HOW TO SELECT AN EYEPIECE
The choice of eyepieces is somewhat subjective, depending on the telescope,
intended targets, budget, etc. The eyepiece is "half the telescope",
in a sense, so it pays to select eyepieces with a practical eye¡ªto avoid
duplication of magnifications, to select the correct eyepiece for the
scope type, and to make observing more fun.
Here are some methods by which to choose additional eyepieces for your
The magnification method:
You say, "I love to see things magnified, but I can't afford to have
so many eyepieces that the change in magnification from one to the next
is hardly noticeable. How would I choose which magnifications provide
meaningful magnification differences?"
Well, start out with a very low power for the widest field in your telescope.
That¡¯s usually from 30X to 50X. In a 10" f/5 telescope, a 32mm eyepiece
would be a good lower power eyepiece, providing about 40X magnification.
Use 1.4X as a multiplier to produce the next higher power. You can convert
to focal lengths by merely dividing the low power eyepiece by that same
So, the set would proceed: 32mm, 23mm, 16mm, 12mm, 8mm, 6mm, producing
magnifications about 40% apart. The raw difference between the 32 and
23mm is small, so, in this case, I¡¯d skip the 23 and go straight to the
The collection didn¡¯t have to start with a 32. If the maximum field size
is not an issue, this telescope could have gone: 25mm, 18mm, 13mm, 9mm,
and 6mm to 7mm.
The exit pupil method:
Your eyes¡¯ pupils don¡¯t open past a certain size. If you use eyepieces
that produce larger exit pupils than your eyes can receive, you are essentially
stopping down your telescope to a smaller diameter as the edge-of-field
rays don¡¯t even make it into your eye. Also, in a reflector, the shadow
of the secondary gets bigger at lower powers, and may become obtrusive.
The upper limit to exit pupil is about 7mm, best visual acuity is often
2 to 3mm, and people¡¯s eyes are often not able to field an exit pupil
smaller than 0.5mm without having minor pieces of flotsam (called floaters)
in the eye interfere with the view of details.
A decent set, by exit pupil, would be 7mm (lowest power), 5mm, 3mm, 2mm,
Multiply your telescope¡¯s f/ratio by the above exit pupils to obtain
the focal lengths of eyepieces to form the set. In the case of the hypothetical
10" f/5, the eyepieces would be: 35mm, 25mm, 15mm, 10mm, 5mm, and 2.5mm
The Compromise method:
If you look closely at the 2 above methods, it's obvious the number of
eyepieces could be reduced by using a good Barlow lens to double the power
(of the eyepieces used with it). The first set could have been a 25mm
and 18mm and a 6mm, with a 2x Barlow lens.
That would be much less expensive than buying all separate eyepieces.
As a matter of fact, the turbulence in the night atmosphere usually limits
our viewing to powers under 200X to 250X, so having eyepieces that produce
50X, 100X, 150X, 200X will guarantee you have an eyepiece for 90% of your
viewing. Add an extra high-power eyepiece, or use a Barlow lens on a lower
power one, and you¡¯re set. In the example just given, 3 eyepieces (50X,
100X, and 150X) and a Barlow would cover all the powers AND provide all
the lesser-used high powers for exceptional conditions.
Whichever process you use to figure out what eyepieces you need, just
be aware that you don¡¯t have to have a whole kit full of eyepieces to
view nearly every object visible. Three and a Barlow will do quite nicely.
But there are other methods, equally valid:
The Eye-Relief Method:
This is the way any person who must wear glasses to look through the
eyepiece has to select eyepieces. Fortunately, few of us have such severe
astigmatism that we have to view with glasses on. You can refocus the
telescope to correct for near-sightedness (myopia) or far-sightedness
(hyperopia), but that won¡¯t work with astigmatism. Though there are additional
astigmatism-correcting lenses that can be added to a few different eyepieces,
by and large people with severe astigmatism must view with glasses on.
Since wearing glasses forces the eye to move back away from the eyepiece,
this means that most eyepieces will lose field of view. Those eyepieces
whose position of best viewing (called Eye Relief) behind the eyepiece
(where your eye is close enough to see the entire field of view, but not
too close) is farther back from the lens are known as Long Eye Relief
eyepieces. There aren't as many models of these as there are of the shorter
eye relief types, but there are some. Glasses wearers usually stick to
long eye-relief models. Some good news for the glasses wearer¡ªalmost all
eyepieces with focal lengths longer than 26mm would be considered Long
Eye Relief. At long focal lengths, glasses wearers have the same choice
as non-glasses wearers.
The Use-Matching method:
This overlaps with the method of choosing by Apparent Field-Size Preference,
so I¡¯ll discuss them both:
Many people find that, after observing a while, they are using primarily
low powers to view large objects, like star clusters, and higher powers
mostly for lunar and planetary viewing. That makes a large apparent field
of view desirable at the lower powers, but makes smaller apparent fields
of view acceptable at higher powers. A judicious selection of eyepieces
would probably have the largest apparent field of view (and perhaps the
most expensive eyepiece) for your low power(s), and a smaller apparent
field of view (which often makes the eyepiece less costly) at higher powers.
A 65 to 82 degree apparent field of view is very desirable when looking
at large nebulas or star clusters at low power, but a 40 to 52 degree
apparent field of view is quite acceptable at high powers, where the objects
viewed are smaller. And some of the high-power eyepieces with smaller
apparent fields of view have exceptional optics (like the Parks Gold Series
Pl?ssl for example).
A user may prefer to have the ultra-wide apparent fields of view at all
powers, or may prefer the narrower apparent fields of view at all powers.
If so, then Apparent Field-Size Preference will become an important component
of choosing eyepieces.