Download astronomical eyepieces - Amateur Astronomers Association Delhi

ON CHOOSING GOOD ASTRONOMICAL
EYEPIECES
K Siva Senthil
Amateur Astronomer Association Delhi
Abstract: An astronomical observing
system’s performance is dependant both on
the optical performance of the primary mirror
as well as the eyepieces. In fact choice of a
good eyepiece becomes a critical factor in
image quality especially with amateur
equipment. In this paper we review basics of
an eyepiece system along with an a survey
of what kinds of eyepiece systems are
available
to
the
average
amateur
astronomer
I-INTRODUCTION
Eyepiece is assembly of lenses placed at
focal point of objective of a telescope. They
are also known as ocular lens. Eyepiece is
used to magnify the image formed by
objective lens or mirror of the telescope.
Eyepieces are specified by their focal length,
apparent field of view (FOV). These
parameters can be used to derive any other
information required. Magnification that an
eyepiece can produce depends on which
telescopes specifications. For telescopes
with longer focal length an eyepiece with
small focal length will produce more
magnification compared to that of when
used with telescope of smaller focal length.
While buying an eyepiece following
properties of eyepiece should be checked
for 1) Optical quality - There are many factors
that influence optical quality. Anti-reflection
coating used to avoid ghost images and total
internal reflection; blackened lens edges to
minimize scattered light, various kinds of
aberrations caused by elements of eyepiece
are few of the factors that should be
considered while purchasing an eyepiece.
2) Field of view - There are two taxonomies
involved with FOV, they are Apparent FOV
and Actual FOV. Actual FOV is expressed in
degrees, minutes, or arc seconds which is
the actual portion of sky that is covered by
looking through the eyepiece. Apparent FOV
is an abstract number that is calculated from
constants which specify the eyepiece.
Formula used to calculate apparent FOV is
as follows S/L = 2 * tan (α/2)
S = Aperture of entrance field stop
L = Focal length of eyepiece
α = Apparent FOV
Apparent FOV and Actual FOV are related
as follows –
FOV actual = FOV apparent / Magnification
Deep sky observations require eyepieces
with low magnification and large FOV. Since,
deep sky objects are very faint and to get
good contrast and light collection eyepieces
with low magnification are preferred.
Planetary observations are usually done in
high magnification and small FOV eyepieces.
Since, in planetary observations we often
like to look in closer details the planetary
object.
3) Eye relief - It is the distance between the
eyepiece and observers eye where the
entire field of view is visible. Observation is
comfortable with eyepieces of fairly large
eye relief. If the eye relief is too large then
observation becomes uncomfortable as it
will be difficult to hold the eye at one position
for a long time. For people wearing glasses
eye relief is of primary concern. As wearing
glasses one will find it difficult to observe
through an eyepiece of low eye relief. As a
rule of thumb eye relief decreases as focal
length
of
eyepiece
decreases
i.e.
magnification increases.
4) Focal length - This is the distance from
the principal plane (for an ideal lens it will be
the plane passing through the center of lens)
of an eyepiece to the point where the rays
converge to a point.
Combined with the focal length of telescope
it yields the magnification that a particular
eyepiece can produce. Plane passing
through the focal point is known as focal
plane. In some eyepiece location of focal
plane is also considered as it helps to place
a graticule without affecting the image
quality.
There are different kinds of
eyepieces. They differ in number of
elements and their assembly. Initial designs
of eyepieces had single elements and are
known as positive eyepiece and negative
eyepiece. These types of eyepiece suffered
from all kind of defects, distortions, and
narrow fields.
First eyepiece was designed by
Hans Lipershy in 1608-1609 using standard
spectacle. It had a positive lens. This was
followed by Galileo Galilei in 1610, who
produced an eyepiece with negative lens.
This lens suffered from all known defects;
surprisingly it is still in use in toy binoculars.
between the two lenses. Huygenians has a
large field lens and small eye lens. Location
of focal plane in this assembly prevents
placement of graticule.
Advantages 1) Absence of cementing in combining the
lenses makes it a perfect choice for solar
projection.
2) Cheap design
Disadvantages 1) Short eye relief, narrow field of view,
2) High image distortion,
3) Chromatic aberration.
Mittenzway Eyepiece This
eyepiece
is
modification of Huygens
eyepiece. This eyepiece
replaces the field plano
convex
lens
with
a
meniscus lens.
II - POPULAR TYPES
Some of the popular types of
eyepieces are listed below with their
functionalities, discussing their advantages
and disadvantages.
Positive Eyepiece - This
is more refined version of
Lipershy's eyepiece. This
has a positive lens. They
are now used in optical
labs for testing telescopes.
