Telescope Buyer's Guide. Part One
An article on choosing and
buying a telescope by Nick Tonkin
Amateur Astronomy is one of the most fascinating hobbies in the world,
but to get really into it, you probably will want to get a telescope. But
to read the adverts in the magazines, you've got one very confusing choice
out there. As well as the type of scope be it a Reflector, Refractor or a
Catadioptric - don't worry at this point, we'll explain the differences
later- there are many different sizes and even some which we wouldn't even
poke a stick at. Finally there's the decision as to how much to spend.
Essential Tip No1: Join a club,
preferably ours, and look through as many scopes that you can. Any member
here will be more than happy to allow you to gaze the heavens through
their scope as well as discussing their own reasons for their eventual
decision. You'll also be able to compare different apertures, which more
than anything will help you choose the size of scope you want.
Telescopes In General:
A telescope has two main functions:
- To gather and focus a large amount of light from an object (much
more than the eye alone can) to form an image of the object, and
- To magnify that image so that distant objects can be better seen.
There are many different designs of telescope that will accomplish
these functions, But the three main types are as follows.
Refractors
In a refractor design, light enters from the left, passes through the
double element objective lens and is then focused at the eyepiece.. The
glass element permits the optical designer to bring all visible
wavelengths (colors) of light to virtually the same focus. Field stops
located at intervals along the inside diameter of the main optical tube
effectively block stray, off-axis light rays from reaching the focal
plane.
Reflectors
In the design of the Newtonian Reflecting Telescope, light enters from
the left and passes the entire length of the telescope tube to a parabolic
primary mirror, where it is reflected back up the tube to the secondary
flat mirror. The secondary mirror, set at a 45° angle to the telescope's
optical axis, reflects the light outside the telescope tube where the
image is brought to a focus at the eye
Catadioptric
In the Schmidt-Cassegrain design, light enters from the right, passes
through a thin lens with 2-sided aspheric correction (correcting plate),
proceeds to a spherical primary mirror, and then to a convex secondary
mirror. The convex secondary mirror multiplies the effective focal length
of the primary mirror and results in a focus at the focal plane, with
light passing through a central perforation in the primary mirror
Basic Telescope Terminology
Telescope performance depends primarily on the diameter of the main
lens or curved mirror, whichever is used. This diameter is commonly known
as the Aperture and is usually measured in inches, although being good
Europeans we will be using the metric system here. The larger the aperture
is, the more light is gathered and the more detailed the image will be.
Another term used in describing telescopes is the Focal Length. It is
simply the distance from the lens or the main mirror's surface to the
point where the light comes to a focus. The two terms aperture and focal
length then give us the F-ratio which is just the focal length divided by
the aperture. So if a telescope has an 200 mm mirror or objective lens and
a focal length of 2000mm, then it has an F-ratio of F10. A manufacturer
usually provides the aperture and F-ratio numbers for a scope, so you can
calculate the focal length of the telescope yourself.
The focal length of a telescope also helps determine another useful
item, the Magnification, of a telescope. The magnification of a telescope
increases the apparent size of an object to a point where the human eye
can easily view it.
If we have two telescopes of the same aperture, but different focal
lengths, the one with the longer focal length produces a large but dimmer
image compared to the smaller but much brighter image of the shorter focal
length.
A long focal length is preferable if your primary targets are the Moon,
the planets, or double stars. And a large objective is a necessity if you
dream of viewing numerous distant galaxies. But if you want to take in
large swaths of the Milky Way or sparkling showpieces like the Pleiades, a
short focal length scope is called for.
From the above you should see that the aperture and focal length are
the two main factors that determine how much of the sky you see, how faint
an object in this field of view you can see, and how clearly you can see
it.
There is also another piece of equipment that can alter the
magnification or Power of your image and that's the eyepiece. This is the
part you actually look through. The power of a telescope is determined by
dividing the focal length of the telescope by the focal length of the
eyepiece. So using our 2000mm focal length telescope with a 26mm eyepiece
gives a power of 77 time or 77x. A 9mm eyepiece would provide a power of
222x and one of 40mm, a power of 50x.
