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Binoculars and Telescopes for Ornithologists

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Abstract

Describes optical instruments for ornithologists and naturalists
OPTICAL INSTRUMENTS
FOR
ORNITHOLOGISTS
"Basic
birdmanship
consisls
in having the
best
pair of binoculars in any
group"
Stephen Potter
Field
ornithologists
consider
binoculars
and
telescopes
to be their most
important
optical
aids as these instruments
enables
them
to
make
observations
under
conditions in which
the
unaided
eye
would
be ineffective.
lt should
be
noted
however
that optics
of
high
quality
are
needed
for
ornithological work
and
instruments
which
are
acceptable for
more
mundane
purposes
(sports,
travel
etc.) may be
unsatisfactory for
bird study. This
article is
intended
therefore
as a compendium
of things
that
an
ornithologist needs
to
know
about these
essential
instruments.
The
advantages
that
binoculars and
telescopes
ofier
(over
the
naked
eye)
may
be
summarized
as
follows
(1) Greater magnification
(larger
image)
(2) lmproved
image
resolutionidefinition
(sharper
image)
(3) Enhanced
light
gathering
power
(better
image
in
poor
light).
Additionally
binoculars
(but
not
telescopes)
produce
an
enhanced
stereoscopic
effect.
Basic Principles
An
understanding
of the basic
principles
governing
them will
assist
the ornithologist
to
select a suitable instrument
as
well
as
obtain
optimum service from
optics
already
at hand.
Performance
is
judged
by image
quality,
which
is in turn
dependent
on
resolution,
contrast
and
brightness.
A telescope
- binoculars
are a form
of telescope
- consists
essentially
of
an
achromatic objective lens
(ie.
one which
brings
Rex L De
Silva,
FZS.,
MBOU
light
of
all colours to
the same focus)
of
relatively large focal
length which
focuses light
from
the
subject, thereby forming
a primary
image. This is in
turn
magnilied
by the eyepiece
or ocular
(a
compound
lens
of short
focus)
which forms
the
image viewed
by the
observer.
When
the
instrument
is
pointed
towards
the
light
and
viewed
from
some
distance behind,
a
bright disk will
be seen in
the center
of the
eyepiece.
This
is known
as the exit-pupil
and
is
an
image
of the objective. The
brightness
of an
image in poor
light is
dependent
on
the size of
the exit-pupil,
a large
diameter
yields
a bright
image
and
vice versa.
Binoculars
Binoculars
are
merely
two low-powered
telescopes
mounted
together with
their
optical
axes
parallel
to one another. The
optical
paths
in
binoculars
are shortened by
the
interposition
of a prism
train between
each
objective and
eyepiece, resulting in shorter
and more
compact
instruments
as shown in figures 1a
and
1b.
(NOTE;
Binoculars
should not
be
confused with
"field-glasses"
which
are
non-
prismatic
instruments
of low
power
and
different
optical design. As field-glasses
are of little
use
to ornithologists
they
will not
be discussed
here). The
specifications
of binoculars
are
marked
on one of the
prism
housings
and
include magnification
and
objective aperture in
mm separated
by an "x"
sign. Thus
an
instrument
designated
as B
x 40
will magnify
eight times and
have
an objective
of 40 mm
aperture
(diameter).
A third
set of
figures
indicating
the
instrument's
field-of-view
is
also
normally
included.
When
selecting binoculars
several factors
must
be considered,
these are discussed
below.
61
MAGNIFICATION
OR POWER. Magnification
is the
first
thing
that
most
users consider when
selecting binoculars.
lt is important
to realize
however
that
magnification
must
be evaluated
in relation
to the objective
aperture
if the
instrument
is to prove
effective
in use
(see
below).
For
ornithological observations
the
power
should
be between 7 x and 12 x. Less
than
7 x will
produce
an image which is often
too small,
whereas
exceeding 12 x will result
in
an
instrument
which is
difficult to
hold
steady
for
any
length
of time
(remember
every tremor
of
the
user's hands will
be
magnified
in
proportion
to the
power).
7 and
I x binoculars are
suitable
for
general
observation whereas instruments
of
12
x will
be
preferred
for long-range
uses such
as sea and
shorebird
watching.
