Last updated: 03/22/03
Telescope eyepieces are the real variable when doing astronomy. Like many
people new to astronomy, when I first started looking through my telescope,
I wanted 1,000,000x magnification of everything. It took a while to realize that
magnification isn't all it's cracked up to be. So, I'd like to provide
a few words of wisdom that I've picked up about eyepieces and magnification that
other beginners may find helpful.
First of all, let's discuss how to calculate magnification. This is pretty
simple, actually. You simply divide the focal length of your telescope by
the focal length of the eye pieces. So, let's look at the combinations
for my telescope. My Meade DS2130 has a focal
length of 1020mm. To make life easier, let's just round it down to 1000mm,
though, which will give us about a 2% error.
Eyepieces:
25mm: 1000mm / 25mm = 40x magnification
15mm: 1000mm / 15mm = 66.6x magnification
12.5mm: 1000mm / 12.5mm = 80x magnification
5mm: 1000mm / 5mm = 200x magnification
4mm: 1000mm / 4mm = 250x magnification
As you can see, the range of eye pieces gives a wide range of magnifications.
So one might ask, why don't I just use the 4mm all the time and get the best
magnification? Well, there are trade-offs. First of all, the more you magnify
and object, the dimmer it will be. Also, as the magnification increases, the
clarity of the image usually begins to deteriorate. Finally, many things you
look at, are simply large enough that high magnification gives you nothing.
For example, many open clusters are so large, that they're best viewed at the
lowest or maybe a medium magnification, to really be able to appreciate them. The
moon is also generally much easier to see at lower magnifications.
Finally, some objects are so dim that you'll never see them at high magnification,
and a lower magnification is needed just to see them at all.
As a general rule, 2 times the aperture in mm, is the maximum magnifications your
telescope can manage, so with my 130mm aperture, 260x would really be the rough limit
of magnification before distortion becomes too bad. The upper limit imposed by the
atmosphere of the Earth is roughly 500x.
In addition to issues of magnification, there are a variety of different types
of eyepieces. Instead of re-inventing the wheel, I'll use the web the way it's
meant to be used and give you a link to
here which gives a ton of information about eyepieces. In particular,
the bottom of the page discusses the different styles and designs of eyepieces and
what they're most appropriate for.
For the beginner, with a small telescope, 1.25" plössl eyepieces will give
really exceptional quality compared to the 0.965" eye pieces that come with
most lower-end scopes. If you want to upgrade and can't afford a bigger telescope,
go for some nice eyepieces. They'll be worth the money, and they don't cost
as much as a new telescope. In addition, you can always use good eyepieces on
your new telescope if you upgrade it later.