At this point you may say, 'I want all the power I can get'. Well
there's a catch. Like the differences between long and short focal length
telescopes, eyepieces that provide more magnification or power also
provide dimmer images. Look at say the Orion Nebula through a 26mm
eyepiece and you'll see a bright and sharp, but not particularly detailed
image. Change to 9mm eyepiece and you see a lot more detail, but it's
fainter and looks a little fuzzy. Push the power even further with a 6mm
eyepiece and the image looks even bigger, but it gets so faint and fuzzy,
it looks as though it's not in focus.
There is no theoretical limit on power, but there is a practical limit
beyond which the images obtained are worthless. Because of the way light
behaves, the maximum usable power is about 50 times the diameter of the
main mirror or lens. For example, a small telescope of say a 60mm aperture
has a maximum usable power of about 120x where's one of 200mm should be
able to cope reasonably well at 400x and with very clear skies, 600x.
Telescope Types
The least expensive types of telescopes such as refractors and
Newtonian reflectors, both have many advantages and disadvantages. The
Refractor is probably the first type of astronomical telescope most people
encounter so we will deal with it first.
Refractors
Because different colours of light are bent differently, known as
Chromatic Aberration, a refracting telescope must use a special compound
main lens which consists of at least two lenses of different shape and
glass type to correct for this unequal bending. In addition, all of the
glass surfaces must be coated with a thin layer of material, which
enhances contrast and reduces unwanted reflections. The main lens is
mounted firmly at the front of the telescope tube, and when the eyepiece
is in place, the tube is closed, keeping dust or air currents out.
Refractors require little maintenance, although they must be handled
with care to prevent scratching or damaging the delicate coatings on all
the glass surfaces. They provide good high-contrast images, and are fairly
popular with those who observe the moon and the planets exclusively.
Advantages
- Closed tube means no air currents, which could degrade the images.
- Gives very good images and long focal lengths allow the use of less
expensive eyepieces.
Disadvantages
- Much more expensive than reflectors.
- Must use a 2-element main lens.
- Tend to be fairly long physically, which gives a long focal length.
- Awkward to look through for objects high in the sky unless star
diagonal is used.
Reflectors
The Newtonian Reflector also has some disadvantages, but it has many
good points as well. It uses a specially curved aluminum-coated glass
mirror to gather and focus the light, and a flat second mirror to direct
the light out the side of the telescope tube to an eyepiece mounted
towards the front end of the tube. The Newtonian is an inexpensive fairly
simple design and is quite rugged. Its open-ended tube does allow air
currents to sometimes degrade the images, and dust can settle on the
mirrors, requiring careful periodic cleaning.
Advantages
- Much less expensive than the refractor or Catadioptric design size
or size.
- More compact than most refractors (short focal length).
- Works well on a simple mounting.
- Easy to use when looking at objects high in the sky.
Disadvantages
- Open tube means dust can get on the thin metal coatings of the
mirrors.
- Mirrors sometimes need to be re-aligned after the telescope is
transported or handled roughly.
- Secondary mirror blocks some light, and its supports cause small
spikes on star images (not really much of problem).
- Cannot use cheap eye- pieces.
A refractor may give slightly better high power views than a reflector,
but the refractor can cost well over three times as much as a reflector of
the same size! It is for this reason that most amateurs prefer the
Newtonian reflector to the refractor for apertures larger than four
inches.
Catadioptric Designs
A newer design which has a price somewhere between that of a refractor
and a reflector is the so-called Catadioptric, or "mirror-lens"
telescope. It uses a curved main mirror, an oppositely curved secondary
mirror, and a special large correcting lens out front to obtain sharp
images over a wide field of view.
Catadioptric telescopes offer large aperture performance in a package
that in some cases is small enough to sit on a table top! Their
portability makes them popular with those advanced amateurs who don't have
room for a large Newtonian or refractor, but their price usually makes
them out of reach of most beginners' pockets. You may see them listed
under the names "Schmidt-Cassegrain", or "Maksutov",
each of which uses a somewhat different optical design to do the same
thing.
Advantages
- Compact in size. A 2000mm focal length can be squeezed into a
package only 500mm long.
- A sealed tube means the inside's stay clear of dust.
- Easy to use when looking at objects high in the sky.
- Because of their fork mounts, these tend to be more rigid and less
prone to vibration from wind.
