Telescopes

The simplest means of observing the Universe is the eye.

The human eye is sensitive to light with a wavelength of about 400 and 700 nanometers.

Collecting Light

In a dark-adapted eye, the iris is fully open and the pupil has a diameter of about 7 mm.

pupil

Limitations of the human eye:

• No ability to build up exposure (equal to about 1/15th of a second)

• Fixed field of view / magnification

• Small collecting area

• Small wavelength coverage

Collecting Light

Around 1600, it was discovered that glass lenses could be used to correct human vision.

During the next decade, several lens makers realized that combinations of lenses could be used to augment normal vision.

The first telescopes used lenses, but later telescopes were developed that used mirrors.

Galileo Galilei built a much improved telescope and used it to survey the night sky.

Collecting Light

A reflecting telescope A refracting telescopeCollecting Light

Modern, large telescopes are all reflectors:

1. Light traveling through glass is bent differently depending on wavelength.

2. Glass is not perfectly transparent. Some light traveling through a lens is absorbed.

3. Large lenses can be very heavy, and can be supported only at their edges. Large mirrors can be supported across their back surfaces.

4. Lenses need two optically polished surfaces, mirrors need only one.

Collecting Light

A small reflecting telescopeNewton’s telescope (3.5 inch diameter)

A big reflecting telescopeMayall 4 meter telescope (Kitt Peak, AZ)

Prime focus

Cassegrain focus

An even bigger reflecting telescopeLarge Binocular Telescope (Mt. Graham, AZ)(Two 8.4 meter mirrors in tandem or solo operation)

A truly huge reflecting telescopeThe Thirty Meter Telescope (Planned for 2016)(492 segments form a 30 meter collecting area)

Collecting LightThe light-collecting power of a telescope equals the area of its primary mirror or lens.

1-meterdiameter

mirror

10-meterdiameter

mirror

How much more area does the larger mirror have over the smaller

mirror?

The recording device may form an image from the light (a camera) or break it into a spectrum (a spectrometer):

Recording Light

The modern standard for collecting visible light is the charge-couple device (CCD).

The CCD chip is made of a silicon wafer that absorbs photons of light and records the electric charge in microscopic “wells”.

CCDs can only measure brightness. They do not measure color directly.

e-

Recording Light

The assignment of color values in an image may not reflect the wavelengths of the spectral lines captured in the image.

“False color” or “Color composites”

Image of Eagle Nebula

Red: red sulfur line, green: red hydrogen line, blue: green oxygen line

This representation gives more information about the nebula than a “true” color image.

Image credit: T. Rector and B. Wolpa (NOAO)

Angular resolution is the ability of a telescope to distinguish neighboring objects in the sky as separate. Here is 4 images of a cluster of stars:

Image Quality

The image on the left has poor “resolution” while the image on the right as high “resolution”.

Image Quality

Angular resolution is limited by the diameter of the telescope and the wavelength of the light.

Telescopes with higher resolution:

• Larger diameter

• Observing shorter wavelengths

Going to Mountaintops: Most observatories are placed at high altitude to minimize the blurring of the atmosphere.

Large Binocular Telescope atop Mt. Graham, AZ, 10700 ft. altitude Kitt Peak National Observatory, AZ,

6900 ft. altitude

Cerro Tololo International Observatory, Chile, 7200 ft. altitude

Mauna Kea Observatories, Big Island of Hawaii, 13700 ft. altitude

Going to Space: The primary reason for placing the Hubble Space Telescope in orbit was to get above the blurring effect of the atmosphere.

Radio telescopes are like optical reflecting telescopes:

• wavelengths are longer, so surface is not polished

• relaxed engineering means larger telescopes

• larger diameter partially offsets long wavelength

Radio Telescopes

Design of 100-m Green Bank Telescope, WV

Largest radio telescope: 300-m dish at Arecibo, PR

Radio TelescopesRadio telescopes observe the range of frequencies

that pass through Earth’s atmosphere (the “radio window”).

The longer wavelength of radio waves means that radio telescopes must be much larger than other telescopes to achieve good resolution.

But there are advantages in radio astronomy:

• Radio telescopes observe 24 hours a day

• Clouds and rain don’t interfere with observing

• Different frequencies, different spectral lines

• Combine signal collected from widely-spread telescopes as if they came from a single antenna

• Resolution will be that of antenna whose diameter equals the separation between dishes

Very Large Array (VLA) in western New Mexico

Interferometry

For example, the Very Long Baseline Array uses 10 radio telescopes spread out across N. America.

Interferometry

Images routinely achieve resolution of a few micro-arcseconds

or

0.00001 arcsecond

Allows parallaxes of Milky Way and other galaxies!

Most of the electromagnetic spectrum is blocked by Earth’s atmosphere, only visible light and radiopenetrate all the way to Earth’s surface.

Atmospheric Absorption

Optical and infrared images of nebulas in Orion. What are you seeing in each image?

Optical image from NOAO Infrared image from 2MASS

Optical and x-ray images of the Crab supernova remnant. What are the temperatures of the gas emitting visible light and x-rays?

Optical image from ESO VLT X-ray image from Chandra Observatory

Optical and ultraviolet images of spiral galaxy M81. What kinds of objects are producing most of the visible light? The ultraviolet light?

You want to observe the emission lines from cold clouds of interstellar gas (at about 10 Kelvin).

What part of the spectrum would you observe?

A. gamma rays

B. ultraviolet

C. visible light

D. radio

Where would your telescope need to be?

You want to observe the thermal spectrum of a newly-born white dwarf (about 100,000 Kelvin).

In which part of the spectrum would the white dwarf be brightest?

A. gamma rays

B. ultraviolet

C. visible light

D. radio

Where would your telescope need to be?

You want to observe the population of cool G-type and K-type stars across our Milky Way galaxy.

In which part of the spectrum would these types of stars be brightest?

A. x-rays

B. ultraviolet

C. visible light

D. infrared

Where would your telescope need to be?

Suppose you wanted to observe the superheated gas (1 million Kelvin) falling into black holes at the centers of galaxies.

What part of the spectrum would you observe?

A. x-rays

B. ultraviolet

C. visible light

D. infrared

Where would your telescope need to be?