Light and Telescopes

Chapter 5

Traditional Telescopes

The 4-m Mayall

Telescope at Kitt Peak National

Observatory (Arizona)

Advances in Modern Telescope Design (I)

Modern computer technology has made possible significant advances in telescope design:

1. Simpler, stronger mountings (“Alt-azimuth mountings”) to be controlled by computers

Advances in Modern Telescope Design (II)

2. Lighter mirrors with lighter support structures, to be controlled dynamically by computers.

Floppy mirror

Segmented mirror

The Keck Telescopes

The two Keck Telescopes on Mauna Kea, Hawaii. Each telescope has a mirror diameter of

10 meters.

Examples of Modern Telescope Design

The Large Binocular Telescope (LBT)

The Very Lage Telescope (VLT) in

Chile

The Future of Optical Telescopes

The European Extremely Large Telescope (E-ELT): 906 segments in a

42-m mirror!

The Giant Magellan Telescope (2016)

Adaptive OpticsComputer-controlled mirror support adjusts the mirror

surface (many times per second) to compensate for distortions by atmospheric turbulence.

Distortions by the atmospheric turbulence are measured using a

laser beam.

InterferometryRecall: Resolving power of a telescope depends on

diameter D:

min = 1.22 /D.

This holds true even if the entire surface is

not filled out.

→ Combine the signals from several

smaller telescopes to simulate one big

mirror → Interferometry

What a telescope does

• Typical resolution of eye is about 0.5 minutes of arc or 30 arc seconds.

• HST has a resolution of 11.6/240 = .048 arc seconds. With HST we can resolve objects that are almost 1000 times smaller.

• NOTE the equation used above was given on page 77 of the textbook.

CCD ImagingCCD = Charge-coupled device

• More sensitive than photographic plates

• Data can be read directly into computer

memory, allowing easy electronic manipulations

False-color image to visualize brightness contours

Negative Images

The galaxy NGC 891 as it would look to our eyes (i.e., in real colors and

brightness)

Negative images (sky = white; stars = black) are

used to enhance contrasts.

The SpectrographUsing a prism (or a grating), light can be

split up into different wavelengths (colors!) to produce a spectrum.

Spectral lines in a spectrum tell us about the chemical

composition and other properties of the observed

object.

Radio TelescopesLarge dish focuses the energy of radio waves onto a small receiver (antenna).

Amplified signals are stored in computers and converted into

images, spectra, etc.

Radio MapsIn radio maps, the intensity of the

radiation is color-coded:

Red = high intensity; Violet = low intensity

Just like optical telescopes, radio telescopes should be built in regions with low average rainfall and cloud cover, and low radio noise.

Radio Interferometry

The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36

km in diameter.

Just as for optical telescopes, the

resolving power of a radio telescope is

min = 1.22 /D.

For radio telescopes, this is a big problem: Radio

waves are much longer than visible

light.

→ Use interferometry to

improve resolution!

The Largest Radio Telescopes

The 100-m Green Bank Telescope in Green Bank, WVa.

The 300-m telescope in Arecibo, Puerto Rico.