Optical Telescopes for Astrophysics Dummies

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Optical Telescopes for Astrophysics Dummies

• Lance Simms
• MASS 7/6/06

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The First Telescope

• 1608 - Jan (or Hans) Lippershey,
  a spectacle maker, invents the
  refractor telescope

Objective
Eyepiece
Refractor - Objective is a lens
Rumor his kids discovered it while playing
around in his shop.
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And Then Galileo
A year Later (1609) Galileo Galilei builds a
scope and looks at the Moon and discovers 4 moons
around Jupiter and phases of venus
Before seeing Jupiters moons. looking mad
After seeing Jupiters moons no more lazy eye
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A Little Terminology

• Objective - lens or system of lenses closest to
  object being viewed
• Eyepiece - lens or system of lenses closest to
  eye/detector
• Focal length - distance of surface of lens/mirror
  to focal point
• Aperture - diameter of objective

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Sorry, More Terminology

• Magnification
• f focal length

Same Magnification, different FOV
M fobjective/ feyepiece
Field of View (FOV)
Amount of sky that can be seen at one time
through telescope Usually expressed in deg2
Same FOV, different Magnification
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Kepler One-Ups Galileo
1608 Galileo Design

• Objective and Eyepiece separated by difference of
  focal lengths
• Upright Image is formed
• Small field of view

Concave eyepiece
1611 Kepler Design

• Objective and Eyepiece separated by sum of focal
  lengths
• Inverted Image is formed
• Large field of view

Convex eyepiece
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They Knew it Then The Bigger The Better
Most important property of a telescope Aperture
!! Larger Objective More light
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8
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Without the light magnification is useless !
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So They Made Bigger Lenses

• But there was a problem
• Spherical Aberration

Light from edges of lens focuses at different
point than light from interior of lens

• All lenses then were spherical lenses
• Descartes proposed two solutions in 1637
• 1. Make Lenses elliptical or hyperbolic
  (not realizable at time)
• 2. Increase the focal length while keeping
  same diameter to lessen
  aberration (means BIG telescope)

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Result HUGE Telescopes

• 1637-1722 objectives of longer and longer focal
  length were made
• 150-200 feet long tubes were not uncommon!

• Largest Refractor is French
• - Stationary Lens
• - 60 m long horizontal tube
• - 1.25m objective lens
• - It was a failure

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New Lens Design Saves the Day

1720s - Elliptical and Hyperbolic lenses finally
feasible - allowed reasonably sized telescopes
to be built
Alvan Clark and Sons built largest refractor lens
at 40 inches
40 in. lens at Yerkes Obs. 1895
36 in. lens at Lick Obs. 1886
Refractor at Yerkes Obs. Now
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Why Stop at 40 inches?

• Large lenses tend to sag under their own weight
  -- distorts image
• Long mounting tubes flex under weight of lens --
  bad for optical alignment
• Alternative Use Mirrors. They
• can be supported
• from below

Light
Gravity
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Reflector Telescopes

• A reflector telescope has a mirror as its
  objective

• James Gregory proposed such a telescope in 1663
  but no optician could build it. He gave up,
  but still got a design named after him.

Gregorian Telescope Concave parabolic
Primary Concave ellipsoidal Secondary -
located beyond focal point of primary
Primary/Secondary/Tertiary/etc. refers to order
in which light strikes surface
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Newtons Reflector

• Isaac Newton designed a reflector in 1672 in his
  attempt to overcome Chromatic Aberration

• Chromatic Aberration
• Each wavelength of light is refracted at
  different angle
• Each wavelength has different focal length
• Only occurs in refraction not reflection

Newton also thought up a way to eliminate the
defect by using two different lenses, but messed
up an experiment and concluded that all
transparent materials refract equally. Now
opticians make double Achromatic lenses
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Newtons Reflector

• Newtonian Reflector
• Concave Spherical Primary
• Flat Secondary Mirror

No Chromatic aberration But still Spherical
aberration
In 1663 John Hadley replaced the spherical mirror
with a parabolic mirror, eliminating the
spherical aberration
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Other Reflectors

• Cassegrain Reflector
• Concave Parabolic Primary
• Convex Hyperbolic Secondary

The design was conceived in about 1672 by the
Frenchman Guillaume Cassegrain Little is
known about him
Popular twist is the Schmidt-Cassegrain
- parabolic primary is replaced with
spherical mirror - corrector plate is inserted to
correct spherical aberration
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More Cassegrains

• Ritchey-Chretien Cassegrain
• Concave hyperbolic primary
• Convex hyperobolic secondary

• Design is free of 3rd order Coma and spherical
  aberration
• Most common type used on research telescopes

Coma is a an inherent property of telescopes
using parabolic mirrors that causes off-axis
images to have fuzzy shapes, like little comets
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More Cassegrains?

