Astronomy 04 The Solar System Chapter 6.1: Radiation Chapter 6.2: Telescopes Please pick up your Test 1 Scantron on front table if you haven’t already.

Electromagnetic radiation is radiation that travels through space and transfers energy from one place to another. Electromagnetic radiation possesses changing electric and magnetic fields. Electromagnetic radiation travels at the speed of light (300,000 km/sec or 186,000 mi/sec)

The wavelength of electromagnetic radiation is the distance between successive peaks of a wave.

The frequency of electromagnetic radiation is the number of cycles (waves) that pass a given point in one second.

The wavelength of electromagnetic radiation varies from a few nanometers to 10,000 meters. A nanometer is 1/1,000,000,000th of a meter. Another very small unit that astronomers use is the angstrom. One angstrom equals 1/10,000,000,000 of a meter.

Electromagnetic radiation can also be thought of as a stream of particles carrying specific amounts of energy. A particle of electromagnetic energy is called a photon.

The amount of energy a photon carries depends upon its wavelength. The shorter the wavelength the more energy a photon carries. The longer the wavelength the less energy the photon carries.

The electromagnetic spectrum is an array of electromagnetic radiation in order of wavelength.

Visible light has wavelengths from 400-700nm. Beyond red light, we have long wavelength radiation Infrared radiation Microwave radiation Radio waves

Beyond violet light, we have short wavelength radiation. Ultraviolet radiation X-rays Gamma rays

Telescopes

Chapter 6.1: “Radiation: Information from Space” The most important function of a telescope is to collect faint light from an astronomical source and to focus this light into an image. The light-gathering power of a telescope is determined by the diameter or aperture of the device acting to gather light – usually the objective lens or mirror. The larger the lens or mirror the greater the light-gathering power of the telescope.

Refracting Telescope

Refracting Telescope – A telescope which uses a large glass lens (objective lens) at one end and a smaller lens (eyepiece ) at the other end.

Focus – The point at which light passing through a lens or reflects off a mirror converges to a point.   Focal Length – The distance from the lens or mirror to the focus.

Chromatic aberration is the inability of the objective lens to bring all colors of visible light to a common focus. This problem of refracting telescopes is solved by using a achromatic lens.

Reflecting Telescope – A telescope which uses a large mirror at one end, a diagonal mirror and an eyepiece lens at the other end.

Reflecting telescopes may have different optical arrangements: Prime focus telescope Newtonian telescope Cassegrain telescope

Telescope Mountings: An alt-azimuth mount uses altitude and azimuth to locate an object in the sky. An equatorial mount uses right ascension and declination to locate an object in the sky.

A sidereal drive moves the telescope to compensate for the motion of Earth’s rotation.

The Powers of a Telescope Light-gathering power is determined by the diameter or aperture of the device acting to gather light – usually the objective lens or mirror.

Resolving Power is the ability of the telescope to reveal fine detail. a (sec of arc) = 11.6 / D (cm)

Magnifying power is the telescope’s ability to make an image larger. The magnifying power equals the focal length objective lens divided by the focal length of the eyepiece lens.

The amount of detail visibility in a telescope is also dependent on atmospheric conditions: Seeing refers to the stability of the atmosphere. Light pollution refers to the brightening of the sky by artificial lights.

A technique called adaptive optics uses a computer to monitor atmospheric distortion and adjust the telescope’s optics to compensate.

The Hubble Space Telescope