Huygens Eyepiece - This
eyepiece has two elements
known as eye lens and
field lens. Both the lenses
are plano convex lenses
separated by air gap with
plane side facing outwards.
Focal plane is located
Ramsden Eyepiece - This
eyepiece has two elements
of same focal length. Two
lenses are separated by air
gap. This distance is
typically less than one
focal length. Lenses are
plano convex lenses with
curved surfaces facing
each other. Separation
between
lenses
is
adjustable to remove the transverse
chromatic aberration. Focal plane of this
eyepiece is outside the lens arrangement
and is thus accessible for a graticule to be
placed.
Advantages 1) It is free from coma,
2) Cheap design
3) Absence of cementing in combining the
lenses makes it a perfect choice for solar
projection.
Disadvantages 1) They suffer from
chromatic and spherical
aberration to an extent,
2) Narrow field of view.
Kellner Eyepiece - This is
a three element design. An
achromatic doublet is used
as eye lens. This doublet is
cemented together. This
doublet helps in removal of
residual
transverse
chromatic aberration.
Advantages 1) Absence of transverse
chromatic aberration,
2) Inexpensive and good image quality,
3) Good field of view (approx - 40 - 50
degrees),
4) Good eye relief,
5) Works well with faster telescopes (f/8, f/6
etc.).
Disadvantages 1) It produces ghost images,
2)
Has
considerable
internal reflection.
RKE Eyepiece - This is a variant of Kellner
eyepiece. This has slightly larger FOV
compared to Kellner eyepiece. It was
developed by Dr. David Rank for Edmund
Scientific. Abbreviation RKE stands for Rank
Kellner Eyepiece.
Orthoscopic Eyepiece This is a four element
design. It has a triplet field
lens with double convex
eye
lens.
This
was
designed by Ernst Abbe.
They
provide
more
magnification compared to
eyepieces mentioned till
now. They are used for
planetary observations.
Advantages 1) Good eye relief,
2) Nearly zero chromatic and spherical
aberration,
3) Fairly flat field of view,
4) Wide field of view,
5) Low internal reflection,
6) Low distortion of image.
Erfle Eyepiece - This is
typically
five
element
design.
However,
six
element design also exist.
This design has typically
two achromats with double
convex lens or three
achromats with double
convex lens. This was
designed by Heinrich Erfle
during world war for war
purposes. Erfle eyepiece with six elements
was designed by Kasperiet, associate of
Erfle. They are perfect for deep sky
observations.
Advantages 1) Large field of view (up to 80 degrees),
2) Good eye relief.
Disadvantages 1) Astigmatism at the edges,
2) Low image quality.
Köing Eyepiece - This is a short focal
length version of Erfle eyepiece. This
contains up to seven elements with
cemented singlets and doublets. This gives
high magnification and FOV up to 70
degrees.
Plössl Eyepiece - This is
a four element design. It
has two sets of doublets
with convex and concave
lenses attached together. It
is one of the most popular
eyepieces
today.
This
eyepiece is used for both
deep sky and planetary
observations.
Advantages 1) Large FOV,
2) Better quality of image.
Disadvantages 1) Short eye relief.
Nagler Eyepiece - This is a patented design
which is also a trademark from TeleVue.
This design has 7 to 8 lenses with varied
density of glasses. This eyepiece, with its
wide (82 degree) field of view and high
optical quality, works well even on
telescopes with fast focal ratios. This is
attributed to the design of eyepiece as the
first two element act like Barlow lens. The
disadvantage of this design is loss of
brightness and contrast due to interaction
with many surfaces.
Barlow Lens is not an eyepiece.
They are attachments to any of the above
mentioned eyepieces. These lenses are
achromatic diverging (convex) elements.
Their functionality is to increase the focal
length of the telescope. They are specified
magnification of typically 1.5x, 2x, 3x etc.
Please note that Lumicon, Orion,
Panoptic, TeleVue, University or Brandon,
Meade are manufacturers. They are not
classified as types of eyepieces. In amateur
astronomy we hardly talk of types of
eyepieces but they are very essential to
know as they are used as standards when
amateur astronomer is submitting reports /
observations to some organisations. There
are few types of eyepieces which are also
trademarks like Nagler and RKE.
In the new age of space
observatories,
even
ground
based
observatories are switching from eyepieces
to CCD chips. These are connected to
Computers
which
generate
images.
However,
eyepieces
maintain
their
importance in the field of Amateur
Astronomy where these new techniques of
observations are not used.
REFERENCES
Content - World Wide Web
1)
2)
3)
4)
Eyepieces on Wikipedia
GATFAQ website
Frosty Drew Observatory
Tom Lougheed ’s formula in
astronomy
5) John Savard ’s pages
6) Astro-tom’s pages
7) The Belmont Society
Image Credit
1) Frosty Drew Observatory