Note
that
on
account of design constraints
so-called
"high-
power"
binoculars
often
produce
relatively
poor
images, this is
another
reason for
avoiding
binoculars of more
than
12 x. Zoom
binoculars
are
available : reject
these
as they
invariably
produce
inferior images. lf high
power
is
important
use a telescope. Note
that some
inexpensive
binoculars
are marked
with
specifications
which
are only approximate
(or
in
a few instances
even
incorrect),
the critical
user
may theref
ore wish to determine the
magnification
of his instrument
and
the
necessary formulae
are
given
here.
OBJECTIVE
APERTURE
(DIAMETER).
Aper-
ture
of the objective
is important
as image
resolution
(sharpness)
and exit-pupil diameter
(see
below) are dependent
on this
factor.
In
general
a large
objective
will
produce
better
resolution than smaller
one
(although
this is
mainly theoretical insofar
as binoculars
are
concerned). Ornithologists' binoculars
should
ideally have
objectives with
apertures between
35 and
50 mm. lf less
than 35 mm resolution
may be inadequate
and
in low
light
images
could be
very
dim
(these
are sound
reasons for
avoiding
so-called
"compact"
and
"mini"
binoculars).
On
the other
hand
objectives
larger
than
50 mm will
result in binoculars
which
are
too bulky
for field
use.
LIGHT.GATHERING POWER
OR
RELATIVE
BRIGHTNESS
INDEX. This
is
a measure
of
image
visibility
under
low-light
conditions and
is
indicated
numerically
as
the exit-pupil diameter
(in
mm)
squared.
(The
exit-pupil can be
measured,
or calculated by dividing
objective
aperture
by power).
Large
objectives and
low
powers give
bright
images,
whereas
small
objectives
and high
powers produce
dimmer
images.
A 5 mm exit-pupil
(relative
brightness
index
25) is probably
the optimum
for
ornithology.
The
exit-pupil should
not
be
greater
than
7.1 mm
(relative
brightness index
50) as
this
would
exceed the
maximum
pupillary
aperture of the dark-adapted eye and
light
would be
wasted.
"Night
glasses"
ie.
those
with
7 mm exit-pupils
(7
x 50
and 8 x 56)
provide
the
brightest
possible
images in low
light, but note
that
in
daylight the
images
are
no
brighter than
in
conventional
instruments.
As the
eye's
maximum
pupillary
aperture
decreases
with
age,
older users
will
not
benefit
from night
glasses
and will be served
better by
conventional
models
which
are lighter
and
more
compact. Specifications of some
popular
binoculars are
listed here.
MA E
7x35 5
7 x50 7.1
8x24 3
8 x 30 3.75
R (T) REMARKS
2s (15.7)
50 (18.7)
9 (13.9) Not recommended for
ornithology
14 (15.5) Not
recommended
for
ornithology
(17.e)
(21.1)
(22.4)
(24.5) For long-range
use
in
good
light
only
M = Magnitication
A = Objective diameter
(mm)
E = Exit-pupil
diameter
(mm)
R = Relative
brightness
index
(T) = Twilight factor
8x40 5 25
8x56 7 49
10x50 5 25
12 x
50 4.17 17.4
62
(A few manufacturers
describe
image
brightness
in
terms of a "Twilight
Factor" which
is derived by calculating the square
root
of
the
product
of the objective
diameter
(mm)
and
power.
Several tests using
a variety
of
instruments of
differing apertures and
powers
suggest
however
that the twilight
factor may
not be
reliable. For example
when
tested at
dusk
an 8
x 40 instrument
(TF
17.9)
gave
better
images than another of
10 x a0
(TF
20), Note
however that as these tests
were
not
carried
out
under strictly controlled
conditions they
could have
been subject to undetected
errors.
In
view
of this ambiguity
the writer
prefers
to
rely
on the
relative
brightness
index:
but
note
this
is a
personal
opinion only.)
FIELD OF
VIEW. In
standard
instruments
the
field of
view and
power
are
interdependent;
a
high
magnification resulting in
a small
field and
low
magnification in a wider one.
Binoculars
of
normal design
have fields ranging from
c. 5 to
7
degrees
or
so.
A wide field is
often useful,
especially
when observing
flying
birds,
communal
roosts
and
large flocks. Wide
angle
binoculars
with fields
of up to 11
degrees are
produced;
unforlunately
the
large f ields
are
sometimes
gained
at the sacrifice of image
quality.
