Telescopes

Optical Telescopes• Ground based and on satellites• Observations are recorded using a camera instead of the

human eye most times. – This is so we can separate colors into light and using times to

study changes over time.• All optical telescopes are reflecting telescopes.

– Mirrors are perfectly shaped and made of high quality, clear glass.– Mirrors are mounted at the bottom of the scope for stability– Mirrors do not produce chromatic aberration.– Lens brings different colors of light into focus at different places.

• Telescopes are like a giant eye.• They collect more light than our eyes allowing us to see faint

objects in greater detail.

• Two major properties of Optical telescopes• Light collection area: how much light the

telescope can collect at one time.– The bigger the area, the more light collection.

• Angular resolution: the smallest angle over which we can tell two stars are distinct.– Large telescopes can have large angular resolution

but is limited due to Earth’s atmosphere.• Diffraction Limit: an interference of light– Depends on the diameter of the telescope mirror and

wavelength being observed– Large telescopes have small diffraction limit and

better (smaller) angular resolution

Ground based telescopes• Pro: Less expensive to build, operate, and maintain• Con: daylight and weather• Can observe Radio Waves, Microwaves, Visible light,

and parts of Infrared

• Problems from our atmosphere– Light pollution– Twinkling/atmospheric turbulence• Wind and air currents are constantly moving

around.• Changes atmosphere’s light bending properties,

or view of things outside Earth’s atmosphere appear to jiggle around• Blurs astronomical images

–Most forms of light do not reach the ground.

Space based telescopes• Pro: above the atmosphere, unaffected by

daylight and weather, light pollution, and atmospheric turbulence.

• Con: More expensive and time consuming to build and maintain

• Can record microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays

Types of Telescopes

• Refracting Telescope• Reflecting Telescope• Multiple-Mirror Reflectors (MMT)• Schmidt Telescope

Refracting Telescope• Developed by Galileo• Very simple, but expensive• Uses Objective lens to bend light to a focus point• Light passes through a lens having an inverted image• Focal length: the distance from the lens to where the

focus will occur. Variables that change this are thickness and curvature.

• Increase both = decrease focal length• Decrease both = increase focal length• Disadvantage: two focal points creates chromatic

aberration.

Reflecting Telescope

• Developed by Newton• Use of concave mirror to reflect light to one point• Very cheap• Light is collected by concave mirror produces small

image• Second mirror reflects the image to the eyepiece

where it is magnified• No chromatic aberration• Example Hubble Space Telescope

Hubble Space Telescope• Named after Edwin Hubble• First launch on April 24, 1990• Job: discover all forms of light in the electromagnetic

spectrum• Problem 1: mirror couldn’t focus, had a spherical aberration

because it was too flat and didn’t account for zero gravity• Problem 2:pin used for stability was put in backwards, NASA

knew about it and ignored it. Made things off by one human hair = 1 micron.

• Now 100% functional• Advantage = easy to work on• Typical maintenance: replace solar units and fuses, and

correct optics on cameras.

Radio Telescopes• Most common type in the world• Similar to a satellite dish– Differences: they look into the sky, larger in size, different

purpose.• Pointed towards cosmic sources that rise and set with

Earth’s rotation.• Atmosphere does not distort radio waves like it does

with visible light.• No advantage to observe from space.• Dish reflects radio waves to an antenna which takes

the signal and amplifies it and then records it. Recorded on paper and makes a contour map.

Interferometry• Allows multiple radio telescopes to be linked

in a way that allows them to obtain the angular resolution of a much larger telescope.– Example: VLA – Very Large Array (in NM)• 27 radio telescopes

Other Telescopes

• Multiple-Mirror Reflectors (MMT)– Several mirror take the place of a single large

mirror– Less expensive to build than a single large mirror.

• Schmidt Telescope– Uses both reflecting mirror and refracting lens– Used to make wide-angle photographs of the sky