Telescopes

Amateur and Professional

Galileo 1609

The Moon as a World

Jupiter has Moons

Refracting telescopes

Long focus refractors were awkward but suffered less from chromatic aberration

Isaac Newton’s reflecting telescope

Mirrors do not havechromatic aberration

Reflecting telescope

Objective mirrors instead of lenses

Three Powers

• Magnifying

• Resolving

• Light Gathering

Magnifying Power

• Ability to make objects appear larger in angular size

• One can change the magnifying power of a telescope by changing the eyepiece used with it

• Mag Power = focal length of objective divided by the focal length of the eyepiece

Resolving Power

• Ability to see fine detail

• Depends on the diameter of the objective lens or mirror

Light Gathering Power

• The ability to make faint objects look brighter

• Depends on the area of the objective lens or mirror

• Thus a telescope with an objective lens 2 inches in diameter has 4 times the light gathering power of a telescope with a lens 1 inch in diameter

Herschel & Lord Rosse

19th century: epoch of the large refractors

Refracting telescopes

Vienna

Lick

YerkesObservatory

Largest refractingtelescope with aone meter objective

20th century Large Reflectors Come of Age

Mount Wilson Observatory 1.5m (1908) and 2.5m (1918)

Palomar 5-m(entered operation in 1948)

4 meter Reflectingtelescope

Objective Mirror

Dome of 4 meterKitt Peak

Keck Telescopes

SOAR Telescope

4.1 meter

SOAR Telescope -- Cerro Pachon

SOAR Observing Room

SOAR Image of the planetary nebula NGC 2440

MSU Campus Observatory

Boller & Chivens reflecting telescope with a 24-inch objective mirror

More on resolution

• Eagle-eyed Dawes• The Dawes Limit

R = 4.56/D

Where

R = resolution in seconds of arc

D = diameter of objective in inches

More appropriate for visible light and small telescopes

A more general expression for the theoretical resolving power

• Imagine that star images look like Airy disks

Minimum Angle that can be resolved

• R = 1.22 x 206,265 / dR = resolution in seconds of arc

= wavelength of light

d = diameter of the objective lens or mirror

Note that the wavelength of light and the diameter of the objective should be in the same units

Examples

• For Visible light around 500nmOur 24-inch telescope

R = 0.20 seconds

This may be compared with the Dawes limit of 0.19 seconds

But with large ground-based telescopes it is difficult to achieve this

Astronomical “seeing”

• Blurring effect of looking through air

• Causes stars to twinkle and planetary detail to blur

– At the SOAR site: good seeing means stellar images better than about 0.7 seconds of arc

– In Michigan, good seeing means better than about 3 seconds of arc

– Not to be confused with good transparency

Bad seeing onthis side

Good seeingon this side

Electromagnetic Spectrum

Radio TelescopesArecibo

Very Large Array

Radio telescope resolution

= 1m d = 100m

R = 2500 seconds = 42 minutes!

Even though radio telescopes are much bigger, their resolving power is much worse than for optical telescopes

Interferometric arrays get around this

Very Large Array

Interferometry

Size of array = 10 km for a VLA

This becomes the effective d

Now R becomes 25 secsec for a

1-m wavelength

For VLBI (very long baseline interfeormetry) the d = 10,000km and R = 0.025 seconds

Observing from space

• No clouds

• Perfect seeing

• Can see wavelengths of light blocked by the earth’s atmosphere

Hubble Space Telescope

Rooftop telescopes