Land Based Telescopes. Telescopes: "light buckets" Primary functions: 1. ___________ from a given region of sky. 2. ______ light. Secondary functions:

Land Based Telescopes

Telescopes: "light buckets"

Primary functions: 1. ___________ from a

given region of sky.

2. ______ light.

Secondary functions: 1. ____________ in image

2. _______ angular size of objects.

Gather light

Focus

Resolve detail

Magnify

Optical Telescopes• Designed to collect ____________ of light

that are ________ to the human eye.• Data observed by human eyes or recorded

on photographs or in computers.

wavelengthsvisible

The Human Eye: Shortcomings

• Eye has limited _____.– limited light gathering power.

• Eye has limited ___________________.– only detects E-M in visible wavelengths.

• Eye distinguishes new image _________ _____________.

– cannot be used to accumulate light over long period to intensify faint image.

• Eye _____________ image for future reference. – unlike photographic plate or CCD.

size

frequency response

multipletimes/second

cannot store

Optical Telescope Design

• Basic telescope has two parts:1. _________

Function: to ____________Materials: __________ of longer focal length & larger diameter than the eyepiece

2. _____________

Function: to ____________ made by objectiveMaterial: ____ with a shorter focal length than the objective

Objectivegather light

Lens/mirror

Eyepiecemagnify image

lens

Optical Telescopes• ___________

–Focus light with _________: bend light path in transparent medium

–Use _______ –First kind made, used by Galileo

• __________–Focus light by __________:

bounce light off a solid medium

–Use ________

–First designed and created by Sir Isaac Newton

• ____________-Uses both ______ and _______

Refractors

Reflectors

Catadioptric

refraction

lenses

reflection

mirror

lenses mirrors

First Optical Telescopes:Refractors

Image of source is formed on focal plane and magnified by eyepiece.

Refracting Telescopes

The Yerkes 40” Refracting Telescope

Refractors: Disadvantages• Quality optics require ______________

– surfaces must be _______– glass will _______ light, especially IR and UV.– changes in _____________________ may flex lenses

– __________ very heavy, hard to support

• ___________________–light passes through glass–refraction a function of wavelength–all wavelengths focus different distances from lens

–correctable with compound lenses, expensive

high toleranceperfect

absorb

orientation, temperature

Large size

Chromatic aberration

Chromatic Aberration• Dispersion of light through optical material causes

blue component of light passing through lens to be focused slightly closer to lens than red component.

• Known as chromatic aberration.

Reflecting Telescopes: Designs

Why build reflectors instead of refractors (advantages)?

1.Mirrors don’t have _________ __________.

2.Mirrors don’t ____________ (especially infrared and UV).

3.Mirrors can be supported by their ____ and back; lenses by ONLY their edge.

4.Mirrors have only ____________ to be machined correctly; lenses have two.

chromaticaberration

absorb light

edge

one surface

Why build reflectors instead of refractors?

5. Telescopes made with mirrors can be _________ in design; reflectors cannot.

6. Telescopes using mirrors can have ______________ ends (because they have bigger mirrors), which means

_______________________.

compact

more light-gathering power

larger objective

Powers of the Telescope

1. Magnifying PowerThe ability to enlarge an image.

2. Light Gathering Power The ability to see faint objects.

3. Resolving Power The ability to see fine details.

focal length of objective

focal length of eyepieceMagnification =

focal length of eyepiece

focal length of objective

Magnification and Focal Length

Light-Gathering Power

•The objective’s area collects light.•The larger the area, the greater the light-gathering power of telescope.

Light-gathering power

proportional to

___________________.

diameter

(objective diameter)2

Resolving Power

• Varies directly with the ____________ _________.

• Also depends on

– ___________ of light being observed and

– __________________ conditions.

diameter ofobjective

wavelength

atmospheric seeing

Resolving Power: Diameter and Wavelength

Site Selection

• Where are the best places for ground-based observatories?

• Important factors

– __________ pollution

– _____ weather

– ___ air

– _____________

dark/light

good

dryair turbulence

Closer to Sea Level, More air to pass through

Higher Altitude, telescopes in the high mountains

Earth At Night

U.S.A. At Night (circa 1994-95)

Detection • Collected light detected in many

ways.–image observed and recorded

• eye, photographic plate, ____–measurements

• intensity and time variability of source–___________

• spectrum of source–____________

CCD

photometer

spectrometer

CCD Imaging• A charge-coupled device(CCD)

– Wafer of silicon divided into a two-dimensional array of many tiny elements, known as pixels.

– When light strikes a pixel, electric charge builds up on device.

– Charge buildup monitored electronically.

Radio Telescopes•Much _______ than reflecting optical telescopes

•Resemble satellite TV dishes

•Used to collect radio waves from space

•AM, FM, and TV signals interfere, so must be in a radio “protected” area

larger

Radio Astronomy: Wavelength Advantages

• NOT __________ on time of day/night• NOT as __________ on weather • Use of interferometry• Information other than _____________

– Quasars, pulsars

• Generally _____________ traveling space– pass through clouds of interstellar dust in our galactic

plane

• __________ of dish shape not as hard to create or maintain– not need to be highly polish, often light weight

dependent

Accuracy

not absorbed

visible light

dependent

Arecibo Observatory:Largest Radio and Radar Dish

•1000-ft radio dish•Used to

–create maps of Moon, Venus, and Mars–discover pulsars and galaxies–measure the rotation

rate of Mercury–discover planetary

systems outside of

our solar system

Very Large Array(VLA) in New Mexico

27 antennas, each 25 m in diameter

Effective diameter = 36 km

Yields radio-image details comparable to optical resolution

Interferometry

• ___________ telescopes used–to observe same object –at same wavelength and –at the same time.

• Uses wave interference to yield high resolution.

• ________ than one (impossibly) large telescope.

• Farthest _ telescopes act like the end of one telescope.–Baseline: • distance between 2 farthest

‘scopes.• equals the relative ‘scope size.

Two or more

Cheaper

2