Wide
angle
binoculars
from reputable
manufacturers
give images which are
satisfactory
for serious
work,
but these
instruments are costlier, somewhat
larger
and
heavier than conventional
models. The field
of
an
instrument may be
indicated
either
in
degrees
or by the
field in feet at a distance
of
1,000
yards. (To
convert the
former into
the
latter
multiply
the
field in
degrees by 52.5).
COLLIMATION.
lt is
essential
that the two
optical
systems
in binoculars
should be
absolutely
parallel
to one another,
in
other
words the
instrument
should
be
in
collimation.
An
instrument
which is
grossly
out of collimation
will
produce
a double
image whereas when
the
collimation
error
is
smaller, the eyes
will
unconsciously
strain
in
an attempt to
fuse
the
two
images
together.
This may result in
considerable
eyestrain accompanied by
headaches and
if such an instrument is
used
intensively over a long
period
could
possibly
have
an adverse effect
on the user's sight.
To
test binoculars
f
or collimation,
focus
on a
distant
horizontal object
(wall,
sea-horizon etc.)
then
slowly
move the
instrument
away
from
the
eyes
while simultaneously
swinging the barrels
outwards
thereby
increasing
the separation
between
the eyepieces.
Binoculars
which
are
in
collimation
will
give
images resembling
figure
1c at all oositions
of the barrels.
lf the
collimation
is
defective
the
images
will resemble
f igure
1d at one or
more
positions
of the
eyepieces.
Some
inexpensive
binoculars
are
not properly
collimated at the time of
manufacture, others
lose their collimation
after
a short
period
of service
and a very few are
almost
impossible to collimate.
This is
just
one
of
the
reasons
for
buying
a reliable make. Note
that
recollimation
is
a skilled
job
which should
only
be carried
out by a competent
repairman.
FOCUSING.
Binoculars come
with
one
of two
focusing systems,
they are
the center-focus and
individual-focus
types.
In
the
former
both optical
systems
are
focused together by rotation of a
large central
focusing
wheel.
One
eyepiece
is
also adjustable
separately
to compensate
for
differences
in the user's eyes
(dioptre
adjustment)
and once
set requires
only
occasional
readjustment thereafter.
Center-
focus binoculars
can be adjusted
quickly
and
are
well suited
for use
in
the
f ield.
The
individual-focus
type
requires that each
eyepiece
is adjusted separately;
a relatively
slow orocess.
hence these
instruments are
not
usually
recommended
for field
use
but as they
are
rugged and
rather
weatherproof
they
could
sometimes
be useful
for
observing
under
adverse
conditions.
OPTICS.
Due
to the
nature of light
optical
systems
are subject
to aberrations
(imaging
errors).
Two aberrations
which concern us
here
are chromatic
aberration
and distortion.
Chromatic
aberration:
This occurs
when
all
colours do
not come to a focus
at the same
point.
By
judicious
selection
of optical
glasses
and
lens design optical
engineers are able to
63
I
I
I
I
I
minimize
this
fault.
An
instrument
suffering
from
this
aberration
will
give
images
with some
spurious
colour.
To test
for this
fault focus on
a
distant
thin object
(antenna, flagpole
etc.)
which
is backlighted
against
a bright
sky.
lf
the
image
shows
fringes
of false colour,
chromatic
aberration
is
present. A small
amount
of
spurious
colour
may be tolerable
at the edges
of
the
field, but
the
central
region
should
be
absolutely
free of this.
lf
chromatic
aberration
is
at
all
obtrusive
the
instrument
will
prove
unsatisfactory
in
use.
Distortion:
This aberration
occurs
when
magnification
is not uniform
throughout
the
field.
lf magnification
increases
from
the
edge
towards
the center
barrel
distortion
results.
lf
on
the
other
hand
it increases
from the
center
towards
the
edge
the
result
is
pincushion
distortion.
To test
for distortion
focus the
instrument
on
a distant
object
with straight
lines
(preferably crossing
at right angles
to one
another
eg.
window,
building
etc.).
lf distortion
is
present
the
straight
lines of
the
object
will
appear
slightly
curved
in the
image;
outwardly
bowed
lines
indicating
barrel
distortion
and
inwardly
curved
ones
pincushion
distortion
(figs.
1e,1f
and
1g).
ln
good
instruments
distortion
should
be
minimal.
Modern
lenses are
coated
at their surfaces
to reduce
reflection
and thereby
increase
light
transmission.
This
results
in images
of
high
contrast
with
little or no
lens
flare. Older
binoculars
were supplied
with
magnesium
flouride
coated
optics
whereas
more
modern
instruments
come
with
multicoated
optics.
