Post on 13-Jan-2016
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Optical Telescope
Faint Light
• Astronomical objects are distant and faint.– Effectively at infinity
• Light collection is more important than magnification.– Refraction
– Reflection
• The Andromeda Galaxy (M31) subtends 3°.– 6 times the moon
– Only visible to the unaided eye in very dark conditions
Refraction
• Light is bent at the surface between two media.– Index of refraction n
• Refraction is governed by Snell’s law.
ttii nn sinsin
v
cn
i
rt
ri
Radius of Curvature
• Lenses shaped like parts of spheres are easy to make.– Easy to calculate rays
• Use Snell’s Law on a small part of a sphere.– Radius of curvature R
– Focal length f
– Index for air is 1
)1( nf
R
R f
Refracting Telescope
• A refracting telescope is designed to concentrate light from a distant object.– Object light rays nearly parallel
– Final image rays also parallel
objective focal point eyepiece
Aperture
• Lenses collect and concentrate light.
• The diameter (D) of the objective lens is the aperture.– Measured in m or mm
– Larger apertures for fainter objects
• The light gathering power (LGP) is related to the area of the lens.– Circular lens: A = (D2)/4
– Intensification relative to eye aperture 5 mm: LGP = D2/(5 mm)2
F-Stop
• The brightness of an image is measured by the focal ratio of the focal length to the aperture.– F-number or f-stop = f/d
– Dimensionless quantity
– Denoted by f/
• Lower f-numbers are “faster” and need shorter exposure times.
Fraunhofer Diffraction
• A single narrow slit creates diffraction.– No minimum for m = 0
ma sin
,2,1 m
Airy Disk
• Fraunhofer patterns are symmetric around the opening.
• A circular hole produces rings around a central maximum.– 84% of energy in center
Angular Resolution
• The limit of resolution is set by the aperture.
• The Rayleigh criterion is calculated from the first minimum of the Airy disk.– Aperture radius a
– Wavenumber k
– Bessel function J1
D
22.1
sin
)sin(2)( 1
0 ka
kaJII
0)(1 xJ ...02.7,83.3,0x
Daka
22.12
83.383.3sin
Tube Length
• The intermediate image at the focal point is a real image.– Long tube accommodates long focal length
– Parallel ray image related to the focal length
objective focal point eyepiece
o
O
oO
iOO s
f
s
sM
Magnification
• The eyepiece magnifies the intermediate image.
• The total magnification is the product from both lenses.
objective focal point eyepiece
E
i
oE
iEE f
s
s
sM
E
OEO f
fMMM
Yerkes Refractor
• The world’s largest refractor is in Wisconsin.
– 40 inch aperture, f/19
– 63 foot tube
Yerkes 40 inch
Chromatic Aberration
• The index of refraction depends on the wavelength.– Longer wavelengths - lower
indexes
– Blue light bends more than red
• Compound lenses can compensate for chromatic aberration.
• Air n(589 nm) =1.00029
• Crown glass 1.52
• Flint glass 1.66
Spherical Aberration
• A spherical surface does not focus all parallel lines to the same point.
• Aspheric lenses can be used to correct the aberration .
f
Curved Mirror
• Light that begins at one focus of an elliptical mirror converges at the other focus.– A parabola for a focus at
infinityfocus
focus
Parabolic Mirror
• A perfect parabolic mirror has a focal length like a lens.
• All wavelengths are focused to the same point.– No chromatic aberration
• The size of the mirror dish is the aperture.
focal lengthfocal point
Newtonian Reflector
• For viewing ray should be parallel on exit.– Combined primary mirror and eyepiece
• The reflecting telescope is cheaper, because a mirror is easier to make than a lens for a given size.
primary mirror
secondary diagonal mirror
eyepiece
Schmidt-Cassegrain Reflector
• A Cassegrain focus uses a flat mirror to make the tube up to three times longer.– Spherical aberration from extra mirror
– Aspheric Schmidt lens corrects aberration
eyepieceSchmidt corrector lens
Keck Reflector
• World’s largest reflector is in Hawaii.
– 400 inch aperture, f/1.75
– Focal length 57.4 feet.
– Telescope height 81 feet.
Keck Observatory
Coma
• Parabolic mirrors focus precisely for rays parallel to the central axis.
• The distortion for off axis objects is called coma.– Greatest for low f-numbers
• Lenses can correct for the coma. Starizona.com
Atmospheric Absorption
• The atmosphere absorbs radiation, except at visible light, infrared, and radio frequencies.
Adaptive Optics
• The moving atmosphere disturbs images.– Wavefront distortions
• Real time corrections are made by feedback to a deformable mirror.– Sample wavefront from beam splitter
– Measure distortion
– Compute necessary compensation for mirrors
Telescope Advantages
REFRACTOR
• Superb resolution
• Good for detail
• Rugged alignment
• Transports well
REFLECTOR
• Inexpensive optics
• Large aperture
• Good for dim objects
• Uniform treatment of colors
SCHMIDT-CASSEGRAIN
• Portable size
• Combines best optical qualities
• Good for photography
Altazimuth Mount
• Telescope mounts should permit two directions of motion.
• Altazimuth mounts directly control altitude and azimuth.
azimuth control
altitude control
Equatorial Mount
• Altazimuth mounts do not track with the star’s movement.
• Equatorial mounts are oriented to the pole.
• Allows control of declination and right ascension.
polar axisdeclination axis
Charge-Coupled Device
• The CCD is an array of photosensitive semiconductor capacitors.– Charge stored proportional
to light intensity
– Transfers charge as a shift register
– Amplifier on last capacitor converts charge to voltage
Hammamatsu.com
Telescope CCDs
• CCDs are sensitive to light from ultraviolet to infrared.
• CCDs are very efficient.– Can be sensitive to
individual photons
• Sensitivity to thermal noise and cosmic rays can blur an image.
• Multiple exposures are averaged to get correct image.– Dark frame closed shutter
Hubble Space Telescope
• The Hubble is an orbiting reflector telescope.
• It has no atmosphere to peer through.
• The onboard computer gives it enhanced optics.
• There are four different
cameras for different views.
Infrared and Ultraviolet
• Infrared is absorbed by water vapor.– Observe at high altitude
• Satellite telescopes avoid the atmosphere.– IRAS (1983) - first
evidence of planets around other stars
– Spitzer Space Telescope (2003-9).
• Ultraviolet is largely absorbed by the atmosphere.– Requires satellites
– HST, GALEX
M81 from GALEX