anyone who has ever seen an episode of the popular BBC science fiction series knows that
the story writers play fast and loose with actual science but this book takes a very
different approach.
Essentially it’s a collection of short stories featuring our favorite Time Lord in his various
incarnations as he travels across time and space in the TARDIS while engaging in all
kinds of fantastic adventures.
The difference is that the authors use real and theoretical science to underpin the stories.
After each story the senior authors take the reader deeper into the science with a short
and engaging essay.
While this book is aimed at teens I think anyone who has both an interest in science
and the Doctor will get a big kick out of this book.
And anything that manages to make young people both enthusiastic about science and which
also makes them stop and think about some pretty heady concepts all in one go gets a
thumbs up from me.
The second book is Welcome to the Universe by Neil deGrasse Tyson, Michael A. Strauss,
and J. Richard Gott.
These three gentleman all team taught a course on the universe for non-science majors at
Princeton University, they have taken their course notes and turned it into Welcome to
the Universe.
But this isn’t a dry textbook, this is a thoroughly engaging work that feels like you
are sitting in that classroom at Princeton soaking up all of that knowledge provided
by three of our premier astrophysicists.
From the life and death of stars to the nature of black holes, and to the beginning and end
of the universe and everything in between, this book will take you there.
This is a great read for anyone with a curious mind and who wants to know more about how
our universe works.
That’s all for now, be sure to visit our web site for all manner of astronomy content
and information and remember: take a little bit of time to step outside and look up in
both awe and wonder.