OF THE ROGER N. CLARK

lens and a piano-convex eye lens. A welldesigned
orthoscopic can be fully corrected
for distortion, and has very good color correction.
It works well with telescopes having
f/ratios as short as f/4.5. The apparent field is
30° to 50°, and the eye relief is as much as 80
percent of the focal length.
Plossl. The Plossl eyepiece consists of two
achromats in a design similar to the type I
and II Kellners. It rates as one of the finest
telescope eyepieces. The apparent field is 35°
to 50°, and the eye relief is abou t 75 percen t of
the focal length. The Plossl works well with
f/ratios as short as f/4.5.
Erfle. This eyepiece has a very wide apparent
field, about 50° to 70°, with 65° common.
It has three achromats and provides good
definition in the center, but aberrations become
obvious toward the edges. The eye relief
is about 30 to 40 percent of the focal
length. The Erfle works well with f/ratios of
f/4.5 and up.
Nagler. The Nagler is a proprietary design
having seven elements and an astonishing 82°
apparent field. It is designed for use with
f/4.5 Newtonian telescopes, though it will
work well with higher f/ratios. The field of
view can be breathtaking, though it is sometimes
difficult to see the entire field at once
VISUAL ASTRONOMY OF THE DEEP SKY
because it appears so large. This is a very
expensive eyepiece, often costing four to five
times as much as a good quality Erfle, Plossl,
or orthoscopic.
In recent years eyepiece sales have become
a very competitive business, and new (and
expensive) proprietary designs are coming to
the market.
The Barlow lens. A Barlow is a negative
lens that enlarges the image plane in a telescope.
It is not an eyepiece but an eyepiece
accessory. Barlow lenses are usually used to
increase the telescope's effective focal length
two to three times so an eyepiece will give a
correspondingly higher magnification. A Barlow
lens is a necessity for short-focus telescopes
if high magnifications are desired.
For example, say your telescope has a focal
length of 1200 mm (47 inches), and your
shortest fo cal length eyepiece is 6mm. That
eyepiece on that telescope gives a magnification
of 200X. Shorter focal length eyepieces
are impractical because their eye relief is too
small. But using a 2x Barlow lens would give
400x and a 3x Barlow 600 x with the same
6mm eyepiece. With three eyepieces and a
Barlow lens that is continuously variable,
almost any reasonable magnification can be
obtained. The Barlow does add more optical
surfaces in the light path, reducing the light
transmission of the telescope. But the nega-
Eye Relief
I Apparent
Eye
-
H uygens
Kellner, Type 1
Plossl
(Ke"ner, Type 3)
Erfle
THE EYE AND THE TELESCOPE
Eyepiece Types
Figure 3.3b.
Normal Focus
,
T
Kellner, Type 2
Orthoscopic
Nagler
Magnified Focus
t
Image
Plane
Field
Lens
Eye
Lens
Iris of Eye at
Exit Pupil
-- - ,... :----
Figure 3.3a. How an eyepiece focuses light for the eye.
Figure 3.3c. The Barlow lens.
26
27