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Telescopes and Optics
NHAS Astro 101
Agenda
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Optics relating to Telescopes, Lenses and Mirrors
Types of Telescopes and their advantages
Focal Length and Focal Ratio
Types of Eyepieces and their advantages
Magnification and Apparent Field of View
Types of Astronomical Mounts and their uses
Types of Finders and their uses
Filters and their uses
Lenses and Mirrors
Properties of Light
Law of Reflection -
Angle of Incidence = Angle of reflection
Law of Refraction - Light beam is bent towards the normal
when passing into a medium of higher Index of Refraction.
Light beam is bent away from the normal
when passing into a medium of lower Index of Refraction.
Index of Refraction -
Speed of light in vacuum
n
Speed of light in a medium
Inverse square law - Light intensity diminishes with square of
distance from source.
Law of Reflection
Normal


Angle of incidence () = angle of reflection ()
The normal is the ray path perpendicular to the mirror’s surface.
Geometry of a Concave Mirror
Focus
Principal axis
Vertex
Focal length
Center of curvature - the center of the circle of which the mirror represents a
small arc
Principal axis - a radius drawn to the mirror surface from the center of
curvature of the mirror - normal to mirror surface
Focus - the point where light rays parallel to principal axis converge; the focus
is always found on the inner part of the "circle" of which the mirror is a small
arc; the focus of a mirror is one-half the radius
Vertex - the point where the mirror crosses the principal axis
Focal length - the distance from the focus to the vertex of the mirror
Index of Refraction
As light passes from one medium (e.g., air) to another (e.g., glass,
water, plexiglass, etc…), the speed of light changes. This causes to light
to be “bent” or refracted. The amount of refraction is called the index of
refraction.
Refraction
Imagine that the axles of a car represent wave fronts. If the car crosses
from a smooth to a rough surface at an angle, one tire of the axle will slow
down first while the other continues at normal speed. With one tire traveling
faster the other, the car will turn in the direction of the slow tire. This is how
refraction works.
AIR
NORMAL
GLASS / WATER
Slower Propagating Speed
AIR
Car
GLASS / WATER
Slower Propagating Speed
( Sand / Gravel )
AIR
GLASS / WATER
Slower Propagating Speed
( Sand / Gravel )
NORMAL
AIR
LIGHT BENDING
TOWARDS THE
NORMAL
LIGHT RAY
GLASS / WATER
Slower Propagating Speed
NORMAL
AIR
LIGHT BENDING
TOWARDS THE
NORMAL
n1
Snell's Law
( Next Slide )
n2
GLASS / WATER
Slower Propagating Speed
( Sand / Gravel )
Slower Propagating Speed
GLASS / WATER
Car
AIR
( Sand / Gravel )
Slower Propagating Speed
GLASS / WATER
Car
AIR
( Sand / Gravel )
Slower Propagating Speed
GLASS / WATER
AIR
Slower Propagating Speed
GLASS / WATER
NORMAL
AGAIN, LIGHT BENDS TOWARDS THE NORMAL
upon entering a region with slower speed.
LIGHT RAY
AIR
AIR
Car
( Sand / Gravel )
GLASS /WATER
Slower Propagating Speed
AIR
Car
( Sand / Gravel )
GLASS /WATER
Slower Propagating Speed
AIR
( Sand / Gravel )
GLASS /WATER
Slower Propagating Speed
Snell's Law
AIR
NORMAL
LIGHT RAY
NOW LIGHT BENDS AWAY FROM THE NORMAL
GLASS /WATER
Slower Propagating Speed
Geometry of a Converging (Convex) Lens
Focus
Optical axis
Focal length
Optical axis - axis normal to both sides of lens - light is not refracted
along the optical axis
Focus - the point where light rays parallel to optical axis converge; the
focus is always found on the opposite side of the lens from the object
Focal length - the distance from the focus to the centerline of the lens
Lens and Mirror Aberrations
SPHERICAL (lens and mirror)
Light passing through different parts of a lens or reflected from
different parts of a mirror comes to focus at different distances from
the lens.
Result: fuzzy image
CHROMATIC (lens only)
Objective lens acts like a prism.
Light of different wavelengths (colors) comes to focus at different
distances from the lens.
Result: fuzzy image
Spherical Aberration in Lenses
Simple lenses suffer
form the fact that light
rays entering different
parts of the lens have
slightly difference focal
lengths. This defect is
corrected with the
addition of a second
lens.
The problem
One focal point
for all light rays
The solution
Spherical Aberration in Mirrors
The Problem
Simple concave mirrors suffer
from the fact that light rays
reflected from different locations
on the mirror have slightly
different locations on the mirror
have slightly different focal
lengths. This defect is corrected
by making sure the concave
surface of the mirror is parabolic
The Solution
All light rays converge
at a single point
Chromatic Aberration in Lenses
Focal point
for blue light
Simple lenses suffer from
the fact that different colors
of light have slightly
different focal lengths. This
defect is corrected by
adding a second lens
The problem
Focal point
for red light
Focal point
for all light
The solution
Coma
• Affects Fast Mirrors with deeply curved reflecting surface
• Causes elongation in one axis if the object is not near
the center of the FOV
• Faster the Mirror the more of an issue.
