3.3 Eyepiece
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The choice of eyepiece will affect your field of view, although that is also, to
a large degree, dependent on the telescope. Some eyepieces are specially designed
to give a larger, more desirable, field of view than others, although this comes at a
financial cost.
Telephoto (also known as zoom) eyepieces are becoming increasingly popular
amongst amateurs. These eyepieces have variable focal lengths, for example, 8–
24 mm. They enable an observer to zoom in and out without changing the eyepiece.
However, keep in mind that every eyepiece design is a compromise between focal
length, optical quality, weight, the field of view, eye relief, and cost, and furthermore,
a zoom eyepiece is never going to compete for accuracy with a fixed focal length
lens with a comparable design.
The simplest eyepiece consists of just a single concave or negative lens. The
Galilean eyepiece is a low-cost design giving a very poor field of view. They are
best used in low magnification scenarios such as toy telescopes.
Like the Galilean, the Keplerian eyepiece consists of a single lens, but in this case
it is a convex lens. It has a wider field of view than the Galilean.
The Huygens eyepiece was the first eyepiece to use multiple lenses. It consists
of two planoconvex lenses facing the same direction, in series, with an air gap. It
suffers from short eye relief, high image distortion, chromatic aberration, and a poor
field of view. However, its low cost means that it is often shipped as the standard lens
for bottom-end telescopes. Since its components are separated rather than bonded,
it is not subject to damage by heat from the Sun; hence Huygens eyepieces are often
used for solar projection.
A slight improvement on the Huygens eyepiece was achieved by the use of two
planoconvex eyepieces placed one focal length apart with both convex sides facing inwards. The Ramsden eyepiece, like the Huygens, can suffer from chromatic
aberration and has poor eye relief.
Eyepiece chromatic aberration was addressed by the introduction of the Kellner
eyepiece . The Kellner consists of three lenses: a planoconvex in front of a bonded
doublet of a convex lens and a planoconcave lens. The Kellner offers a significant
reduction in chromatic aberration compared to earlier designs as well as good eye
relief and apparent field of view. It is often shipped with smaller telescopes, and it is
quite possible that one is to be found in your observatory.
The Plössl eyepiece is a compound eyepiece consisting of two bonded convex
concave-plano doublets. The Plössl has a good field of view but relatively poor eye
relief, and it is well suited for deep sky and planetary viewing. It is one of the most
common eyepieces in use, and there will almost certainly be one in your observatory.
For good performance, the bonded doublet must be well matched to prevent internal
reflections, although this is often mitigated by the use of coatings. As a result, there
is a significant difference between the top-end and bottom-end Plössl eyepieces.
The orthoscopic eyepiece is a high field of view, low distortion eyepiece, ideal
for observing the Moon and other solar system objects. The orthoscopic consists of
two doublet convex lenses bonded to either side of a concave lens, with the bonded
triplet sitting in front of a planoconvex lens.
33
The choice of eyepiece will affect your field of view, although that is also, to
a large degree, dependent on the telescope. Some eyepieces are specially designed
to give a larger, more desirable, field of view than others, although this comes at a
financial cost.
Telephoto (also known as zoom) eyepieces are becoming increasingly popular
amongst amateurs. These eyepieces have variable focal lengths, for example, 8–
24 mm. They enable an observer to zoom in and out without changing the eyepiece.
However, keep in mind that every eyepiece design is a compromise between focal
length, optical quality, weight, the field of view, eye relief, and cost, and furthermore,
a zoom eyepiece is never going to compete for accuracy with a fixed focal length
lens with a comparable design.
The simplest eyepiece consists of just a single concave or negative lens. The
Galilean eyepiece is a low-cost design giving a very poor field of view. They are
best used in low magnification scenarios such as toy telescopes.
Like the Galilean, the Keplerian eyepiece consists of a single lens, but in this case
it is a convex lens. It has a wider field of view than the Galilean.
The Huygens eyepiece was the first eyepiece to use multiple lenses. It consists
of two planoconvex lenses facing the same direction, in series, with an air gap. It
suffers from short eye relief, high image distortion, chromatic aberration, and a poor
field of view. However, its low cost means that it is often shipped as the standard lens
for bottom-end telescopes. Since its components are separated rather than bonded,
it is not subject to damage by heat from the Sun; hence Huygens eyepieces are often
used for solar projection.
A slight improvement on the Huygens eyepiece was achieved by the use of two
planoconvex eyepieces placed one focal length apart with both convex sides facing inwards. The Ramsden eyepiece, like the Huygens, can suffer from chromatic
aberration and has poor eye relief.
Eyepiece chromatic aberration was addressed by the introduction of the Kellner
eyepiece . The Kellner consists of three lenses: a planoconvex in front of a bonded
doublet of a convex lens and a planoconcave lens. The Kellner offers a significant
reduction in chromatic aberration compared to earlier designs as well as good eye
relief and apparent field of view. It is often shipped with smaller telescopes, and it is
quite possible that one is to be found in your observatory.
The Plössl eyepiece is a compound eyepiece consisting of two bonded convex
concave-plano doublets. The Plössl has a good field of view but relatively poor eye
relief, and it is well suited for deep sky and planetary viewing. It is one of the most
common eyepieces in use, and there will almost certainly be one in your observatory.
For good performance, the bonded doublet must be well matched to prevent internal
reflections, although this is often mitigated by the use of coatings. As a result, there
is a significant difference between the top-end and bottom-end Plössl eyepieces.
The orthoscopic eyepiece is a high field of view, low distortion eyepiece, ideal
for observing the Moon and other solar system objects. The orthoscopic consists of
two doublet convex lenses bonded to either side of a concave lens, with the bonded
triplet sitting in front of a planoconvex lens.