Multicoated
optics have higher light
transmission
factors but the
coatings
are
rather
easily
scratched
and also
attacked
by
lens
fungus,
hence they
should
be
treated
with extra
care.
Although
binoculars
may be
described
as
"fully-coated"
the
prisms, in
all
but
the
very best
instruments,
are
normally
left uncoated'
DESIGN
AND CONSTRUCTION.
Binoculars
come
in a variety
of
forms, all
are based
on one
of
two
designs
viz. the
porro
prism
and
roof
prism models,
each
of which
has
its
particular
advantages
and
disadvantages.
(Refer
to
figures
1a
and
1b).
Porro
prism
binoculars
are
sturdy,
easy
to
repair
and
give
good
stereoscopic
effect.
lf
well
made
they
give images
of high
quality.
Disadvantages
are
that
they
are
not
very
compact,
especially
if
of
the
"American"
(B)
single
unit
body
type.
The
"German" (Z)
two
piece
body
types
are
more compact,
although
not
as compact
as
roof
prism
models.
Roof
prism
binoculars
are
light, compact
and
usually
more dust
and
weatherproof
than
most
porro
prism
models.
They are
optically
more
comolicated
and
can
be
difficult
to repair'
nevertheless
some
of the
best
binoculars
are
of
this
design.
The main disadvantage
is that
good roof
prism
models can
be
very expenslve.
Binocular
bodies
are
made of
aluminium,
zinc
die-castings
or plastics. Metal bodies
are
usually
covered
with textured
plastic
coatings'
Several
models
come
fitted
with
rubber
coverings
(armour) for
protection. In some
instruments
this
is
not
removable
and
may
have
to be cut
away
to effect
repairs.
While
the
rubber
can
usually
be
refitted,
appearance
may
be marred
hence
reputable
makes
have
provision for repairs
without
damage
to
the
rubber
covering.
Some
binoculars
are
fitted
with
threaded
bushings
for
mounting
on camera
tripods.
This
is a useful
feature but
most
binoculars
can
be
adapted
to a tripod
with a
simple
home-made
device.
Binoculars
come
with
fitted carrying
cases
and
lens caps.
These are
for
protection
of the
instrument
whilst
in
transit.
Lens caps
are
easily
lost
but as
these
protect
the
optics
it is well
worth
looking
after
them.
A neck strap
is an
essential
item,
however
most are
too
long
allowing
the
binoculars
to swing
about
with the
obvious
risk of damage,
hence they
should
be
kept
as
short
as
is convenient.
Note
that
narrow
straps
are
best,
broad
(camera-type)
straps
are
not
a
good
choice
for
binoculars.
64
PORRO PRIS}I BINOCULAR
Binoculars
Eyepiece
- Prisns
- Objective
FIGURE 1a
FIGURE 1d
FIGURE 1b
ROOF
PRISU BINOCULAR
FIGURE 1c
- EyePiece
-Prisns
-Obj ective
collination
Obj ect
FIGURE
le
LR
Out of collimation
L
In
8arre1
dis tort ion
FIGURE
1f
Pincushion
di s tort ion
FIGURE 1g
Perera )
(Illustrations p. T. G.
65
FIGURE
FIGURE
2c
Telescopes
FIGURE
2a. A SPOTTING
CATADIOPTRIC
TELESCOPE
TERRESTRIAI-
REFRACTOR
66
(Illustrations by P. T. G. Perera)
TELESCOPES
When
high power
is rquired
a telescope
should
be used. A disadvantage
with
telescopes is
that
they
cannot
easily
be hand-
held hence
they
should
always
be
mounted
on
tripods.
Telescopes
provide
high
magnification
combined with
good
image-quality
if
well
made,
but their
fields-of-view
are
rather
small
and they
give
best
performance
in
fairly
bright conditions.
Three
basic
designs
are
used
by ornithologists;
they are
spotting
scopes,
terrestrial
refractors
(both
of which
use lenses)
and
catadioptrics
(using
mirrors
and lenses).
Note that
conventional
newtonian
and cassegrain
astronomical
reflectors
cannot
be used
for
ornithological
purposes.
Spotting
scopes are popular with
ornithologists
as
they
are short,
compact
ano
hence
easily
carried
about. They
usually
are
prismatic
instruments
which,
when
fitted
with
the appropriate
eyepieces,
can
encompass
relatively
wide
fields-of-view.
