July 12, 2006 1

Optical Telescopes for Astrophysics Dummies

Lance SimmsMASS 7/6/06

July 12, 2006 2

The First Telescope

1608 - Jan (or Hans) Lippershey, a spectacle maker, invents

the refractor telescope

Rumor: his kids discovered it while playing around in his shop.

Refractor - Objective is a lens

Objective Eyepiece

July 12, 2006 3

And Then Galileo

Before seeing Jupiter’s moons….

looking mad

After seeing

Jupiter’s moons…

no more lazy eye

A year Later (1609) Galileo Galilei builds a scope and looks at the Moon and discovers 4 moons around Jupiter and phases of venus

July 12, 2006 4

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 5

Sorry, More Terminology

Magnification

f = focal length

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

Same Magnification, different FOV

July 12, 2006 6

Kepler One-Ups Galileo

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

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

1608 Galileo Design

1611 Kepler Design

Convex eyepiece

Concave eyepiece

July 12, 2006 7

They Knew it Then: The Bigger The Better

Most important property of a telescope:

Aperture !!Larger Objective = More light

5’’

8’’

14’’Without the light magnification is useless !

July 12, 2006 8

So They Made Bigger Lenses

•But there was a problem:

Spherical AberrationLight 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 9

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 10

New Lens Design Saves the Day

Alvan Clark and Sons built largest refractor lens at 40 inches

40 in. lens at Yerkes Obs.

189536 in. lens at Lick Obs.

1886

1720’s - Elliptical and Hyperbolic lenses finally feasible

- allowed reasonably sized telescopes to be built

Refractor at Yerkes Obs.Now

July 12, 2006 11

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 LightGravity

July 12, 2006 12

Reflector TelescopesA A reflector reflector telescope has a mirror as its objectivetelescope 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 TelescopeConcave 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 13

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 14

Newton’s ReflectorNewtonian Reflector Concave Spherical Primary Flat Secondary Mirror

No Chromatic aberrationBut still Spherical aberration

In 1663 John Hadley replaced the spherical mirror with a parabolic mirror, eliminating the spherical aberration

July 12, 2006 15

Other Reflectors

Cassegrain ReflectorConcave Parabolic PrimaryConvex 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 16

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 17

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 18

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 19

A Comparison of PointsPoint 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 20

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

•Example of Alt-Az Mount

•Herschel didn’t like using it; he preferred his 20 footer

1) Up/Down -- Altitude 2) Left/Right -- Azimuth

Alt-Az Mount: 2 axes

July 12, 2006 21

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!

1

2

July 12, 2006 22

And How to Keep it Dry?

CFD simulation showing turbulence generated by 3m/s wind

• Turbulence is the enemy!

Put it in a Dome!Put it in a Dome!

- - protects telescope protects telescope from elements, from elements, bird droppingsbird droppings

- care must be taken - care must be taken to avoid large to avoid large temperature temperature gradients/turbulence gradients/turbulence

Dome of SOAR telescope

July 12, 2006 23

The Big Guns: Gemini Twins

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

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

Gemini North: (background) Location: Mauna Kea in Hawaii Elevation: 4200 meters

July 12, 2006 24

The Keck TelescopesLocation: Mauna KeaPrimary Mirrors: 10 m, 36 hexagonal concave hyperbolic

segmentsOptical Design: Ritchey-Chretien Cassegrain Na Laser Guide Star Adaptive Optics

8 Stories high

Both telescopes can be used together as an optical interferometer85 m baseline gives 0.005’’ resolution at 2 microns

Alt-Az Mount

July 12, 2006 25

Sloan Digital Sky Survey (SDSS)

Location: Apache Point Obs. Sacramento Peak, NM

Primary: 2.5 meterSecondary: 1.08 meterDesign: 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

July 12, 2006 26

Wider Fields Wanted: LSST

Still in the works…Large Synoptic Survey Telescope

Location: Cerro Pachon,ChileElevation: 2700 meters

Primary: 8.4 meters concave Secondary: 3.4 meters convex Tertiary: 5.0 meters concave FOV:

10 deg2

Design: Paul-Baker 3 element

.5 deg

.25 deg2

July 12, 2006 27

And Who Could Forget HST?

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)

Bigger is better on earth, but location trumps size

Above atmosphereBelow it

July 12, 2006 28

And Right in Our BackyardStanford 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 !We’ll be using it soon

July 12, 2006 29

In Honor of the Soccer Champs

GRAZIE