This page covers a range of
subjects and looks at the different factors that effect a telescope. It is to help enlighten you to areas you need to consider when
purchasing a telescope. This list is by no means complete.
the most common question people
as is:
What type of telescope should
I buy?
This is a common questioned asked
frequently by people who have a interest in Astronomy and wish to purchase their
first telescope. So the best way to go through some of this is to start at the
beginning.
There are a few questions you
should ask yourself before you look at the telescopes:
1. How often
will I be observing and for how long each session?
This is more a cost factor comparison. You don't really
want to spend $5000 on a telescope you use 3 times a year. You also don't want a
telescope that takes about an hour to set up and dismantle for a quick 10-15
minutes observation. some types of mounts can take time to set up. Then going to
the other end of the scale, in your haste you have purchased a $300 Department
Store telescope which has quickly becomes unsuitable to your needs
2. Where will I
be conducting my observing?
There is
several answers to this, whether you wish to observe from your backyard or
nearby park. You may travel out of town to a dark site, you may be a member of a
club or group. The factor here may be size of the telescope for transporting.
You will not fit a 12" Dobsonian mounted Reflector in a Mini Minor. Also you
will quickly accrue ancillaries such as table, chair, maps etc.
3. Where will I
store my telescope?
This is also important for size and
security. Do you live upstairs/downstairs. Do you have a garage. Is there
moisture or humidity?
4.
What do I want to achieve?
Do you want explore the sky? Are you just
interested in the planets and the Moon? Do you want to take photo's? These all will
dictate the type of telescope and mount you need, as some types of telescopes are more
suited than others. Remember want and need are not always the same thing.
Now you have answered these
questions your choice will be easier to make.
As with any product that you buy, the old saying
"you
get what you pay for" , this is no different when buying a telescope. There
are, however, a number of ways to insure that you get your money's worth when
buying a telescope. Whether you are buying a child's first telescope or one for
an adult, if you want to observe planets or deep sky objects, you want to do
astrophotography. Having an understanding of the different types of products
available will help you make the right choice.
So before parting with your hard earned cash, it is highly recommend that
you learn more about telescopes by reading the the information below.
What is a Telescope?
This might sound like a simple question. There are,
however, many ways this question can be answered. It would be easy to say that a
telescope is a device that is used to look at objects that are distant. While this is true, it
is only part of the answer. There are telescopes for Terrestrial viewing and for
Astronomical viewing.
Terrestrial
- Earth bound, these types of telescopes are good for
observing ships at sea, mountain ranges etc. These telescopes do not have to
consider light gathering as a factor.
Astronomical
- These telescope have better light gathering
capabilities, which allow better viewing of the night sky.
Note:
This tutorial only pertains to Astronomical
Telescopes
A typical telescope is made up of one or two lenses (or mirrors) which collect
visible light. The light is then magnified by an eyepiece for enhanced viewing.
Most people believe that a telescope provides enhanced viewing of stars and
other distant objects because it magnifies those objects. As you will see as you
read on, magnification plays a role in the quality of a telescope, but it is not
the major factor.
A bucket provides a good analogy to a telescope. A bucket can hold much more
water than your hand. It is therefore more efficient for collecting water. Hang
around an Astronomy group and you will here some telescopes referred to as a
"Light bucket". A telescope gathers light. Since it is much larger in diameter
than your eye, it can gather more light. The light is then focused down to a
smaller point where it can be viewed. The focused image can then be magnified
for enhanced viewing by an eyepiece.
Understanding the history of the telescope can help give some insight into how
people began to scientifically study the stars.
History
The telescope was one of the most important inventions of
the Seventeenth century. While lenses that could change the magnification of
objects were known in the Western world at the end of the thirteenth century, it
was not until 1608 that the first telescope was officially used. Both Hans
Lipperhey and Jacob Metius were recognized in the Netherlands for creating the
first low power telescopes. The instruments were largely proclaimed to be of use
in spotting one’s enemy on the battlefield but, others soon found new uses for
the device.
Galileo made the telescope famous for his astronomical observations in 1609.
He was the first to
provide drawings of the moons of Jupiter and document the phases of Venus. Since
the time of Galileo, the craft of telescope making has been continually refined.
We now have telescopes that can see deep into space. Even today advances are
being made in giant telescopes around the world. The Hubble Space Telescope is
orbiting the Earth capturing images from what scientists believe is the edge of
our universe.
For most of us, however, seeing the moons of Jupiter or Nebula's is
just as exciting as it was to Galileo. There are many different Types of
astronomical telescopes available.
Types
There are three basic types of astronomical telescopes,
with variations within those types. Each type has its own strengths and weaknesses. Depending on your budget
and the images you want to view, one of these types could fulfill your
requirement from the questions that you answered before.
Refractors
When you think of a telescope, the most likely picture
that comes to mind is the refractor. Refractors have eyepieces at the back end
of an elongated tube. At the front of the tube is an objective lens that focuses
the light to a small image. The eyepiece enlarges the image to make it viewable.
Refractors are excellent for viewing the Moon, planets, double stars and other
objects requiring high-power viewing. Refractors, however, are quite expensive.
