Telescopes

• Magnification (make things look bigger)easy to make a telescope with good magnification

• Collection of large amounts of light (see fainter things)most important feature of a telescope

• Sharp images (see more detail and structure)• Map large areas of sky (search for rare objects)• Detect light across the electromagnetic spectrum (see

new phenomena)• Sophisticated analysis of light (e.g., spectroscopy)• Record images (e.g., photographs, digital pictures)

Functions of Telescopes

Types of Telescopes

A telescope collects light over a large area and focuses it in a small area where it can be viewed with an eye or a camera. This can be done in 2 ways:

• By using a lens to bend, or refract the light• By using a mirror to reflect the light

The size of a telescope is characterized by the diameter of its lens or mirror: a “36-inch telescope” has a lens or mirror that is 36 inches in diameter.

Refracting Telescopes

Refracting TelescopesThe first telescopes were refractors, but they have limitations:• A larger lens collects more light and allows one to see fainter

objects. However, it is difficult to physically support a big lens because it must be held at the edges.

• Lenses focus different wavelengths of light at different locations. As a result, images cannot appear in focus at all wavelengths simultaneously.

Reflecting TelescopesIt is much easier to build and support a large mirror than a large lens. Mirrors also bring all wavelengths to a common focus, unlike refractors. As a result, all modern telescopes used by professional astronomers are reflectors.

Comparison of refractors and reflectors

Day Night

The pupil of the eye expands at night in order to collect more light. Similarly, astronomers build telescopes with larger mirrors so that they can gather more light and see fainter objects in the sky.

The largest telescopes can collect more than a million times as much light as the human eye.

The mirrors in the largest optical/infrared telescopes have diameters of 10 meters. Rather than use single mirrors that are even larger, future telescopes will contain several mirrors.

currentfuture

Smaller mirrors are easier and cheaper to build, so large telescopes also use segmented mirrors.

The next generation of telescopes will have multiple mirrors that are equivalent to a single 30-50 meter mirror.

In addition to collecting more light and seeing fainter objects, larger mirrors also provide sharper images.

Twinkle, twinkle, little starAlthough large mirrors have the potential to produce sharp images, turbulence in the Earth’s atmosphere causes stars to appear blurry.

The mirror of the Hubble Space Telescope is 2.4 meters in diameter, smaller than the largest ground-based telescopes. But it is above the atmosphere, so it produces very sharp images.

Sharper images from space

ground-based telescope Hubble

ground-based telescope Hubble

ground-based telescope Hubble

Some telescopes on the ground now have the ability to partially correct for the blurring by the atmosphere.

normal blurry image of a star corrected for blurring

normal blurry image of Neptune

corrected for blurring

Mapping the SkyHubble provides very sharp images of tiny patches of sky. In comparison, some telescopes are designed to photograph the entire sky, making it possible to search for new rare objects.

Telescopes allow us to collect light across the entire electromagnetic spectrum

We see different phenomena at different wavelengths

Atmospheric WindowsGamma-ray and x-ray telescopes must be in space since those wavelengths don’t penetrate the atmosphere. Optical, infrared, and radio telescopes can operate from the ground, although the atmosphere makes optical and infrared images blurry.

Where to Put a Telescope on the Ground

For an optical observatory, you need • Dark skies (avoid city lights)• Clear skies/few clouds• Stable air for sharper images• High altitude to get above as much of the atmosphere

as possible

For a radio observatory, you need• An area far from cell phones and TV/radio stations

Optical (= Visible) & Infrared Observatories

Hawaii Chile

Arizona Chile

Texas

Radio Observatories

Very Large Array (VLA)

Aricebo

Square Kilometer Array

The Square Kilometer Array will be an array of thousands of radio dishes with a total collecting area of 1 km2 and a baseline of 3000 km. It will be built in Australia and Africa and will be 100 times more sensitive than the VLA.

Space Observatories

Hubble (visible) Chandra (x-ray)

WMAP (radio)Spitzer (infrared)Galex (UV)Compton

(gamma ray)

Space Observatories Chandra X-ray Observatory

• X-ray wavelengths• Can see things at very hot temperatures

Space Observatories Spitzer Space Telescope

• Infrared wavelengths• Can see things at very low temperatures

James Webb Space Telescope