• Maksutov-Cassegrain
• Concave spherical primary
• Convex spherical secondary

• - Spherical corrector lens plate removes first
  order spherical aberration
• - Tend to have narrower field of view than
  Schmidt-Cassegrains due to longer
  focal length
• Invented by Dmitri Maksutov (1896-1964)
• Does not scale very well with large aperture
  since meniscus corrector plate becomes
  prohibitively large and expensive

An excellent telescope for lunar and planetary
observations!
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Enough with the Cassegrains!
Dall-Kirkham Cassegrain Concave parabolic
primary Convex spherical secondary

• Under corrected primary removes first order
  spherical aberration of the spherical secondary
• Large coma makes its usable field of much smaller
  than true Cassegrain
• Developed in 1930s by Horace Dull of Luton,
  England

That about covers Cassegrainsexcept for minor
tweaks
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A Comparison of Points

• Point Spread Function (PSF)
• The irradiance distribution resulting from a
  single point source (e.g. a star) in object space

Simulated PSF for LSST telescope
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Large Mirrors Large Mount

• William Herschels 40 foot long, 4 foot mirror
  telescope in Slough, England 1789
• It took 2 assistants to point while he observed
• They had speaking tubes to communicate
• Example of Alt-Az Mount
• Herschel didnt like using it he preferred his
  20 footer

Alt-Az Mount 2 axes
1) Up/Down -- Altitude 2) Left/Right --
Azimuth
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Bigger Mirrors Better Mounts

• Mirrors continued to get bigger and optical
  quality improved
• Equatorial Mount introduced

Equatorial Mount 2 axes 1) Right Ascension -
celestial longitude 2) Declination - celestial
latitude Turning one knob follows a star!
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2
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And How to Keep it Dry?
Put it in a Dome! - protects telescope from
elements, bird droppings - care must be taken to
avoid large temperature gradients/turbulence
Dome of SOAR telescope

• CFD simulation showing turbulence generated by
  3m/s wind
• Turbulence is the enemy!

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The Big Guns Gemini Twins
Secondary Mirror Diameter 1.023 metres/3.36
feet. Central Hole Diameter 0.168 metres
Optical Surface Convex, hyperboloid
Gemini South (above) Location Cerro Pachon,
Chile Elevation 2700 meters
Primary Mirror Outside Diameter 8.10 metres
Central Cassegrain Hole 1.18 metres Thickness
20 cm/7.87 inches Optical Configuration
Ritchey-Chretien Cassegrain Optical Surface
Concave, hyperboloid
Gemini North (background) Location Mauna Kea
in Hawaii Elevation 4200 meters
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The Keck Telescopes

• Location Mauna Kea
• Primary Mirrors 10 m, 36 hexagonal concave
  hyperbolic segments
• Optical Design Ritchey-Chretien Cassegrain
• Na Laser Guide Star Adaptive Optics

Alt-Az Mount
8 Stories high
Both telescopes can be used together as an
optical interferometer 85 m baseline gives
0.005 resolution at 2 microns
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Sloan Digital Sky Survey (SDSS)

• Location Apache Point Obs. Sacramento Peak,
  NM
• Primary 2.5 meter
• Secondary 1.08 meter
• Design Gascoigne-Ritchey Cassegrain

COSMIC MAP
With its wide field, SDSS will map 1/4 of the sky
Small Scope, Large Field of View 3 deg2 of sky
in one image
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Wider Fields Wanted LSST

• Still in the works
• Large Synoptic Survey Telescope

.25 deg2
Location Cerro Pachon,Chile Elevation 2700
meters
.5 deg
Primary 8.4 meters concave Secondary
3.4 meters convex Tertiary 5.0 meters concave
FOV 10 deg2
Design Paul-Baker 3 element
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And Who Could Forget HST?
Bigger is better on earth, but location trumps
size
Above atmosphere
Below it

• Hubble Space Telescope
• Telescope style Ritchey-Chretien Cassegrain
• Diameter 2.4 m (94 in)
• Collecting area approx. 4.3 m² (46 ft²)
• Effective focal length 57.6 m (189 ft)

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And Right in Our Backyard

• Stanford Student Observatory Scope
• Telescope style Cassegrain/Newtonian
• Diameter 0.61m (24 in)
• Secondary mirror is convertible to accommodate
  Cassegrain and Newtonian foci

Capable of research science ! Well be using it
soon
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In Honor of the Soccer Champs

• GRAZIE