• Its Not a mistake in workmanship
• This off-axis distortion is called coma, named after the
term for a comet’s head
Types of Optical Telescopes
Basic Telescope Designs
Refractor
• Uses a lens to gather the
light to a point
• Most rugged design - easy to
care for
• Gives the sharpest views especially of planets and the
moon
• Most expensive for any
given aperture
• Usually the tube is quite
long, although short tube
designs are now available
• Inexpensive models suffer
from chromatic aberration –
achromatic vs. apochromatic
Basic Telescope Designs
Reflector
• Uses a mirror to gather the
light to a point
• Open tube collects dust,
mirror eventually tarnishes
• Requires periodic alignment
(collimating) of the mirrors
• Least expensive for any
given aperture
• Available in both long and
short tube design
• Generally no chromatic
aberration
• Most “bang for the buck”
Basic Telescope Designs
Compound
Schmidt-Cassegrain, Maksutov
• Uses mirror and lens to
gather the light to a point
• Sharp views, Maksutov are
almost as good as refractors
• Closed tube protects optics
• Moderate cost for any given
aperture
• Tube is shortest for any
given aperture
• Most portable for any given
aperture
Refracting Telescope
Uses lens to focus light from
distant object - the eyepiece
contains a small lens that
brings the collected light to a
focus and magnifies it for an
observer looking through it.
Focal Ratio= FL/Obj Diam
Obj Diam
FL= Focal Length
Types of Reflecting Telescopes
Each design incorporates a small mirror just in front of the prime focus to
reflect the light to a convenient location for viewing.
Focal Length and Focal Ratio
• Focal Ratio= Focal Length/Objective Diam
• Faster = Shorter= is smaller ratio
• Shorter Focal Ratio Optics (F6 and below)
• Wider Fields of View
• More Compact
• More Expensive or More Distorted
– Optics must be close to perfect
– Fast Optics are difficult to make
Telescope Specs
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100mm F7 Refractor
100mm F10 Refractor
200mm F10 Schmidt Cass
400mm F4.5 Newtonian
16 inch F4.5 Newtonian
The Powers of a
Telescope
Light Gathering Power: Astronomers prefer *large*
telescopes. A large telescope can intercept and focus more starlight
than does a small telescope. A larger telescope will produce
brighter images and will be able to detect fainter objects.
Resolving Power: A large telescope also increases the sharpness
of the image and the extent to which fine details can be
distinguished.
Magnification: The magnifying power is the ability of the
telescope to make the image appear large in the field of view.
Three Fundamental Properties of a Telescope
Light-Collecting Area
think of the telescope as a “photon bucket”
The amount of light2 that can be collected is dependent on the
mirror area
A =  (D/2)
Resolution
smallest angle which can be seen
 = 1.22  / D
The angular resolution of a reflecting telescope is dependent on
the
diameter of the primary (D) and the wavelength of the
light being viewed
()
These properties are much more important than magnification which is
produced by placing another lens - the eyepiece - at the mirror focus.
Light Gathering Ability: Size Does Matter
1. Light-gathering
power: Depends on
the surface area A of
the primary lens /
mirror, proportional
to diameter squared:
A = (D/2)2
D
Angular Resolution
• The ability to separate two
objects.
• The angle between two
objects decreases as your
distance to them increases.
• The smallest angle at which
you can distinguish two
objects is your angular
resolution.
Eyepieces
• Used to magnify the image at the focal
plane for viewing by the naked eye
• Your image will only be as good as the
weakest chain in your optical system
• Many Different designs
– All specified with an Eyepiece FL and an
AFOV
Types of Eyepieces
• Old designs (limited use)
– Huyghenian, Ramsden, Kellner, Erfle
– Low Cost, with distortion
• Gold Standards, (52deg AFOV)
– Plossl, Orthoscopics
– Med Cost, without distortion
• Widefields, ( up to 82deg AFOV)
– Naglers, Panoptics, Radians, Swans
– High Cost: Distortion Free AFOV correlates to Cost
– More money vs more distortion
Magnifying Power
Magnifying Power = ability of the telescope to make the
image appear bigger.
The magnification depends on the ratio of focal lengths of
the primary mirror/lens (Fs) and the eyepiece (Fe):
M = Fs/Fe
A larger magnification does not improve the resolving
power of the telescope!
Rule of Thumb- Maximum useful Mag is 50x per inch of
Objective diameter under ideal seeing
- 20x to 30x per inch of Objective is more common in NE
Field of View: FOV
• Each eyepiece design has a specified Apparent Field of
View, AFOV
• AFOV/ Magnification = effective FOV
– Expressed in angular degrees
Ex…
A 25mm Plossl with 52deg FOV is being used on a
refractor with a 1000mm FL.
What is the magnification and FOV:
1000mm FL/25mm Ocular= 40x mag
52deg AFOV / 40 Mag = 1.3 deg effective FOV
Examples
• 100mm F7 Refractor, w 32mm Plossl (52deg AFOV)
• Mag
• FOV
• 200mm F10 Schmidt Cass, w 32mm Nagler (82deg AFOV)
• Mag
• FOV
• 400mm F4.5 Newtonian, w 32mm Widefield (66deg AFOV)
• Mag
• FOV
Eye relief
• The distance from the last surface of the eyepiece eye
lens (the lens closest to your eye) to where the image is
formed.
