02 light and telescopes mc neely 2008

Astronomy

Light & Telescopes

Edwin Hubble and the 48-inch Palomar Telescope in 1949

Light

A form of wave motionWaves:

Rise and fallTransfer energy, but not materialFeatures: Crest, trough, wavelength, frequency

Photon: Light can also behave as a particle named a photon

Frequency: The number of waves that pass a fixed point in a given time

Waves

Visible Light

Human eye responds to “visible light” which is just one portion of the electromagnetic spectrum

Visible Light = 4000-7000 AngstromsVisible Spectrum: ROYGBIV

Wavelength Relationship

EM Spectrum

Short wavelengths

Long wavelengths

Speed of Light

The speed of light is represented as “c” in Einstein’s famous equation (E=mc2)

“c” = 186,000 miles per second (300,000 km per second)

“Speed limit of the universe,” nothing can travel faster

Light Years

Light Year: Distance measure of light travel in one year, about 6 trillion miles

Light year is a measure of distanceLight from sun = 8 light minutes

Light Travel Time

Light from nearest star = 4.3 light yearsDiameter of Milky Way Galaxy =

100,000 lyDistance to Andromeda Galaxy = 2.3

million lyDistance to Virgo Galaxy Cluster = 50

million ly

Time Travel

Light takes time to travel through spaceThe farther away we look in distance, the

further back we look in timeEx: The star Sirius lies 8 light years away.When we look at Sirius, we are seeing the

star as it was 8 years ago

Types of Telescopes

Three types: Refractors

Use lenses to collect light

ReflectorsUse mirrors to collect light

CompoundBoth lenses and mirrors

Telescope Designs

http://www.aw-wrdsmth.com/scuttlebutt/telescope-daigram.jpg

Refractors

“Department store” refractor

Modern APO refractor

Types of Reflectors

The Newtonian reflector was first designed by Isaac Newton and uses two mirrors to collect light

In recent years, Newtonians have been popular in the Dobsonian design where the telescope tube is mounted like a cannon

Newton’s original telescope

http://telescopemaking.org/images/newtontele.jpg

Newtonian & Dobsonian

Meade Dobsonian telescope

http://www.nachohat.org/images/static/meade_starfinder.jpg

Eyepiece

Newtonian optical diagram

John Dobson

6-in Newtonian on a Dobsonian Mount

“6-in” indicates that the telescope uses a 6-inch diameter mirror as its main light gathering optic

This Orion Telescopes XT6 is an excellent scope for beginners and is reasonably priced

Compound Scopes:Schmidt-Cassegrain

Cutaway view of an SCT

Maksutovs are similar yet use a more curved front lens

Main mirror

Lens

Meade Telescopes 8-

inch SCT

Eyepiece

Properties of Telescopes

Objective: Main mirror or lensAperture: Diameter of the objective,

determines amount of light gathered by the scope

Eyepiece: Set of small magnifying lenses that forms the image viewed through a telescope

Focal Length: Distance from the objective to the image in the eyepiece

Refractor Objective Lens

http://www.rocketroberts.com/astro/refractor.htm

Magnification

Magnification =Telescope focal length ÷ Eyepiece focal length

Ex: 2800mm focal length Schmidt Cassegrain telescope, with 32mm and 25 mm focal length eyepieces:

2800mm ÷ 32mm = 87.5x2800mm ÷ 16mm = 112x

Useful Magnification

Highest useful magnification usually equals 50 times the aperture of the scope in inches:

Useful magnification = 50 * Aperture (inches)

Ex: What is the highest useful magnification of a 2.4-inch department store telescope and a 6-inch reflecting telescope?