Inexpensive refractors are not worth their reduced price. If budget is a primary
concern, then consider purchasing the next type, a reflector.
Reflectors
Reflectors use a large mirror in the back of the telescope to focus light
toward the front of the tube. The most common type of reflector is a Newtonian.
A Newtonian reflector has a second mirror that bounces the light toward the
eyepiece that is located at the front end of the tube. Quality reflectors can be
fairy inexpensive and are a good choice for beginners or those on a budget.
Reflectors do require higher maintenance because the mirrors must be
occasionally realigned to maintain optimal performance. This maintenance is not
complicated and can be easily carried out by the user.
SCT/Catadioptic
The Catadioptic telescopes offer excellent high-power viewing without
having the long tube associated with the Reflector and Refractor telescopes. This is
accomplished by using a complex arrangement of mirrors. The corrector
plate is located at the front of the telescope and there is a large mirror
located at the back of the telescope. There is a smaller mirror behind the front
lens (corrector plate) which reflects the light back into
the eyepiece at the back of the telescope. The compact design makes this type of
telescope one of the most popular for serious amateurs. There are two different
types of catadioptic scopes: the Schmitt-Cassegrains and the
Maksutov-Cassegrains. Both are considerably more expensive than similar size
Newtonian telescopes.
All telescopes share some common features and terminology. When looking at
telescopes, one of the most often mentioned characteristics is magnification or
power.
Magnification / Power
When you see "Power", "500X" or similar numbers on a
telescope,
BEWARE!!
When a small telescope advertises an extremely high
magnification, there is good reason to be skeptical, especially if it is
advertised as an Astronomical Telescope. The amount of magnification
a telescope offers is not a good way to judge how good the telescope will be for
actual viewing. In fact, lower power may mean better viewing in many cases.
A number such as "400X" represents the theoretical ability of the telescope to
magnify an image. This number is only a mathematical representation of what is
possible, not what the telescope will be able to achieve. Most inexpensive
telescopes, however, are unable to achieve their theoretical "power" to magnify
because they cannot collect enough light. Therefore, images will be too dim for
adequate viewing.
Additionally, higher magnification means that the field of view will be smaller.
In other words, you will be looking at a smaller portion of the sky. For
beginner astronomers this can make it difficult and frustrating to find specific
objects in the sky. Without a tracking system (motorized mount usually run by
computer) a small field of view will also mean that objects will not stay in
view for very long, thus requiring constant adjustment to look at a specific
object.
Many experienced amateur astronomers prefer a lower "powered" telescope. The
most important number to pay attention to when buying a telescope is its
aperture. A telescope’s aperture determines the brightness and sharpness of what
you see.
A good rule of thumb to follow is not to exceed 50X per inch of
aperture. Remembering this whilst in a shop looking at a telescope, you can quickly
determine if the advertising on the packaging is false. A telescope with a
4-inch aperture should have no more than 200X. Anything over 300X on any size
telescope will probably lessen your viewing experience.
This "Power" is not the advertised "Power" on the box. You can, by using a
simple formula ascertain the "Power" of the telescope. Actually i will
show you how to find the
Optimal Magnification Range OMR .
This is the range from minimum Magnification to maximum that the respective
telescope can operate without distorted or unsatisfactory views
To
find your maximum optimal magnification
Aperture X 2 = MAG
(aperture is the diameter
of your primary lens)
So for the 8" and 10" reflectors
the formula is as below:
8" reflector scope 8" is
200mm
200mm X 2 = 400X magnification
12" reflector scope 10" is
300mm
300mm X 2 =
600X magnification
Note* - If you
check back 2 paragraphs where it stated "A
good rule of thumb to follow is not to exceed 50X per inch of aperture."
this formula does just that.
To find your minimum optimal
magnification
So
for an 8" reflector scope 8" is 200mm
| 200mm |
=
|
29X
|
Magnification
|
|
7 |
|
|
|
And
for
a 12" reflector scope 12" is 250mm
|
300mm |
=
|
43X
|
magnification
|
|
7 |
|
|
|
Now by using these simple formula's you can ascertain the
OMR of your telescope, this will also become clearer and of more
importance when you read the Eyepiece Section
So recap this section,
Magnification is a contributing factor to a telescope but is not the deciding
factor.
Aperture
When looking at telescopes the most important specification is not "power" or
the magnification but aperture. Depending on what type of telescope you are
looking at, aperture refers to the diameter of either the main lens or mirror.
The larger the aperture the brighter and clearer the image will be.
The ability of a telescope to gather light is proportional to the diameter of
the aperture. Thus making increasingly larger jumps for each additional
increment of aperture. Even a three-inch telescope will be able to view 90 times
the amount of light seen by the naked eye. A ten-inch telescope can gather
nearly 900 times more light than your eyes alone!!
Though it might seem that the easiest way to buy a telescope is simply to buy
the largest size you can afford, you also need to think about what is
realistically portable. If the scope is so big that you cannot easily transport
it to a viewing site, then you will be less likely to use it. You should also
take into account the fact that larger scopes often require more setup time.