The
prisms
sometimes introduce
a small
amount
of
false
colour and
to
counler
this
some
(expensive)
models have
aluminized
first-surface
mirrors
in
place
of
prisms.
When
used with
low
and
medium
power
eyepieces,
spotting
scopes
produce
images
which
are adequately
sharp
and
of
satisfactory
contrast.
Their
rather
srow
focusing
movements
could however
be
disadvantageous
at times.
Spotting
scopes
are
always identifiable
by their
characteristic
"bent"
orofiles.
The
best
modern
terrestrial
refracrors
(apochromats)
produce
extremely
sharp, high-
contrast images.
Many
are
small
astronomical
telescopes
adapted
for
terrestrial
use. They
are
usually
fitted
with
either
rack-and-pinion
or
sliding drawtube
focusing
movements:
both
systems
permit
fast
focusing.
Although
relatively
light,
refractors
have
long
tubes
and
this together
with
their
small fields
could
be
a
drawback
but
exceptional
image
quality
makes
them
ideal
for
critical
work.
(Beware
though
of
similar-looking
"department
store"
telescopes,
often
with
zoom
eyepieces
- these
are no more
than toys).
Catadioptrics
are
compound
telescopes
which
use spherical mirrors
in
conjunction
with
meniscus
lenses
or aspherical
correcting
plates.
They
are compact
but not
light
and
give
sharp images
with
good
contrast;
drawbacks
are
high
costs
and exceptionally
slow
focusing
movements.
Smaller models
(ie.
apertures
of
125
mm
or
less)
are suitable for
ornithology.
Most telescopes
are provided
with
interchangeable
eyepieces
giving
a range
of
magnilications.
Ornithologists
normally
use the
lower
powers
(15
- 30 x) much
of
the time
and
reserve higher
powers
for
use
when
conditions
are ideal.
This is because high
power
eyepieces
give
small fields,
magnify
defects in
the optics
and are
severely
affected
by
atmospheric
scintillation
(see
below). Variable
power
or zoom
eyepieces
are
sometimes
encountered
and
it is
claimed
that a
single
such
eyepiece can replace
a set
of
conventional
ones.
Despite
the
apparent
convenience
this
is
a fallacy
as many
zoom
eyepieces
produce
inferior images
of low
resolution,
poor
contrast
and sometimes
severe
chromatic
aberration.
Telescope
performance
is
always
dependent
on atmospheric
conditions ("seeing")
when
image
quality
could
be seriously
compromised
by turbulence
or scintillation
resulting
f rom
rising heat-waves
(sometimes
miscalled
"mirage"),
large
apertures
and high
mag-
nifications
are more
seriously
affected
than
smaller apertures
and
low
powers.
TRIPODS.
To
use a telescope
effectively it is
necessary
that it
be
securely mounted
on
a
tripod.
In
general
heavy
tripods
are
steadier
than
light
ones. As
portability
is
a prime
consideration
for field
use
a compromise
must
often be made
between firmness
and weight.
The
steadiest field
tripods have
tubular
("O"
cross section)
alloy legs. For
firmness
each leg
should
preferably
be
in
two
(or
not more
than
three) sections. Tripods
with
channel legs
("C"
cross section)
are
popular
as they
can be
easily
67
*rlf,?hs
set up and
dismantled
on account
of the fast-
acting lever-locks
fitted
to their legs.
On
a
weight-for-weight
basis however
they
are not
as
firm
and
steady
as those
with
tubular
legs.
Steadiest
of all are
the
old-fashioned
wooden
and modern
professional
video
tripods,
regrettably
these
are often
too heavy
for use
under field
conditions.
For
field
use
trioods
with
tubular legs
would
ideally
weigh
between
2.25
and 4.5 kgs (5
and
10
lbs).
Tripods
with
channel
legs would
require
to be 1 to 2 kgs
more
in
order
to have
equal
stability.
Many
tripods have
crank-operated
centre
posts
for
convenient
height
adjustment,
however
a telescope
will
be
steadiest when
the
centre
post
is retracted
to its
lowest
position.
A wide variety
of heads
are
available for
better
tripods;
the
standard
"pan
and
tilt" units
are
quite
adequate,
but
avoid
"ball
and socket
heads"
which
are not
suitable
for
field
use. Lastly
the
tripod
shoutd
be of
adequate
height.
