Skywatch - 2021 - 62

Intro to Scopes
are quite large, requiring the wide fields
of view oﬀered by low magnification.
Because eyepieces are largely standardized, you can use the same ones
in multiple telescopes. Indeed, most
observers have only one set of eyepieces
even if they have several scopes. So,
that same 20-mm eyepiece that gave
me 35× in my little reflector would produce a magnification of 50× if used in a
telescope with a 1,000-mm focal length
(1,000/20 = 50).
The Eyepieces Have It
It has been said that the eyepiece is half
the telescope. I might quibble with that,
but there's no question that good eyepieces are crucial for good views. You
don't have to spend much time looking
into the topic to discover that there's a
bewildering array of eyepieces on the
market. How does one choose? You
already know that focal length is the
main specification, but there are other
factors to consider.
First, there's the barrel size. You'll
encounter three diﬀerent configurations: 0.965-inch, 1¼-inch, and 2-inch.
Check the diameter of your telescope's
focuser - you don't want to buy an
eyepiece that doesn't fit. My Tasco has a

u Look Here! You may have only one telescope, but chances are you'll end up with several
eyepieces to achieve a sufﬁciently wide range of
magniﬁcations and ﬁeld sizes. Eyepieces come in
many different optical designs and focal lengths,
but only three barrel sizes. The smallest (0.965inch) is typically found only on older, imported
instruments. Most eyepieces available today are
produced in the 1¼-inch size, and some models
(those typically designed for low magniﬁcation)
have 2-inch barrels.

0.965 focuser, so I'm restricted to using
eyepieces of that rather uncommon size.
If you have a 1¼-inch focuser (the most
popular size) you have a wide range of
choices. If, however, your scope has a
2-inch focuser, you have the best of all
worlds because you can use 2-inch eyepieces and (with the addition of an inexpensive adapter) all the smaller sizes too.
Second, there's something called
apparent field. This is how big the "window" of your eyepiece looks. Eyepieces
with a narrow apparent field of view
give a kind of tunnel-vision eﬀect - it's
like looking down a long tube. At the
opposite end of the range, with some
eyepieces you can barely even see the
edge of the field. Apparent field is
expressed in degrees - and generally,
the bigger the number the better. A 50°

apparent field is common with basic eyepieces, while more exotic (and expensive)
eyepieces boast apparent fields of view
that exceed 100°!
Although they're interrelated, don't
confuse apparent field with true field, however. True field is determined by dividing the eyepiece's apparent field by its
magnification. So, a 50° apparent field
eyepiece working at 100× would yield a
true field of ½° - roughly the width of
the full Moon.
The third consideration is the eyepiece's optical design. Of the various
parameters, this is the one that's hardest to quantify. At one time there were
only a handful of popular eyepiece
types: Kellners, Plössls, Orthoscopics,
and Erfles. Recent decades have seen
an explosion of new, exciting configurations. Each type has its pluses and
minuses. The best way to figure out
which works best for you, your scope,
and your budget, is to try out as many as
possible when you observe with friends
or go to a star party.
When you buy a new telescope, it
usually comes with one or two basic
tt Classic Glass When most people visualize a telescope, they usually think of something
like this refractor - an instrument with a big
lens at one end and an eyepiece at the other. This
particular unit features a 4-inch objective lens and
rides on an alt-azimuth mount.
t Compact

Cat Schmidt-Cassegrain
telescopes, like this 8-inch model, are members
of a family of instruments known as compound
or catadioptric ("cat" for short) telescopes. The
designs feature a folded optical path, resulting in
comparatively short tubes.
62 SK Y WATCH 2021

Skywatch - 2021

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