Remember the questions at the beginning!
While a telescope’s aperture might be the most important number to understand
when buying a scope, other factors are important as well. The focal ratio
(f/some number) is also a key part of understanding a telescope.
Focal Ratio
A telescope’s focal ratio is
its focal length
(FL)
divided by the aperture (A).
Where the focal length is the distance from the
main lens (or mirror) to where the light converges to a focus. For example, if a
telescope has a focal length of 1000mm and a 200mm aperture, then its focal
ratio is f/10.
The formula is
This might sound a bit confusing and what you probably want to know is what the
focal ratio has to do with deciding on what telescope to buy. Unlike
photography, the focal ratio does not have a direct meaning in terms of viewing
quality. In most cases, however, a larger f/number will provide better quality
at a lower cost.
Here is a real life example that will make this easier to understand. If two
telescopes have an equal aperture and one is f/4 and the other is an f/8, then
the f/8 will almost always provide better quality than the f/4. The f/8,
however, will be twice as long, making it more difficult to carry than the f/4.
There are f/4 scopes that will provide outstanding quality but, the tight
tolerances needed to produce such a quality instrument make it quite expensive.
So, while a larger f/number does not always mean a better instrument, it does
generally mean a better image for an equal amount of money.
The qualities of the telescope itself are extremely important. Most people
buying their first telescope, however, tend to underestimate the importance of
related items that will make the experience more enjoyable. Purchasing a quality
mount for your telescope can make the difference between enjoyable viewing and
constant frustration.
Mounts
One of the things many people do not think about when buying a telescope is the
mount, the manufacturers at times can do the same. Telescope mounts can make a huge difference in the quality of the viewing
experience. An unstable, shaky mount can make it difficult to see detail on
close objects and nearly impossible to see faint sky objects.
There are two main types of mounts and a number of sub-types. The alt azimuth
mounts are configured much like a camera tripod. There are controls for moving
the telescope up and down (altitude) and controls for moving left to right
(azimuth). One popular type of alt azimuth mount is known as the Dobsonian.
Dobsonian mounts are usually found on medium or large size reflecting
telescopes. They are very stable and very user friendly.
The other main type or mount is called an equatorial mount. Equatorial mounts
are designed to follow the motion of the sky. This is especially useful since
the rotation of the earth can cause objects to quickly move out of view.
Equatorial mounts are often configured with motorized drives which match the
rotation of the Earth. This makes it easier to observe sky objects for longer
periods of time.
There are now sophisticated drives and computer software that can allow
automated viewing with either type of mount. Simply align the scope with a
couple of known stars and the computer can guide your scope to any visible
object in the night sky. These systems can add quite a bit of cost to your
telescope. For beginners, however, finding sky objects manually can be one of
the real joys of astronomy, and helps learn the night sky.
No matter what type of telescope you choose, having a quality mount will vastly
improve your viewing experience. Another important accessory is the eyepiece.
Eyepieces
Most telescopes will come with at least one or two different eyepieces.
Eyepieces are rated by millimeter (mm). The smaller the number, the higher the
magnification. Most telescopes come with at least a 25mm eyepiece. This is an
excellent size for beginners.
As stated earlier, magnification or power, is not the best way to judge a
telescope. Having a higher power eyepiece does not guarantee better viewing.
Instead of thinking of higher and lower as better or worse, think of them as
just different tools for observing. So objects are better viewed at a low
magnification, as so to get the whole effect.
Lower power eyepieces have an advantage in finding objects and keeping them in
view. They also do not require as much light, making them better for viewing
some dim objects.
Higher power eyepieces can provide more detail of the object in view. It is, however, more
difficult to keep the object in view (if you are not using a motorized mount) and
requires the telescope to have more light gathering capability to see the object
clearly.
Remember, an eyepiece only magnifies the image your telescope can capture.
Having a scope that can gather more light will be a more important factor in
determining the quality of viewing.
The type and size of your telescope will determine the range of your eyepieces.
We can determine this by a formula.
The formula is
So now armed with this information
we decide we want to use a 4mm EP
|
1000 |
= 250X
magnification |
|
4 |
|
We are using an 8" reflector and we
know our OMR is between 29 and 400X. So the 4mm EP is a good EP for us
to use.
If for example you were to add a
Barlow ( a Barlow is a magnification tool that is inserted between the EP and
the Focuser ) we would add to our formula an 2X or 3X etc, depending on
the type of Barlow where are using.
|
1000 |
X 2 |
= 500X magnification
|
|
4 |
|
|
Now as you can see we have exceeded
our OMR. The view will be distorted, very dim as we won't be getting enough
ambient light. We can achieve 500X magnification, but it is unusable. Ambient
light is the light from the sky that when reflected from the lenses make the
view visible. This is why when we increase the power of magnification ( i.e. go
from using a 25mm EP to a 10mm ) the view dims and the lower the EP the dimmer
the view.
Overall all these factors will need
to be taken in to consideration when making a purchase of a telescope, or even
thinking of buying some new Eyepieces.
We hope this information has been of
help, if you would like more information, you can contact us.
