Ch 5 -how do telescopes work

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How do Telescopes Work?

How Telescopes Work? Tool of the trade

The whole idea is to collect radiationfrom distant objects, not just observe them.

We have come a long way with telescopes.

Now telescopes are not only on earth but in space as well. Modern TelescopeWhat is measured:1. Brightness2. Spectra3. Position

8 meter mirrorat Mauna Kea,Hawaii

This picture shows the primary mirror blank, after fusing, of one of the Gemini 8 meter optical/infrared telescopes to be installed by the end of the decade (roughly 1998-2000) on mountaintop observatories at Mauna Kea, Hawaii, and Cerro Pachon, Chile. Both sites offer clear weather and good atmospheric stability. The large mirror sizes and good atmospheric conditions, in combination with the use of adaptive optics and other engineering advances, this should allow these telescopes to achieve even better image quality than that presently afforded by Hubble Space Telescope at wavelengths above about 1 micron. 4Tools:Astronomers are rarely at the eyepiece. They are on computerterminals.

Powers of a TelescopeCollecting Power -Bigger the telescope, more light collected. Focusing Power -Use mirrors or lenses to bend the path of light rays to create imagesResolving Power - Picking out the details of an imageLight Gathering Power

Light collected brighter image is proportionalto the collectorarea.

Small changesin collector radius gives large change in number of photons caught. Because A = pr2 Refracting Telescopes


Refraction is

The bending of light becauseof lightchanging speed in different mediums. Refraction is responsible forDispersion: causes different colors to travel at different speeds through the same material. Responsible for the distortion of the Sun near the horizon.

Twinkling or Scintillation

Temperature and density differencesin pockets of air shift the image of a star. Refracting TelescopesTelescopes that use lenses to collect and focus light called refractors.

Disadvantages: 1. Large lens: Expensive2. Large lens: sags in the center3. Dispersion causes images to have colored fringes. 4. Many lens materials absorb short wavelength light. Chromatic aberrationThe colors of white light refract differently so they focus differently.

Reflecting TelescopesUse Mirrors.Can make BIG, BIG Mirrors. Used almost exclusively by Astronomers today.

Multiple mirrorsand very thin.

Remember Resolving Power?For a given wavelength resolution is increased for a larger telescope. Interferometers will increase resolution by combining observations from two or morewidely spaced telescopes.

Interferometer difference

McGraw Hill Picture See two stars in B.Light Gathering Power Compare Two Telescopes: A to B

LGA = Diameter of A LGB Diameter of B2Example: Suppose we compare a 6 cm telescope with a 32 cm telescope. How much more light will the large telescope gather?

Use A as the larger telescope: (32/6)2 = 28.6 times