Telescopes help us resolve details about celestial objects and phenomena
The Nature of Light
3-1 (p.55) Early discoveries explained white light and revealed the speed of light. Dispersion bends different wavelegths through different angles: prism -> rainbow. 1675 Roemer measured earth orbit diameter = 16.5 light-minutes from eclipses of Jupiter's moons (delayed when Earth is furthest from Jupiter) -> c=3 x 108 m/s
3-2 (p.57) The complex nature of light only became apparent this century. ~1650 Huyghens: waves. ~1700 Newton: particles. 1801 Young: interference -> waves. ~1860 Maxwell: EM waves
3-3 (p.57) Light sometimes behaves as particles, sometimes as waves. See Quantum Tour. Einstein + Planck: quantization: E=hf where v=fl and Planck's h=6.63 x 10-34 kg m2/s (angular momentum units). Photoelectric effect demonstrated quantization of light energy: electron ejection from metals depends not on intensity, but on energy of incident light.
3-4 (p.58) Light is only one type of electromagnetic radiation. All light travels with the same speed (in vacuum), but longer wavelength = lower frequency = lower energy. Wavelengths: Radio (m) - microwave (cm-mm) - IR (700-1000 nm) - visible (400-700 nm) - UV (10-400 nm) - X rays (0.01-10 nm) - gamma rays (<0.01 nm)
| radiaton | wavelength | frequency | passes through: | absorbed by: |
| gamma | <0.01 nm | atmosphere, skin | ||
| X | 0.01-10 nm | flesh | bone, skin | |
| UV | 10-400 nm | clouds | glass, ozone | |
| visible | 400-700 nm | atmosphere | "opaque" materials, clouds | |
| IR | 700-1000 nm | atmosphere, water vapor | ||
| microwave | cm-mm | atmosphere | ||
| radio | m | atmosphere, clouds | ionosphere, metal |
An Astronomer's Toolbox 3-1 (p.59) Photon Energies. E=hf where c=fL and h=6.63 x 10-34 kg m2/s, c=3 x 108 m/s. Exercise: find frequencies in table above.
Optics and Telescopes
3-5 (p.60) A refracting telescope uses a lens to concentrate incoming light. Refraction = light bending due to density change in medium (around corners, spreading out through holes, focussing through lens at focal point f= R/2). Large objective lens collects and focuses light at fo. Small eyepiece at fe away from real image directs parallel rays to eye. (Fig.3-10 p.62)
3-6 (p.62) Telescopes brighten, resolve, and magnify.
3-7 (p.63) Refracting telescopes have several severe problems.
| pros | cons | |
| Spherical lenses | easy to grind | spherical aberration |
| Parabolic lenses | no spherical aberration | hard to grind |
| Lenses | easy to make | dispersion -> chromatic aberration
bubbles -> blurring opaque to UV... |
| large aperture lenses | better power, resolution | sag |
3-8 (p.65) Reflecting telescopes use mirrors to concentrate incoming starlight, with eyepiece on incoming end or side (except Cassegrain, such as our Meade LX 200). No refraction -> no bubble blurring, chromatic aberration, but some light is blocked, and you still have spherical aberration (unless you grind parabolic mirrors or use a Schmidt corrector plate on top, as on our Meade: more light, but extra reflection surfaces -> optical distortions).
3-9 (p.67) Reflecting telescopes also have limitations.
3-10 (p.68) Earth's atmosphere hinders astronomical research. Twinkling is due to density differences(e.g. from temperature gradients or near horizon). Air pollution reduces seeing.
3-11 (p.69) Advanced technology is spawning a new generation of telescopes.
Eyes on... Your first telescope should be binoculars. Looks like our Meade on p.72: also an Equatorial Mount
Radio Astronomy - and beyond
3-12 (p.72) A radio telescope uses a large concave dish to reflect radio waves to a focus. Radio gets through atomosphere and clouds.Radio signals from Sagittarius (center of MW galaxy: Jansky 1932), 3K background radiation (Penzias ad Wilson), pulsars (Jocelyn Bell Burnell, 1968), black hole in cygnus X-1, LMC X-3, M87... (pp.295-97). Long wavelengths can get great resolution with multiple scopes. Ex: VLBA has a scope in WA.
What's the name of that B-sci-fi flick where an enterprising conspiracy theorist links neighbors' TV dishes together in array so he can track signals broadcast by aliens taking over the Earth? Alien Invasion?
3-13 (p.74) Observations at other wavelengths are revealing sights previously invisible. Space-based scopes can detect
3-14 (p.76) The HST is clear at last.grinding error -> imperfect shape -> aberration (1990). Corrective optics installed in Dec.93 shuttle spacewalk
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Derive angular resolution: Rayleigh criterion: an aperture of width d (or two eyes separated by a distance d...) can resolve light of wavelength L from two stars (or two headlights...) separated by an angle Q if
sin Q > 1.22 L/d (this is the angle of the first diffraction minimum).For small angles (under 10 degrees), this is equivalent to Q<1.22 L/d. For light of wavelength 500 nm (visible), this yields Q(rad)~ 6 x 10-7 cm / d. Translate the angle from radians (which are huge) to arcseconds (which are common measures of separation between stars), using 360 degrees = 2 * Pi radians, 60 arcminutes = 1 degree, 60 arcseconds = 1 arcminute... to find about 2 x 105 arcsec per radian. Finally, Q(arcsec) ~ 12 / d(cm)
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Discussion Question: DQ#23 (K.79) Discuss the advantages and disadvantages of using a small scope in Earth orbit versus a large scope on a mountaintop. (what criteria? strengths and weaknesses of each design?)
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Maintained by E.J. Zita
Last modified: 14.Apr.98