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.
