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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

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Optical Telescope

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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.

qi

qt

qr


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

Radius of Curvature

f

R


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.


A single narrow slit creates diffraction.

No minimum for m = 0

Fraunhofer Diffraction


Fraunhofer patterns are symmetric around the opening.

A circular hole produces rings around a central maximum.

84% of energy in center

Airy Disk


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

Angular Resolution


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


Magnification

  • The eyepiece magnifies the intermediate image.

  • The total magnification is the product from both lenses.

objective

focal point

eyepiece


Yerkes Refractor

  • The world’s largest refractor is in Wisconsin.

    • 40 inch aperture, f/19

    • 63 foot tube

Yerkes 40 inch


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.

Chromatic Aberration

  • Airn(589 nm) =1.00029

  • Crown glass1.52

  • Flint glass1.66


A spherical surface does not focus all parallel lines to the same point.

Aspheric lenses can be used to correct the aberration .

Spherical Aberration

f


Light that begins at one focus of an elliptical mirror converges at the other focus.

A parabola for a focus at infinity

focus

Curved Mirror

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 length

focal 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.

secondary diagonal mirror

primary 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

Schmidt corrector lens

eyepiece


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


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

Telescope Advantages


Altazimuth Mount

  • Telescope mounts should permit two directions of motion.

  • Altazimuth mounts directly control altitude and azimuth.

altitude control

azimuth 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.

declination axis

polar 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 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

Infrared and Ultraviolet

M81 from GALEX


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