# Telescopes. Tomorrow night Viewing session # 1 7:00 – 8:00 p.m. DRESS WARMLY! Bring paper and writing utensil for taking notes.

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Telescopes

Tomorrow night Viewing session # 1 7:00 – 8:00 p.m. DRESS WARMLY! Bring paper and writing utensil for taking notes

When light goes from air to glass, it slows down. This causes it to bend We call that bending “refraction” There is also some reflection from the surface. Reflection and Refraction Inbound light Reflected light Refracted light fasterslower

Telescopes, microscopes, binoculars, and magnifying glasses take advantage of that bending to bring light to a “focus” The focal length of a lens depends on its curvature An “image” is formed at the focal point, or “focus” (when the light comes from very far away)

A telescope that uses a lens here is called a refracting telescope or This is called the “objective” lens The objective lens creates an image here The eyepiece is used as a magnifier to examine the image simply a refractor

Mirrors can also bring light to a focus “focal length”

And so mirrors are also used to make telescopes You should know these telescope types! Telescopes that use mirrors are called “reflectors”

The most important “powers of a telescope” are… Resolving Light gathering

Angular resolution is the smallest angle over which we can tell that two points – stars, craters on the Moon, etc. – are distinct Resolving power

It’s limited by “diffraction” and “interference” Interference Diffraction A diffraction pattern is formed whenever light goes through an opening, like when it enters a telescope! Points of light form bulls-eye patterns Then the diffraction patterns from nearby points of light “interfere” with each other Resolving power

It’s limited by “diffraction” and “interference” Diffraction A diffraction pattern is formed whenever light goes through an opening, like when it enters a telescope! Points of light form bulls-eye patterns Resolving power

The most important “powers of a telescope” are… Resolution Light gathering Bottom line: Larger telescope “aperture”  Smaller diffraction pattern  Better resolution

Light gathering Aperture

Light gathering Everything else being equal, a larger aperture will show more faint details

How much more light will a 20” telescope gather than an 8” telescope? A) 2.5 times more B) 6.25 times more C) 8 times more D) 20 times more E) 160 times more

The most important “powers of a telescope” are… Resolution Light gathering The LEAST important power of a telescope is magnification Over-magnification can lead to horrible results!

Still, telescopes DO magnify, so we need to know how to change that. Magnification Focal length of objective Focal length of eyepiece

Still, telescopes DO magnify, so we need to know how to change that. Magnification Focal length of objective Focal length of eyepiece Example: A telescope mirror has a focal length of 3000 mm. What is the magnification if we use a 25 mm eyepiece?

Still, telescopes DO magnify, so we need to know how to change that. Magnification Focal length of objective Focal length of eyepiece Example: A telescope mirror has a focal length of 3000 mm. What focal length eyepiece should I use if I want the magnification to be 250?

What are the three powers of a telescope? A) Magnifying, photographing, light gathering B) Area, length, diameter C) Resolving, refracting, reflecting D) Light gathering, resolving, magnifying E) Light gathering, area, magnification

What is the least important power of a telescope? A) Resolving B) Magnifying C) Light gathering

Spectroscopy Astronomers almost never look through telescopes! A “spectrograph” Spectral resolution

Astronomers almost never look through telescopes! Photometry Measuring the brightness of a star over time

Telescopes and the atmosphere Light pollution

Telescopes and the atmosphere Light pollution

Telescopes and the atmosphere Atmospheric turbulence “Twinkling” Seeing Bad “seeing” Good “seeing” The atmosphere bends the light passing through it, just like a pool’s surface bends the light passing through it.

Telescopes and the atmosphere Atmospheric turbulence Adaptive optics can fix the twinkling

Telescopes and the atmosphere Atmospheric absorption The only bands that get through our atmosphere are Visible and Radio

Telescopes and the atmosphere And so we put telescopes in space!

Which problem cannot be improved by placing a telescope on top of a mountain? A) Bad seeingB) Light pollution C) DiffractionD) Absorption

Disadvantages: Susceptible to radio noise (car ignitions, radio stations, etc.)  Locate them in remote places, away from dense populations Low angular resolution  Build them BIG  “Interferometry”

Radio telescopes Advantages: Can be used day or night Easy to build large Can easily do “interferometry”

Interferometry with radio telescopes The VLA in New Mexico

Interferometry with radio telescopes VLBI uses radio telescopes all over the world Has the angular resolution of a single telescope the size of the Earth!

Astro-Cash Cab! Ashley Lopez Rick Maxfield Jade Moreno Stevon Cornish Jesse Ling

1) What are the two main types of telescopes?

2) What happens to light when it goes through an opening, thereby limiting the telescope’s resolution? (one word)

3) What eyepiece should you use with a telescope of focal length 2000 mm, if you want a magnification of 100?

4) True or False ? In general, the seeing is better on top of a tall mountain than at sea level.

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