Light, Optics and Telescopes Trip to Lick Observatory
Light, Optics and Telescopes Trip to Lick Observatory
Light, Optics and Telescopes Trip to Lick Observatory
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<strong>Light</strong>, <strong>Optics</strong><br />
<strong>and</strong> <strong>Telescopes</strong><br />
<strong>Trip</strong> <strong>to</strong> <strong>Lick</strong> Observa<strong>to</strong>ry<br />
• <strong>Lick</strong> Observa<strong>to</strong>ry trip: Thursday, May 6<br />
! <strong>to</strong>ur starts at 6:30pm<br />
! leave campus no later<br />
than 4:30pm<br />
! return about 11:00pm<br />
• Fill out info form (if you<br />
haven’t already)…<br />
! return it <strong>to</strong>day<br />
• If you need transportation,<br />
we’ll need $10 deposit by<br />
5/4 <strong>to</strong> guarantee a spot<br />
! it will be returned if<br />
you go on the trip
Next few classes…<br />
• This week’s labs…<br />
! in Nat. Sci. II Room 110/190 (just around the corner)…<br />
! bring your telescope kit<br />
!<br />
• Thursday’s class -- more telescope observing…<br />
! meet at 7:30pm at the Porter Squiggle<br />
! some of the same object’s as last week (be there if you<br />
didn’t h<strong>and</strong> in observations <strong>to</strong> Corey last week)<br />
! some new ones<br />
• Next Tuesday’s lab<br />
! make up missed labs<br />
How we see:<br />
• We see an object when light from the object reaches our<br />
eyes.<br />
• Either because the object emits light...<br />
! the Sun, a star, a light bulb<br />
• Or because the object reflects light<br />
! the Moon, the planets, your desk
Eyes <strong>and</strong> brain process information<br />
• We figure out where an object is<br />
! from the direction light enters the eye...<br />
• We get color information from the light's wavelength<br />
• An object appears red if it emits red light (a traffic light…)<br />
! or if it reflects red light (an apple).<br />
Why is the sky blue…<br />
• It’s not that air has a bluish glow…<br />
• Air molecules scatter sunlight, particularly blue parts of the<br />
spectrum.<br />
• Blue light reaches us<br />
from all directions.
At sunset, even more “reddening”…<br />
In space the sky is dark…
The electromagnetic spectrum<br />
• “White” light contains a<br />
mixture of colors…<br />
! similar <strong>to</strong> that in sunlight.<br />
• Our eyes are insensitive <strong>to</strong> most of the electromagnetic<br />
spectrum.<br />
Early studies of light…<br />
• Showed that it traveled at a finite speed…<br />
! Galileo tried <strong>to</strong> demonstrate this with lanterns<br />
! Olas Roemer succeeded with Jupiter’s moons<br />
• New<strong>to</strong>n developed a “particle” theory of light<br />
• Christian Huygens developed a “wave” theory<br />
! light waves analogous <strong>to</strong> sound waves<br />
In fact, they were both right…
<strong>Light</strong> as a wave…<br />
• <strong>Light</strong> has wave-like properties…<br />
! diffraction<br />
! interference<br />
Pho<strong>to</strong>ns: light as a particle<br />
• <strong>Light</strong> has particle-like properties…<br />
! travels in straight line<br />
! <strong>and</strong> “bounces”<br />
! i.e. reflects<br />
! transfers energy in discrete “quanta”<br />
Pho<strong>to</strong>electric Effect: blue<br />
pho<strong>to</strong>ns knock electrons<br />
off metal; red pho<strong>to</strong>ns don’t,<br />
even lots of them.<br />
diffraction of<br />
water waves<br />
diffraction of light
Wave fronts <strong>and</strong> light rays<br />
• Waves travel circularly<br />
(spherically) from source<br />
! light “rays” show<br />
direction of travel<br />
• If source is distant…<br />
! wavefronts are flat<br />
! light rays are parallel<br />
Why are distant objects hard <strong>to</strong> see?<br />
• They are faint<br />
! light spreads out as it<br />
travels from source<br />
• They are small<br />
! angular size is small
The spectrum, again<br />
• Different regions of the spectrum….<br />
! have different wavelengths…<br />
! have pho<strong>to</strong>ns that carry different amounts of energy<br />
A look at the eye…<br />
• <strong>Light</strong> enters through the pupil...<br />
! is focused by the lens...<br />
! on<strong>to</strong> the retina.<br />
• <strong>Light</strong> entering at different angles<br />
is focused in different places.<br />
• The information your brain gets is<br />
! wavelength(s)<br />
! direction
