Skywatch - 2021 - 61
ered comes to a sharp focus. The same
specification is given for all telescope
types (and camera lenses).
You'll often hear a telescope
described by its focal ratio. My Tasco
is an f/9. You can figure out the focal
ratio by dividing the focal length by the
aperture. My Tasco's mirror is 76 mm
across, so dividing 700 by 76 we get
a focal ratio of f/9. Where focal ratio
comes into play is as a shorthand way of
characterizing an instrument's general
capabilities. Broadly speaking, focal
ratios of f/6 and lower (so-called fast
telescopes) reveal a bigger area of sky
than same-sized scopes with higher focal
ratios (slow telescopes).
Focal length matters most when it
comes to figuring out your telescope's
magnification. Here again, there's some
simple math involved.
The Power of Power
My little Tasco came equipped with
three eyepieces of different focal lengths:
20 mm, 12.5 mm, and 5 mm. (An eyepiece's focal length is always engraved
on top.) The 20-mm eyepiece on my
Tasco delivers a magnification of 35×.
How do I know that? Because figuring
out magnification is simply a matter of
dividing a telescope's focal length by the
focal length of the eyepiece you're using:
700 divided by 20 yields 35×. The other
eyepieces produce magnifications of 56×
and 140×, respectively. Notice that the
smaller the eyepiece focal length, the
higher the power.
Reflector
Is there such a thing as too much
magnification? You bet! At a certain
point, more magnification doesn't result
in more detail, just a bigger but blurrier
image. That's why excessive power is
referred to as "empty magnification."
As a rule of thumb, the upper limit is
roughly 50× per inch of aperture (or,
2× per millimeter). That means my
little reflector tops out at about 150×. In
practice, however, you'll fi nd yourself
using magnification much lower than
that most of the time. Why? Because
as magnification increases, the field of
view - the size of the piece of sky you
see in your scope - decreases. It's very
difficult to locate observing targets when
you can see only a tiny portion of the
night sky; some objects you'll want to see
Compound
ADVANTAGES
DISADVANTAGES
ADVANTAGES
DISADVANTAGES
* Most economical
design - biggest
bang for the buck
* Requires occasional
realignment of mirrors
(collimation)
* Folded light path yields
a compact optical tube
assembly
* Some designs require
occasional collimation
* Convenient eyepiece
position
* In larger sizes, can take time
for optics to cool and provide
sharpest views
* Eyepiece usually
conveniently positioned
* Can be highly portable
even in larger sizes
* Mirrors may need re-coating
after many years of use
* Highly versatile
* Front corrector lens
prone to dewing
* Closed tube can mean
long cool-down times
SK Y WATCH 2021
61
Skywatch - 2021
Table of Contents for the Digital Edition of Skywatch - 2021
Contents
Skywatch - 2021 - Cover1
Skywatch - 2021 - Cover2
Skywatch - 2021 - 1
Skywatch - 2021 - Contents
Skywatch - 2021 - 3
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