• Eye relief should be fairly long for comfortable viewing,
– if you must wear eyeglasses, you will need a minimum of 15mm
of eye relief to see the entire field of view
– Eye relief usually decreases as eyepiece focal lengths get
shorter
• More $$ for more eye relief
Barlow Lens
• x2 or x3 increase in your mag or a /2 or
/3 decrease in your eyepiece FL.
• Using a x2 barlow you can make a
32mm eyepiece also serve as a 16mm
eyepiece.
– (But you keep the 32mm eye relief)
• Slight decrease in image brightness
due to extra elements.
Altitude-Azimuth (Alt-Az)
• Simple, easy to use
• Inexpensive
• Most portable
Equatorial
• Easy to keep objects in
the field of view
• More difficult to setup
• Usually heavy
• Usually driven
Dobsonian (Dob)
• Very easy to use
• Least expensive ??
• Very stable
Most important: Stability!
Telescope Mounts
Many mounts
are motorized,
some are
computerized!
Finders
• Why? most telescopes have a 1 to 2 deg
FOV at their lowest magnification
• Types
– Reflex Sight:
• Zero Power
• dovetail, red dot, telrad (concentric circles)
– Magnifying 30mm, 50mm and 70mm
• Correct view
• Telescope view
Finder Protocol
• Use a star map to define area to observe
• Use the finder to point the scope to the
general area
• Use your eyepiece with the widest
effective field to locate your target.
• Happy Observing
Filters
Filter Basics
• Filters are designed to block
light.
• This inherently darkens the
image, so the scope must be
able to pull in enough light to still
allow you to see the object you
are interested in.
• Due to this fact, small telescope
often do not benefit from filters.
• The Moon looks better through a
filter in any size telescope.
• The Sun can be viewed directly
with the proper filter.
• Most filters are threaded for
attaching to the bottom of
eyepieces, the front of
diagonals or to the visual
back of an SCT telescope.
Solar Filters
• Conventional solar filters come in
two varieties (glass and Mylar film)
and allow us to see sunspots on
the surface of the sun.
• Most Mylar filters show the sun as
a blue disk. Glass filters generally
show the sun in yellow. Baader
Solar Film (Mylar) show the sun as
a white disk and has the best
contrast.
• H-Alpha filters are expensive, but
allow us to view the flares and
other features in the Sun’s
chromosphere.
These conventional solar filters
mount on the front of the scope.
Never use a solar filter that mounts
on the eyepiece!
Moon Filters
• The Moon is very bright,
especially at lower
magnifications. This makes
it difficult to see fine detail.
• A standard lunar filter may
block 80% or more of all
visible light.
• A polarizing filter uses two
polarized elements that can
be rotated to vary the
amount of light blocked.
Color Filters
• Color filters are mostly used for
the planets.
• By blocking certain wavelengths
(colors) of light, they help to bring
out faint details.
• To learn what colors work well for
which planets, visit the Learning
Center at www.telescope.com.
• Other than Jupiter and Venus
(two very bright objects) color
filters will not provide much
benefit for scopes smaller than
4.5”.
Deep Sky Filters
• Designed to pass only certain
wavelengths of light in order to
show faint objects while
blocking manmade light and
skyglow.
• Broadband filters allow most
light to pass, but block
wavelengths commonly
produced by exterior lighting.
They improve most faint
objects.
• Narrowband filters block much
more light, but pass the light
emitted by many faint nebulae.
• Oxygen III (O-III) filters block
all but the one specific
wavelength common to just
a few nebulae (the Veil
nebula for example).
• Hydrogen Beta (H-Beta)
filters block all but the one
specific wavelength common
to just a few nebulae (the
Horsehead and California
nebula for example).
• These filters will not provide
much benefit for scopes
smaller than 6”.
Credits
• Phillip Anderson, University of Texas
• Joseph Howard, Info Technology
• Michael Swanson, US Naval Hospital Okinawa
Appendix
Snell's Law
Where:
VL1 is the longitudinal wave velocity in
material 1.
VL2 is the longitudinal wave velocity in
material 2.
Snell's Law describes the relationship between the angles and the
velocities of the waves. Snell's law equates the ratio of material
velocities VL1 and VL2 to the ratio of the sine's of incident and
refracting angles.
Snell's Law
n=(c/v) where :
C is the velocity
of light and
v is the velocity
of light in that
medium
where 1 and 2 are the angles from the normal
of the incident and refracted waves, respectively.
n1, n2 are indices of refraction of the two media
respectively.
Refracting vs Reflecting Telescopes
Reflecting telescopes are primary astronomical tools used for research:
1. Lens of refracting telescope very heavy - must be placed at end of
telescope - difficult to stabilize and prevent from deforming
2. Light losses from passing through thick glass of refracting lens must be very high quality and perfectly shaped on both sides
3. Refracting lenses subject to chromatic aberration