Cameras <strong>and</strong> telescopes<br />
• Cameras are the simplest “artificial” eyes...<br />
! film takes the place of your retina...<br />
• <strong>Telescopes</strong> can be thought<br />
of as large cameras...<br />
! bring light <strong>to</strong> a focus...<br />
! on some sort<br />
of detec<strong>to</strong>r...<br />
• As in eye, light entering from different angles…<br />
! is focused in different places on detec<strong>to</strong>r<br />
Lenses <strong>and</strong> mirrors<br />
• Lenses <strong>and</strong> curved mirrors described by focal length<br />
! distance from for parallel light rays <strong>to</strong> come <strong>to</strong> a focus<br />
f f<br />
! For both: more curved = shorter focal length<br />
• Telescope optics (without eyepiece) also have focal length<br />
! (effective) distance from primary mirror/lens <strong>to</strong><br />
location of focal point
<strong>Telescopes</strong> for visual observing…<br />
• If your eye is the “detec<strong>to</strong>r”…<br />
! optics need <strong>to</strong> be more complicated<br />
• Eyepiece lens makes<br />
light rays parallel again…<br />
! your eye will<br />
focus them on<br />
retina<br />
Refrac<strong>to</strong>rs<br />
• Use a lens for primary focusing<br />
of light…<br />
• Many backyard<br />
telescopes are<br />
refrac<strong>to</strong>rs…<br />
! but its hard<br />
<strong>to</strong> make<br />
large ones
Reflec<strong>to</strong>rs<br />
• Use mirror for primary focusing of light…<br />
New<strong>to</strong>nians, Cassegrains, etc<br />
• Much jargon surrounding ways of arranging focus <strong>and</strong><br />
detec<strong>to</strong>rs…<br />
prime focus<br />
New<strong>to</strong>nian<br />
Cassegrain
Catadioptrics<br />
• Use combination of lenses <strong>and</strong> mirrors…<br />
! Schmidt-Cassegrain<br />
! Maksu<strong>to</strong>v-Cassegrain<br />
• Design allows construction of compact<br />
telescopes with long<br />
focal lengths<br />
• Which allows for<br />
! high magnification<br />
viewing<br />
! high resolution imaging<br />
• xx<br />
An optical telescope A radio telescope
Telescope mounts:<br />
• Alt-Az<br />
! azimuth axis points <strong>to</strong> zenith<br />
! altitude axis parallel <strong>to</strong> horizon, usually east west<br />
" mechanically simple, intuitive <strong>to</strong> point<br />
! for tracking need two mo<strong>to</strong>rs, computer<br />
! a camera will turn compared <strong>to</strong> sky<br />
• Equa<strong>to</strong>rial (a.k.a. “polar”)<br />
! “hour-angle” axis points <strong>to</strong> (north) pole<br />
" allows tracking with one simple mo<strong>to</strong>r<br />
" camera will turn in same way sky does<br />
! declination axis is parallel <strong>to</strong> equa<strong>to</strong>r (east-west)<br />
! mechanically difficult<br />
(the late) alt-az mounted 300ft<br />
Green Bank WVa<br />
polar mounted 140ft<br />
Green Bank WVa
Imaging: what you care about<br />
• Aperture size: light collecting area<br />
! larger diameter telescopes can see fainter objects<br />
• Angular resolution: how much angular detail you can see<br />
! larger telescopes have better angular resolution<br />
! but blurring by sky is usually the limiting fac<strong>to</strong>r<br />
• Field of view: how much of the sky you can see at once<br />
! depends on design of telescope (“plate scale”)<br />
! size of your detec<strong>to</strong>r<br />
Large telescopes collect more light…<br />
• Telescope funnels light <strong>to</strong> camera<br />
! the larger the aperture, the more light collected<br />
35mm camera at<br />
“prime focus” on<br />
a Schmidt-Cassegrain<br />
telescope
<strong>and</strong> have better resolving power…<br />
• <strong>Light</strong> passing through any opening is<br />
bent slightly…<br />
• Which makes images slightly fuzzy<br />
! instead of a pin-point dot<br />
! you get a wide dot <strong>and</strong> rings<br />
• This limits angular resolution<br />
Angular resolution<br />
• The smallest angular separation a telescope can measure<br />
! its ability <strong>to</strong> see detail<br />
low angular resolution high angular resolution
Diffraction limit<br />
• Diffraction limit: best<br />
possible angular resolution<br />
of a telescope<br />
radio telescopes<br />
need <strong>to</strong> be very<br />
large for good<br />
! wavelength of radiation: angular resolution<br />
shorter gives better<br />
angular resolution<br />
! aperture size: larger gives better<br />
