Telescopes and Optics

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Telescopes and Optics

• NHAS Astro 101

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Agenda

• 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

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Lenses and Mirrors
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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 -
Inverse square law - Light intensity
diminishes with square of distance from source.
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Law of Reflection
?
Normal
?
Angle of incidence (?) angle of reflection
(?) The normal is the ray path perpendicular to
the mirrors surface.
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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
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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.
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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.
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AIR
NORMAL

GLASS / WATER
Slower Propagating Speed
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AIR
Car

GLASS / WATER
Slower Propagating Speed
( Sand / Gravel )
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AIR
Car
GLASS / WATER
Slower Propagating Speed
( Sand / Gravel )
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NORMAL
LIGHT BENDING TOWARDS THE NORMAL
AIR
LIGHT RAY

GLASS / WATER
Slower Propagating Speed
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n2
NORMAL
LIGHT BENDING TOWARDS THE NORMAL
AIR
n1

Snell's Law ( Next Slide )
GLASS / WATER
Slower Propagating Speed
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GLASS / WATER
( Sand / Gravel )
Slower Propagating Speed

Car
AIR
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GLASS / WATER
( Sand / Gravel )
Slower Propagating Speed

Car
AIR
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GLASS / WATER
( Sand / Gravel )
Slower Propagating Speed

Car
AIR
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Slower Propagating Speed
GLASS / WATER
NORMAL

AGAIN, LIGHT BENDS TOWARDS THE NORMAL upon
entering a region with slower speed.
LIGHT RAY
AIR
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AIR

Car
( Sand / Gravel )
GLASS /WATER
Slower Propagating Speed
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AIR

Car
( Sand / Gravel )
GLASS /WATER
Slower Propagating Speed
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AIR

Car
( Sand / Gravel )
GLASS /WATER
Slower Propagating Speed
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NOW LIGHT BENDS AWAY FROM THE NORMAL
Snell's Law
AIR
NORMAL

LIGHT RAY
GLASS /WATER
Slower Propagating Speed
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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
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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
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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
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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
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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
Focal point for red light
The problem
Focal point for all light
The solution
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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 comets head

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Types of Optical Telescopes
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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

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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

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Basic Telescope Designs

• CompoundSchmidt-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

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Refracting Telescope
Obj Diam
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.
FL Focal Length
Focal Ratio FL/Obj Diam
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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.
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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

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Telescope Specs

• 100mm F7 Refractor
• 100mm F10 Refractor
• 200mm F10 Schmidt Cass
• 400mm F4.5 Newtonian
• 16 inch F4.5 Newtonian

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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.
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Three Fundamental Properties of a Telescope

• Light-Collecting Area
• think of the telescope as a photon bucket
• The amount of light that can be collected is
  dependent on the mirror area A ? (D/2)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.

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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
D
A ?(D/2)2
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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.

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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

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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

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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
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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

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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

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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

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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.

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Telescope Mounts

• 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!

Many mountsare motorized,some arecomputerized!
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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

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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

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Filters
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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.

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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 Suns
  chromosphere.

These conventional solar filters mount on the
front of the scope. Never use a solar filter
that mounts on the eyepiece!
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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.

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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.

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Deep Sky Filters

• 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.

• 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.

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Credits

• Phillip Anderson, University of Texas
• Joseph Howard, Info Technology
• Michael Swanson, US Naval Hospital Okinawa

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Appendix
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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.
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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.
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Refracting vs Reflecting Telescopes

• Reflecting telescopes are primary astronomical
  tools used for research
• Lens of refracting telescope very heavy - must be
  placed at end of telescope - difficult to
  stabilize and prevent from deforming
• Light losses from passing through thick glass of
  refracting lens - must be very high quality and
  perfectly shaped on both sides
• Refracting lenses subject to chromatic aberration

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