This Set of Slides

• This set of slides deals with telescopes.

• Units covered: 26, 27, 28, 29, and 30.

Telescopes

• Telescopes have been used for hundreds of years to collect light from the sky and focus it into an eyepiece. An astronomer would then look through the eyepiece at planets, nebulae, etc.

• The human eye is not very sensitive to dim light, and was replaced in astronomy by the film camera.

• Film is sensitive to only around 10% of the impinging light, and is often today replaced by a…

The Charge-Coupled Device (CCD)

• The CCD, similar to those found in commercial digital cameras and cell phones, can collect around 75% of the visible light that is focused on it.

• It has revolutionized astronomy – images can be recorded and downloaded to a computer anywhere in the world for analysis.

• The science of developing new methods for sensing, focusing and imaging light in astronomy is called instrumentation.

Outside the visible spectrum

• Many objects of astronomical interest are visible only in frequencies other than the visible.

• Much can be learned from studying a star, planet or nebula in different frequencies.

• Radio telescopes can be used from the ground to image pulsars and other bodies.

• Observations in other wavelengths require instrumentation to be lifted above the Earth’s atmosphere.

• X-ray, Gamma ray and infrared wavelength telescopes are currently in orbit.

• Displaying these require false color.

• Modern telescopes are designed to collect as much light as possible.

• Collected visible light is of nanometer wavelength; the telescopes must be extremely precise to keep the images clear and sharp.

Modern Telescopes

• Radio telescopes, like the one in Arecibo, Puerto Rico, collect radio waves from astronomical objects.

• Radio waves have long wavelengths. The telescopes must be large or arranged in arrays of smaller telescopes.

Radio Telescopes

Radio Telescope Project at OSU

• OSU physics department has a radio telescope project.

• Utilizing old satellite dish antennae (for TV) and homebuilt electronics, radio signals have been detected from space.

• See Dr. Bill Hetherington or Jim Ketter for more info.

www.physics.oregonstate.edu/~hetheriw/astro/index.html

Size Matters!

• Aperture size is very important when collecting light.

• A large collecting area allows astronomers to image dimmer and more distant objects.

• For a telescope with an aperture diameter D:

2

4Area Collecting D

Telescopes continued…

• How do telescopes focus light? There are two main ways this is done.

• Refracting telescopes (or refractors) use refraction to focus light through lenses to form images.

• Reflecting telescopes focus light by reflecting it off curved mirrors (for visible) or surfaces (for radio.)

Refraction

• The speed of light changes depending on what substance, or medium, it moves through.

• The speed of light in vacuum is around 300,000 km/s. Its speed through glass or water, for example, is slightly slower.

• If a beam of light enters a new medium at an angle, light on the side of the beam that enters first slows down.

• This slowing of one part of the beam causes the light to change direction.

• This bending of the path of light is called refraction.

Lenses

• A lens is a specially shaped piece of glass that bends light rays passing through it so that they focus a particular distance away (the focal length) at a particular location (the focal plane).

• A sensor - such as the human retina, camera film, or a CCD - placed in the focal plane can image the light.

– Glass is heavy, and glass lenses must be supported only by their rims, a difficult engineering problem.

– Glass sags under its own weight, distorting the lens and image.

– Refractors suffer because of dispersion, a blurring effect due to changes in the focal plane of the lens for different wavelengths of light.

Refracting Telescopes

Large refractors are difficult to build and have other limitations.

Dispersion

• The amount that light is diffracted depends on its frequency.• A prism or raindrop spreads white light out into colors.• This dispersion of light is a problem in refracting telescopes (and cameras

too), as the focal plane will be at a slightly different location for each wavelength (color) of light.

• This leads to chromatic aberration, a blurring effect.

• Mirrors can be supported from behind, and so can be much larger than refractors.

• Larger sizes mean that more light can be collected and focused, allowing astronomers to image dimmer or more distant objects.

• Most modern telescopes are reflectors.

Reflecting Telescopes

Different styles of reflectors

• X-rays only reflect at glancing angles, otherwise they are absorbed or pass through the mirror.

• X-Ray mirrors are designed to gently reflect the incoming photons, focusing them at the end of a long tube-shaped array of mirrors.

X-Ray reflectors

• Reflectors can be made even larger if multiple mirrors are used as the primary mirror.

• The Keck Telescope uses 36 large mirrors to create a single huge primary reflecting surface.

• The positions of the mirrors are precisely measured by lasers, and can be individually adjusted to keep them perfectly aligned.

Very Large Mirrors

• Some stars that appear to be a single object to the unaided eye are, when viewed through a telescope, discovered to be two separate stars.

• The telescope is able to separate the two stars, while the human eye is not.

• The telescope has better resolution than the human eye.

• The telescope’s resolution is better because it has a larger aperture, and there is less diffraction as light passes through that aperture.

• Diffraction is a “spreading” effect due to the finite size of an aperture.

• Light waves approach the aperture as flat plane waves, similar to the straight water waves seen above.

• As the waves pass through the aperture, the waves become curved as seen here.

Diffraction and Resolution

Diffraction Effects

• Diffracted light waves spread and interfere with each other.

• This results in a diffraction pattern, a blurring of the image as it passes through the telescope.

• Larger apertures have less diffraction, and therefore have higher resolution than smaller apertures.

• For observing light of wavelength nm, the smallest separation angle arcsec a telescope can resolve is related to the telescope aperture Dcm by:

cm

nm

D

02.0arcsec

Interferometers

• To counter diffraction effects (and obtain higher resolution), astronomers use interferometers.

• Signals from these arrays of widely-separated telescopes are added together to create images with very high resolution.

• In fact, the resolution is equivalent to that of a single telescope with an aperture as large as the separation between telescopes in the array!

• Image from a single radio telescope. – What looks like a single star…

• Image from an array.– …is actually two stars!

Single versus Array

• The Earth’s atmosphere absorbs most of the radiation incident on it from space.

• This is a good thing for life – high energy photons would sterilize the planet!

• This is not a good thing for astronomy, however!

• Visible, radio and some infrared wavelengths are not absorbed readily by the atmosphere.– Optical and radio telescopes work

well from the ground.

• Gamma Rays, X-rays, UV and IR photons are absorbed.– Observatories for these wavelengths

must be in orbit above the Earth’s atmosphere!

Atmospheric Absorption

Ground- and Space-based Observatories