Title: Optical Telescopes for Astrophysics Dummies
1Optical Telescopes for Astrophysics Dummies
2The 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.
3And 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
4A 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
5Sorry, 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
6Kepler 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
7They Knew it Then The Bigger The Better
Most important property of a telescope Aperture
!! Larger Objective More light
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Without the light magnification is useless !
8So 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)
9Result 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
10New 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
11Why 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
12Reflector 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
13Newtons 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
14Newtons 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
15Other 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
16More 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
17More 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!
18Enough 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
19A 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
20Large 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
21Bigger 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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22And 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!
23The 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
24The 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
25Sloan 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
26Wider 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
27And 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)
28And 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
29In Honor of the Soccer Champs