ACCESSORIES.
Filters
can
be of vatue
rn
certain circumstances
but very
few
telescopes
have
the necessary
threaded
objective
cells
to
take them;
a simple home-made
adaptor
can
however
be easily
devised
to accept
standard
camera filters.
The
writer
finds
the filters
listed
below
to be
of some
value
for long-range
observation
of
seabirds.
K2
(Yellow):for
penetrating
tight
haze.
Polarizer:
for
better haze-penetration
with
some
light
loss,
also
eliminates reflections
from
water
surfaces
and for
glare
reduction.
ND4
(neutraldensity
4 x):
for
glare-
reduction.
'1A,
1B
or UV: for lens
protection.
CARE AND
MAINTENANCE
Binoculars
and
telescopes
are
delicate
instruments
which
can
be easily
damaged,
they
should therefore
be handled
with
care
- not
always
easy
in the field.
Following
the
guidelines
below
will keep
these instruments
in
good
working
order.
(1) Protect
from
excessive
heat,
vibration.
sea-
spray,
dust and moisture.
(2)
Use
lens-caps
and case
and
store
the
instrument
in a dry
place
when
not in use.
Protect
a telescope
objective
with
UV,
1A
or
1B filter
especially when
used near
the
sea
and in
dusty
conditions.
(3)
Clean lenses
only when absolutely
necessary.
Use a blower lens-brush, good
quality
lens
tissue
and lens
cleaning
fluid
(use
the fluid
very
sparingly).
DO NOT
use
substitutes
such
as cotton wool
and
toilet
paper
for lens
tissue;
or xylene,
benzine
and other
solvents in
place
of lens
cleaning
fluid.
lnternal
cleaning,
collimation
and
repairs
should
be
carried
out by
a qualified
person.
lf you
do not have
the
correct tools
and expertise resist
the
temptation
to
"do-it-
yourself":
a screwdriver
in unskilled hands
can be
a
weapon
of destruction!
FORMULAE
The formulae
given
below may
be of assistance
to
readers.
F
(1)
Todeterminemagnification(power):
M = -
I
A
0r: M=-
E
A
(2)
To
calculate
exit-pupil diameter: E = -
M
(3)
To
calculate relative
brightness index: R = E2
NOTES:
A
=
Objective
diamter,
E
=
Exit-pupil
diameter,
F
= Focal length
ol objective
f =
Focal length
of
eye-
piece.
M
=
Magnification.
R
= Relative
brightness index.
6B
ACKNOWLEDGEMENTS
I am gratef
ul to Thusitha
Perera who
produced
the
illustrations.
My thanks
also
go out to Brindley
de Zylva
tor
constructive comments on the text.
BIBLIOGRAPHY
The undernoted references
provide
more information
on the
subjects dealt
with
here.
Anon, 1990,
Pupil
size and aging. SKY AND TELESCOPE
80
(6).
Bowen, K P. 1991
, Aging
eyeballs.
SKY
AND TELESCOPE
82
(3).
Brown,
S, 1975.
All
about telescopes. Banington, Edmund's
Scientific Corp.
Conrady,
A.E. 1952,
Applied
optics and optical
design
(2
vols.). New York,
Dover Publications Inc.
De Silva, R.l. 1980, Binoculars
and
telescopes lor
ornithologists. LORIS 15
(3).
Fiegg,
J. J. M. 1972, Binoculars,
telescopes and cameras
for the birdwatcher. BTO
guide 14, Tring, BTO.
Hanna, A. 1952, Binoculars in Ingalls,
A. (ed,)
Amateur
lelescope making
(advanced).
Vol.
3, New
York, Scientific
American.
Hill,
S.B. & Clayton,
D.H. 1985. Wildlile
after
dark:
a review
ol nocturnal
observation techniques.
Minneapolis.
Bell
Museum
of Natural History.
Johnson,
B.K. 1960. Practical
Optics. New York, Dover
Publications Inc.
Lowenfield, l.E. 1987, Night vision. Washington, National
Academy Press.
Rutten, H.
and
Venrooij,
M. 1988.
Telescope
Optics:
evaluation and design.
Richmond.
Willmann-Bell.
Minnaert, M. 1954. The nature
of light
and colour
in
the
open air.
New York. Dover Publications Inc.
Sidgewick, J. 1971
. Amateur astronomers' handbook. New
York.
Dover Publications lnc.
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