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Optical Telescopes for
Astrophysics Dummies
Lance Simms
MASS 7/6/06
July 12, 2006
1
The First Telescope
1608 - Jan (or Hans) Lippershey,
a spectacle maker, invents
the refractor telescope
Objective
Eyepiece
Refractor - Objective is a lens
July 12, 2006
Rumor: his kids discovered it while playing
around in his shop.
2
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 Jupiter’s
moons….
looking mad
After seeing
Jupiter’s moons…
no more lazy eye
July 12, 2006
3
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
July 12, 2006
4
Sorry, More Terminology
Magnification
Same Magnification, different FOV
M= fobjective/ feyepiece
f = focal length
Field of View (FOV)
Amount of sky that can be seen
at one time through telescope
Usually expressed in deg2
July 12, 2006
Same FOV, different Magnification
5
Kepler One-Ups Galileo
1608 Galileo Design
Concave eyepiece
•Objective and Eyepiece
separated by difference of
focal lengths
•Upright Image is formed
•Small field of view
1611 Kepler Design
•Objective and Eyepiece
separated by sum of focal
lengths
•Inverted Image is formed
•Large field of view
July 12, 2006
Convex eyepiece
6
They Knew it Then: The Bigger The
Better
Most important property of a telescope:
Aperture !!
Larger Objective = More light
5’’
8’’
14’’
July 12, 2006
Without the light
magnification is useless !
7
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)
July 12, 2006
8
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
July 12, 2006
9
New Lens Design Saves the Day
1720’s - 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.
36 in. lens at Lick Obs.
1895
1886
July 12, 2006
Refractor at Yerkes Obs.
Now
10
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
July 12, 2006
11
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
July 12, 2006
12
Newton’s 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
July 12, 2006
13
Newton’s 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
July 12, 2006
14
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
July 12, 2006
15
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
July 12, 2006
16
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!
July 12, 2006
17
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 Cassegrains…except for minor tweaks
July 12, 2006
18
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
July 12, 2006
19
Large Mirrors = Large Mount
•William Herschel’s 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
Alt-Az Mount: 2 axes
1) Up/Down -- Altitude
2) Left/Right -- Azimuth
July 12, 2006
•Example of Alt-Az Mount
•Herschel didn’t like using it; he
preferred his 20 footer
20
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
1
2) Declination - celestial latitude
Turning one knob follows a star!
July 12, 2006
2
21
And How to Keep it Dry?
Put it in a Dome!
- protects telescope from
Dome of SOAR telescope
elements, bird droppings
- care must be taken to
avoid large temperature
gradients/turbulence
CFD simulation showing turbulence
generated by 3m/s wind
• Turbulence is the enemy!
July 12, 2006
22
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
12, 2006
OpticalJuly
Surface:
Concave, hyperboloid
Gemini North: (background)
Location: Mauna Kea in Hawaii
Elevation: 4200 meters
23
The Keck Telescopes
Location: Mauna Kea
Primary Mirrors: 10 m, 36 hexagonal concave hyperbolic segments
Optical Design: Ritchey-Chretien Cassegrain
Alt-Az
Na Laser Guide Star Adaptive Optics
Mount
8 Stories high
Both telescopes can be used together
as an optical interferometer
85 m baseline gives 0.005’’ resolution at
2 microns
July 12, 2006
24
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
Small Scope, Large Field of View
2 of sky in one image
~ July
3 deg
12, 2006
With its wide field,
SDSS will map 1/4 of
the sky
25
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
July 12, 2006
26
And Who Could Forget HST?
Bigger is better on earth,
but location trumps size
Above atmosphere
Below it
Hubble Space Telescope
•
•
•
•
Telescope style:
Diameter:
Collecting area:
Effective focal length:
July 12, 2006
Ritchey-Chretien Cassegrain
2.4 m (94 in)
approx. 4.3 m² (46 ft²)
57.6 m (189 ft)
27
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
July 12, 2006
Capable of “research” science !
We’ll be using it soon
28
In Honor of the Soccer Champs
GRAZIE
July 12, 2006
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