2.4-in * 50 = 120x6-in * 50 = 300x

Telescope Formula

A useful relationship for describing telescopes is the following:

f/number =

Focal length ÷ Aperture

Compare:8-inch reflecting telescope of 900mm focal

length70 mm refractor of 480 mm focal length

Telescope Formula Examples8-inch Reflector 2.7-inch

Refractor

Aperture (mm) 200mm 70mm

Focal Length (mm)

900mm 480mm

f/Number 900/200=f/4.5 480/70=f/6.8

Magnification (32mm eyepiece)

900/32=28x 480/32=15x

Two Scopes

Telescopes and Light Collecting

Small increases in aperture can dramatically improve telescopic views

This is because area is proportional to the square of a telescope’s diameter

Telescopes promoted as having high magnification are meant to deceive consumers because aperture is the true way to access a telescope’s ability

Aperture Demo

http://www.clarkvision.com/visastro/m51-apert/index.html

The animation compares sketches of the Whirlpool Galaxy (M51) through 6, 8, and 12.5 in telescopes

Telescope Aberrations

Chromatic: Inability of a lens to focus all colors of the spectrum. Ex: Color error or chromatic aberration in

refracting telescopes

Spherical: Inability of a mirror to reflect all light to a single point.Ex: Poorly made reflecting telescope mirrors. Original Hubble Space Telescope mirror

BinocularsUseful for stargazingTwo telescope tubes

mounted side to sideUsually have fixed

magnificationsEx: Pair labeled 7x50,

means 7x magnification, front objective lenses of 50mm diameter

Milky Way starfield

Telescope “Seeing”

The term “seeing” refers to the steadiness of the atmosphere overhead

Poor atmospheric seeing produces “twinkling” (star scintillation)

Unsteady air produces poor telescope images without sharp focus

Telescopes need to acclimate to outside temperature

http://en.wikipedia.org/wiki/Astronomical_seeing

Lunar crater Clavius in

poor seeing

Star Scintillation

This montage of photographs shows how a single star’s image is distorted over time by atmospheric seeing or turbulence

Ideal star image

Why do Star’s Twinkle?

Turbulent air causes a star’s image to distort

Light Pollution

Stargazing is difficult in the cityExcess artificial light that enters the night

sky is termed light pollutionObservatories are built in remote places

away from cities if possible

http://www.apstas.com/astrotas/glow.jpg

Effects of LP

http://www.spaceweather.com/swpod2003/20aug03/Carlson1.jpg

The view from Kitt Peak The view from Kitt Peak National Observatory of National Observatory of the Tuscon, Arizona the Tuscon, Arizona skyline in 1959skyline in 1959

The same skyline in 1972The same skyline in 1972

Kitt Peak LP

US at Night

http://archives.cnn.com/2000/US/08/24/dimming.the.lights.ap/large.usa.lights.jpg

Eastern US

http://www.seds.org/~aschultz/images/light-pollution/us_nite.gif

Europe

http://www.clocktower.demon.co.uk/stockgrove/light/europe.jpg

•Notice how brightness can indicate wealth and development; Poor countries have much less outdoor lighting

Earth at Night (Click Below)

http://veimages.gsfc.nasa.gov//1438/earth_lights_lrg.jpg

Good and Bad Lighting

•Good light fixtures shine their light only toward the ground, not toward your eyes or the sky

•A bad light fixture is one in which the uncovered bulb is visible

Light Fixtures

•Billboards that emit light straight into the sky are bad

Observatories

Observatories provide a permanent installation to house a telescope

Modern, professional observatories are usually located on mountain tops to take advantage of better seeing

Ex: Keck Observatory on Mauna Kea in Hawaii

Amateur astronomers build backyard observatories of many types

Keck Observatory

http://www.wainscoat.com/astronomy/keck-moonlight.jpg

•The twin Keck 10-meter telescopes are the largest in the world

•The telescopes are located on the 14,000 foot elevation summit of Mauna Kea in Hawaii

•Each telescope uses a “mirror” composed of 36 hexagonal segments arranged in a mosaic pattern

•The individual mirrors act together like a single mirror

Keck “Mirror”

http://www.astro.ucla.edu/~seth/albums/images/mirror3.jpg

Backyard Observatory

Roof rolls off for easy access to sky

SCT mounted on a permanent pier

aka Mini Keck

Summary: Telescope Formulas

Magnification =

Telescope Focal Length (mm) ÷ Eyepiece Focal Length (mm)

“Useful” Magnification =

50 * Aperture (in)

f/Number =

Telescope Focal length (mm) ÷ Telescope Aperture (mm)