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Transcript
The Beginners Telescope.
This a simple guide to buying the beginners telescope. With hundreds of models from a score of
manufacturers, it can be a confusing experience for the first time buyer. These notes should make
things easier.
Telescopes come in three designs. The REFRACTOR is the traditional design that most people
will be familiar with. The telescope uses a LENS at the front of the telescope tube to gather light,
and the image is viewed through an eyepiece at the other end.
BORG 70mm Refractor
Refractors come in apertures ranging from 60mm (2.4") to 150mm (6.0").
Because it is more expensive to produce a good quality lens, compared
with an equivalent sized mirror, refractors tend to be more expensive per
inch of aperture than the second telescope design, the NEWTONIAN REFLECTOR.
Siberia 150 Reflector
The Reflecting telescope uses a MIRROR to capture the
light. The mirror is situated at the bottom of the telescope
tube and the image is viewed using an eyepiece near the top of the telescope. A
small mirror (the secondary) diverts light out through the side of the telescope. Reflectors have apertures ranging from 114mm (4.5") to 250mm (10"). Larger apertures are available but at the expense of portability.
Orion StarMax 127
The third telescope design is the CATADIOPTRIC. These telescopes use a mixture of lenses and mirrors and are of two types,
the Maksutov-Cassegrain and the Schmidt-Cassegrain. The main feature of
these telescopes is their compact tubes, typically half the length of a comparable
refractor or reflector.
All telescopes produce an inverted image, but refractors and catadioptrics use a
star diagonal to give a comfortable viewing position, and this has the effect of giving an erect image that is flipped left-to-right. Use of an erecting prism will give a
correct image if the telescope is required for terrestrial viewing, i.e. bird watching
etc. Generally speaking reflecting telescopes cannot be used for terrestrial viewing.
Telescopes require a mount and tripod in order to function. Mounts come in two
types, ALT AZIMUTH and EQUATORIAL. Alt azimuth mounts allow the telescope to move up and down and left to right. They are suitable for terrestrial
viewing and low power, wide field viewing. A variation of this mount is the DOBSONIAN, a design primarily for large aperture Newtonian reflectors. The main
limitation of the alt azimuth mount is that it does not track objects as they move
across the sky and is therefore unsuitable for astrophotography.
The second design of mount is the equatorial. This type of mount will allow the
telescope to track objects by making small adjustments to the telescopes two
axis. The mount can be fitted with a motorised clock drive, making long exposure photography an option.
Meade DS2-114 GOTO
Recently there has been an addition to the range of mount options, namely the GOTO computerised mount. These mounts
overcome the greatest difficulty encountered by beginners,
finding objects in the sky. There is nothing more frustrating
than waving a telescope around the sky for an hour in the
freezing cold and not finding the planet, nebula or galaxy that
you are looking for! GOTO telescopes are easy to setup and
Orion SkyQuest XT10
Meade ETX70-AT GOTO
use and are ideal for the novice astronomer. GOTO telescopes are available in a wide range of
models, from 70mm refractors to 300mm Schmidt-Cassegrains. All can be used for photography.
In the astronomical world aperture is everything. The larger the aperture, the
brighter the image you will see through the eyepiece. A 70mm aperture will deliver 100x more light than the naked eye, a 114mm aperture 265x and a
200mm aperture 816x. The downside to larger aperture is the increase in size
and therefore the weight of the telescope. If portability is an issue then consider
a smaller, lighter telescope. You will get more use out of it.
What about MAGNIFICATION? Rule one - NEVER BUY A TELESCOPE SOLD
ON IT'S MAGNIFICATION. A telescope can magnify as many times as you
want, but it's useable magnification is likely to be restricted to roughly 200x. A
60mm refractor telescope magnifying 525x is going to be a load of junk. As a
general rule useful magnification is limited to 50x per inch of aperture, and this
is with good optics and good atmospheric seeing conditions. You will see more
detail in a small crisp image than in a large fuzzy one.
Celestron NexStar 8i
Most telescopes are supplied with one or more eyepieces. The purpose of the
eyepiece is to magnify the image. Objects such as planets require medium to
high powers, while the larger nebulae and star clusters require a low power,
wide field of view.
We often get asked "Which is the best telescope?".
The answer to that is THE ONE YOU USE!
Orion ED Epic Eyepieces
Tele Vue TV-102 Refractor
Copyright SCS Astro Ltd 2002. www.scsastro.com