Disadvantages
- More expensive compared to similar sized reflector.
- Tend to suffer more than other designs with dewing on Corrector
plate during.
- All that glass tends to make them quite heavy. A medium sized 200mm
can weigh as much as 20 kilos.
Technology
There's also another major decision to be made. All these three kinds
of telescope can be pointed at objects manually, either by turning
adjustment knobs or driving small electric motors that guide the telescope
via a hand box. As in all other walks of life, technology plays and
important role in Astronomy and you can now buy any of these designs with
what is called Go-To. These are controlled by a simple internal computer,
which once the telescope is aligned, can point you to nearly every
observable object in the night sky, and keep it in the field of view as
the object tracks from east to west: All by pressing a few buttons!
However this technology can double or treble the cost of a similar sized
manual scope of the same aperture.
Telescope Mountings:
Most astronomical telescopes cannot be hand-held like binoculars, so
some sort of mount or stand must be used to hold the instrument and allow
it to be pointed at any object in the sky.
There is nothing more frustrating than having a telescope on a mount
that makes the images jiggle and dance every time the wind blows, so a
good solid steady mount is a must.
Many cheap telescopes found in stores have small metal tripods with
thin legs and small screw-tightened pivot points for bearings. These are
the worst possible kind to mount a telescope. The single tiny support
bearings are so small that the telescope vibrates like a tuning fork when
touched. Most camera tripods are almost as bad.
The mount must be designed to support the telescope well and to damp
out vibration, while at the same time make the instrument easy to point.
There are two basic types of mountings: the Altazimuth , and the
Equatorial
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The altazimuth is a good simple design
that allows the telescope to tip up and down (Altitude) and to
rotate around in a circle (Azimuth) In the same way as a battleship
gun turret does.
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The equatorial or polar mount is the design favoured
by serious amateurs because it allows the instrument to follow the
motion of the stars using movement around one axis instead of two.
If you look closely, you'll see that the vertical pivot (that's the
rod at 90 degrees to the telescope body) is inclined to the
vertical. This angle is such that as the earth rotates around it's
inclined axis, the telescope mount allows the telescope to track any
object by just moving the telescope in a left to right movement, or
Right Ascension. If equipped with a motor drive, it will
follow the stars automatically without the user's help. This is a
real convenience at high powers where the earth's rotation causes
many objects to drift out of the field of view in only a few
seconds. The equatorial mount is heavier, a bit more expensive, and
somewhat harder for the beginner to get used to than the altazimuth.
Both mounts will work well if the bearings are large enough and the
tripod or pedestal used with the mount is sturdy. |
Telescope Choice
Unless you are very strapped for cash, don't buy any scope with an
aperture smaller than 60mm. Apart from observations of the moon or larger
star clusters, anything smaller will not provide any useful views of the
smaller planets or fainter galaxies. If you can, start your selection in
the 70mm to 90mm sizes. Prices start at as low at £80 to £100 mark for a
60mm refractor with a basic Altazimuth mount, to £300 for a 70mm Go-To
computer model.
At the 100mm to 150mm apertures, you can start to do some serious
observing. All the major planets are observable. Saturn's rings are
visible - the Cassini division can just be made out with a 150mm and
Jupiter's major clouds can now be clearly seen.
Prices range from £200 for a 100mm refractor with altazimuth mount; £400
for a 150mm Reflector on an equatorial mount and up to £1000 for a 125mm
Go-To computer model.
At 200mm to 250mm the serious amateur probably has all the scope they
probably need. Anything larger tends to be cumbersome, and takes time to
set up if not permanently mounted in it's own observatory. Prices range
from £500 for a Newtonian reflector with Equatorial mount to over £3000
for a 250mm computerised Go-To model.
Essential Tip No2: Only buy your
telescope from a specialist dealer. Toy shops, petrol stations,
departments stores and the like are not suitable places to obtain quality
optical hardware. We would suggest you look through as many telescopes as
possible - remember essential Tip No1. Once you've an idea of the
particular kind of telescope that suits you best, then hit the web, and
the various manufacturers web sites, and what each has to offer of your
chosen type.
Here's a list of some major US and UK manufacturers.
Meade: www.meade.com
Celestron: www.celestron.com
Orion Optics: www.orionoptics.co.uk
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