Choosing and using a telescope
large scope with a long focal length. The larger the diameter of the telescope
the more resolving power it will have, enabling you to see smaller and
smaller detail. Unfortunately, with increased aperture size comes increased
cost, so this is where your budget limit kicks in!
The brighter planets and the Moon require a reasonably high
magnification to see significant detail. As magnification is calculated by
dividing the focal length of the telescope by the focal length of the current
eyepiece, it’s clear that a telescope with a naturally long focal length will be
able to get up to high magnifications more easily than one with a relatively
short focal length. Similarly, if your interests are in imaging these objects,
a long-focal-length instrument will give you a better opportunity to use
higher magnifications.
As an example, consider a telescope with a 400mm focal length using a
10mm eyepiece. Here the magnification will be 400/10 = 40x. Compare this
with a telescope with a 2000mm focal length using the same eyepiece; the
equivalent magnification is 2000/10 = 200x, which is much better suited
for planets.
You cannot go on increasing the power indefinitely though, and the rule of
thumb for the maximum useful magnification you can achieve is obtained
by multiplying the aperture in inches by 50. So for an 8-inch scope, the
maximum useful magnification would be 400x, but this would only be useful
if the atmosphere you were looking through was very stable.
The ideal telescope for looking at the Moon and planets would be a
long-focal-length, large-diameter, colour-corrected refractor, but here the
budget limit is easily exceeded with alarming speed. An alternative would be
a large-diameter, long-focal-length reflector, but these can be quite bulky
and difficult to mount and handle. Scopes that use folded optics, known as
catadioptric telescopes, can offer a good compromise here, combining a
relatively large aperture with a long focal length, all for more realistic prices.
At the other end of the range, a telescope designed to look at the stars
and deep-sky objects needs be large and have a relatively short focal length.cheaper than a refractor of equal power because a large mirror is easier to
make than a large lens; all the world’s largest telescope are reflectors. The
largest refractor, the Yerkes, is 40 inches across; the largest single mirror
reflectors have mirrors more than 323 inches in diameter.
Modern manufacturing techniques have opened the market for new and
improved, cost-effective telescope designs. These include large-diameter
reflectors, colour-corrected refractors and hybrid telescopes known as
catadioptrics, which incorporate both lenses and mirrors.
Choosing a suitable scope out of what has become a rather bewildering
array of instruments has actually become more complicated over time
because of the extended choice. In reality there is probably no one simple
answer to the question of which telescope is best for you, but a simple bit of
self-questioning may help to narrow your options quite considerably.
The first question is one of budget and how much are you prepared to
spend? For a first scope purchase this is a pretty serious question. Do you
spend lots of money on a top-of-the-range model which you’ll keep and use
for many years, or do you spend a small amount on a telescope to let you
dip your toe in the water? The only person that can answer this question
honestly is you, but experience suggests that the second choice is probably
the right one. Spending a lot of money on what is essentially a blind purchase
may lead to disappointment, an unused scope and wasted money.
If you are looking to upgrade from an existing scope to a more advanced
one, the choice is easier because you will know about the foibles and
limitations of your current instrument and have a greater sense of the
direction you want to move in.
Once your budget has been set, the next decision needs to be geared
toward what you want to look at in the sky. The choice ranges from the
Moon and brighter planets to the stars, galaxies, nebulae and clusters that
pepper the night sky.
Let’s deal with the simple situation first, when your interest is either Solar
System or deep-sky objects. For Solar System observing you will need a
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