02 light and telescopes mc neely 2008
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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)