Lecture 11: Lecture 11:Telescopes - All the Better to See You With

"...I'll be watching you."
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Date: October 6, 1994
Reading Assignment: pp. 106-120

Description : Introduction to optical telescopes and detectors

Objectives

understand the differences between reflector and refractor telescopes

be able to identify the 4 main types of optical telescopes

list the types of optical detectors and the advantages and disadvantages of each type

understand the importance of resolution, light gathering power, and magnfication

Lecture Outline

Slide # 1: Lecture 11: Telescopes- All the Better to See You With

Click here for subtitle

refractors and reflectors

image detectors

why use telescopes?

Slide # 2: Lecture 10 : Atomic Physics for Kids of All Ages

absorption and emission lines

KirchhoffÕs Laws

Bohr atom

formation of spectral lines

temperature, line broadening and molecules

Slide # 3: The Electromagnetic Spectrum

Radio

Infrared

Visible

Ultraviolet

X-ray

Gamma-ray

Slide # 4: Two Processes

Emission

Gas emits light

Energy is lost from the gas

Absorption

Gas absorbs light

Energy is absorbed by the gas

Slide # 5: KirchhoffÕs Laws

describe the relationship between

emission lines

absorption lines

continuous spectra

Slide # 6: The Bohr Model

the hydrogen atom

Slide # 7: Line Intensity

lines of a given element change intensity due to temperature

temperature determines the fraction of atoms at each excited level

temperature is related to collisional rate

collisional rate determines excitation rate

Slide # 8: Two Types of Optical Telescopes

refractors

objective lense

reflector

objective mirror

Slide # 9: Refractors

objective lens

Slide # 10: Reflector

objective mirror

Slide # 11: Telescope Terms

prime focus

where objects are viewed

objective

main mirror or lense

focal length

distance between objective and prime focus

Slide # 12: A Problem with Refractors - Chromatic Aberration

different colors focus at slightly different distances

Slide # 13: Other Problems with Refractors

cannot support lenses from the back

need very clear glass

two optical surfaces

Slide # 14: Reflectors

can support back of mirror

glass can be less clear

one optical surface

Slide # 15: Types of Reflectors

prime focus

Slide # 16: Types of Reflectors

Newtonian focus

Slide # 17: Types of Reflectors

Cassegrain focus

Slide # 18: Types of Reflectors

Coude focus

Slide # 19: Field of View

telescopes objects near where they are pointed

ÒComaÓ occurs for objects off-center

Slide # 20: Coma

coma distorts images near the field edge

Slide # 21: Types of Detectors

photometers

image brightness

may be function of time or position

spectroscopes

spectrum of objects

Slide # 22: Image Detectors

the eye

photographic plates

CCD detectors

Slide # 23: Image Detectors- the Eye

advantages

cheap and low tech

disadvantages

no permanent record

low efficiency

not electronic

Slide # 24: Image Detectors- photographs

advantages

relatively low cost and low tech

disadvantages

relatively low efficiency

not electronic

Slide # 25: CCD Detectors

pixels are picture elements

Slide # 26: CCD vs Photographs

Photographs record 3% of photons

CCDÕs record 75% of photons

CCDÕs are 20-30x more sensitive than photos!

Slide # 27: Detectors- CCD detectors

advantages

great efficiency

electronic images

disadvantages

not cheap as cheap as photos

requires computers

Slide # 28: Which is the Best Detector?

CCD cameras are BEST for professional astronomy

Amateurs are using CCD cameras too!

Probably the future for astronomy!

eye is still best for public viewing

seeing the sky is fun, but not very useful

Slide # 29: Why use a telescope?

magnification

resolution

light gathering ability

Slide # 30: Light Gathering

how much brighter is the object?

the human eye can only gather light through the pupil

average size = 0.5 cm

Slide # 31: Telescopes

telescopes gather light across their objective

Slide # 32: Light Gathering

the light gathering power depends on the collecting area of the objective

Slide # 33: Light gathering

human eye = 0.5 cm diameter

amateur telescope = 18 cm

362 times more light gathering power!

professional telescope = 10,000 cm

(20,000)2 times more light gathering power!

Slide # 34: Magnification

how large is the image?

not very important

Slide # 35: Resolution

how clear is the object?

Slide # 36: Resolution Limits of Telescopes

Diffraction

limit of the mirror size and wavelength

Seeing

atmospheric limit

Slide # 37: Diffraction Limit

human eye = 0.5 arcminutes

10 cm optical telescope = 10 arcsecs

1 meter optical telescope = 0.1 arcsecs

5 meter optical telescopes = 0.02 arcsecs

Slide # 38: Diffraction Limits

human eye, 10cm telescope, 1 meter telescope

Slide # 39: Diffraction Limit

1 meter optical telescope = 0.1 arcsecs

1 meter infrared telescope = 2.5 arcsecs

1 meter radio telescope = 1 degree!

Slide # 40: Telescope Resolution

diameter of telescope

wavelength of EM energy

Slide # 41: Atmospheric Blurring

1 arcsec is resolution of most big, optical telescopes

atmosphere blurs images

Slide # 42: Seeing Disk

smeared out star image in long exposure from atmospheric blurring

Slide # 43: High Resolution Astronomy

telescope in space

adaptive optics

interfereometry

Slide # 44: Telescopes in Space

Hubble Space Telescope

resolution 0.05 arcseconds

20x better than normal telescopes on the ground

Slide # 45: Adaptive Optics

possible to compensate for atmospheric blurring

a few tests currently underway

Slide # 46: Interferometry

combine light from many telescopes

different blurring at each telescope

Slide # 47: Lecture 11: Telescopes- All the Better to See You With

refractors and reflectors

image detectors

why use telescopes?