angular resolution<br />
• Depends on…<br />
• Diffraction limit of telescope of diameter D :<br />
! angular diameter of central “maxima”<br />
(dot) of diffraction pattern: " # 70° ($ / D)<br />
Field of view…<br />
1000-foot Arecibo telescope<br />
• For imaging, how much of the sky you can see is set by…<br />
! “plate-scale” of telescope<br />
! depends only on focal length<br />
! arcseconds of sky per millimeter of film/detec<strong>to</strong>r<br />
! S= 206265/f mm<br />
! size of film, pho<strong>to</strong>graphic plate,<br />
or detec<strong>to</strong>r
Visual observing: what you care about<br />
• Aperture size: light collecting area<br />
! larger diameter telescopes can see fainter objects<br />
• Magnification: how much larger object appears through<br />
eyepiece than <strong>to</strong> naked eye<br />
! depends on design of telescope <strong>and</strong> eyepieces<br />
! changing eyepieces changes magnification<br />
• Field of view: how much of the sky you can see at once<br />
! depends on design of telescope <strong>and</strong><br />
Large telescopes collect more light…<br />
• Telescope funnels light <strong>to</strong> <strong>to</strong> your eye<br />
! the larger the aperture, the more light collected
<strong>Telescopes</strong> magnify…<br />
• View through eyepiece…<br />
! increases apparent angular size of object <strong>and</strong> features<br />
! allowing eye <strong>to</strong> see more detail<br />
Telescope magnification:<br />
• For a telescope (or binoculars) magnification depends on…<br />
! focal length of telescope<br />
! focal length of eyepiece<br />
M = f telescope /f eyepiece<br />
• Telescope magnification changes by changing eye pieces<br />
! “low power” = long focal length (e.g. 50mm)<br />
! “high power” = short focal length (e.g. 7mm)
Telescope focal length:<br />
• Telescope focal lengths are not usually stated directly<br />
! usually via the aperture diameter<br />
! <strong>and</strong> focal ratio (ratio of focal length <strong>to</strong> aperture)<br />
• Example: Lab telescope…<br />
! aperture diameter: 10”<br />
! focal ratio: f/6.3<br />
! focal length is 10 x 6.3 = 63” = 1600mm<br />
! magnification with a 50mm eyepiece = 1600/50 = 32<br />
! magnification with a 10mm eyepiece = 1600/10 = 160<br />
• Various accessories will change telescope’s focal length<br />
! focal reducers shorten it<br />
! Barlow lenses lengthen it<br />
Field of view: visual observing<br />
• For visual observing field-of-view is set by…<br />
! eyepiece design, primarily<br />
! focal length<br />
! diameter<br />
! Magnification of telescope/eyepiece combination<br />
• Eyepieces have an apparent field of view<br />
! e.g. 50° -- full field of view of eyepiece looks like 50° <strong>to</strong><br />
your eye<br />
• True field of view<br />
! apparent view divided by magnification<br />
! e.g. 50° eyepiece <strong>and</strong> 100x magnification focal length<br />
! you can see 0.5° of sky through eyepiece
Optical telescopes <strong>and</strong> “seeing”<br />
• Angular resolution of optical telescopes on Earth is limited<br />
! by changing atmospheric refraction<br />
! not diffraction<br />
• The effect that makes stars twinkle blurs telescope images<br />
Twinkling…<br />
• Stars twinkle because…<br />
! angular size of star is very small % …<br />
! smaller than “bouncing”<br />
due <strong>to</strong> seeing<br />
• Angular size of planets is larger…<br />
! they are blurred by seeing….<br />
! but don’t seem <strong>to</strong> twinkle<br />
% Stars look point-like through a telescope even with very<br />
high magnification
Better seeing in space<br />
• Best seeing on Earth is a fraction of an<br />
arcsecond<br />
• Even small telescopes in space<br />
are useful<br />
Adaptive <strong>Optics</strong><br />
• Astronomers now try <strong>to</strong> track<br />
atmospheric wobbling...<br />
! with fraction-of-a-second images...<br />
! <strong>and</strong> artificial guide stars...<br />
2.4m HST
More atmospheric problems…<br />
• Many wavelengths can only be observed from space…<br />
! ultraviolet<br />
! x-ray<br />
! gamma-ray<br />
! many infrared wavelengths<br />
• The atmosphere doesn't just blur these wavelengths...<br />
! it absorbs them.<br />
Next week: Observing Stars<br />
• Parallax <strong>and</strong> distance measurements<br />
• The magnitude scale<br />
• Spectra, <strong>and</strong> what you can learn from them
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