J' H' Burge University of Arizona

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Telescope opticsJim BurgeCollege of Optical
SciencesUniversity of Arizona

• History of telescopes
• Types of telescopes
• Astronomical telescope instrumentation
• Modern telescopes

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Telescope optical configuration
Early reflector designs Mersenne 1636 Gregory
1663 Cassegrain 1672
Early refractorsGalileo (Lippershey)
1609Kepler 1645
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Early refractors

• Huygens eyepiece 1661
• Refractors limited by glass quality
• 1800s, improved glass, chromatic compensation
• 40 Yerkes refractor 1897
• Increased sensitivity requires larger telescopes

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36-in Lick Refractor Note size of Dome/Aperture

• King Fig 125

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40 Yerkes refractor (1897)
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Newtonian Telescope 1668
Optical design does not satisfy the sine
condition aberration of coma limits the field
of view
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Herschel Telescope 1789

• Wilson Fig 1.11

Newtonian design 48 metal primary mirror
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Rosse Telescope 1845
Newtonian design 72 metal primary mirror

• Wilson Fig 5.3

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Foucault

• 1857
• Glass mirror withsilver coating
• Glass is stable,metal is not.
• Direct measurementof mirror surface

Wilson 5.8
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Mt. Wilson100 Hooker telescope (1917)
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Palomar 200

• Wilson, Fig 5.16, 5.17

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Why are big telescopes difficult?

• Primary mirror
• Scale up, mass goes as D3 , deflection D1,
• Solid glass is too heavy thermal problems
• Glass technology to make large homogenious blanks
• Mechanics to hold mirror
• System
• Moving mass is large
• Drive, encoders difficult
• vibration
• Requires large building
• Alt-AZ
• Fast PM

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Reflective telescope designs

• Cassegrain and Gregorian (add field correcting
  lenses)
• Parabola with prime focus corrector
• Dall Kirkham (Coma city)
• Ritchey-Chretien fixes coma
• Couder aplanat, ,anastigmat, diffiicult
  geometry
• Bouwers limited to slow telescopes by 5th order
  SA
• Schmidt limited by spherochromatism
• Maksutov
• Solid Schmidt
• Schmidt Cassegrain
• Maksutov Cassegrain
• Three mirror anastigmats

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Equatorial mount
Polar axis is aligned to the Earths axis of
rotation
Declination axis
Polar axis
(Note the Coude path)
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Alt Az mounting
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Alt Az dome
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Prime focus
Wide field (gt1) Lenses correct field aberrations
from primary
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Schmidt

• Very wide field (gt5)
• Uses spherical symmetry
• Aspheric corrector plate compensates for
  spherical primary mirror

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LAMOST
Reflective Schmidt, 5 FOV, 4-m
aperture Siderostat mirror used for pointing,
also has Schmidt correction
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Solid Schmidt
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Bouwers telescope
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Schmidt-Cassegrain
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Maksutov-Cassegrain
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HET, SALT
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Arecibo
300 m fixed spherical dish Receiver and
correction optics and moved
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Couder aplanatic anastigmat

• Aplanatic coma is corrected by satisfying the
  sine condition
• Anastigmatic astigmatism is balanced by the two
  mirrors

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Hubble Space Telescope

• Ritchey-Chretien design
• Aplanatic coma is corrected by satisfying the
  sine condition
• Primary mirror is not quite paraboloidal
• Secondary is hyperboloid

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Spitzer Telescope
Ritchey-Chretien design 85 cm aperture Cryogenic
operation for low background
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TMA (Three Mirror Anastigmat) SNAP, annular FOV,
1.4 sq degrees, 2 m aperture, diffraction
limited for gt 1 um
1 Gpixel
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JWST TMA
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James Webb Space Telescope
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JWST launch/deployment
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Historical use of telescopes

• Pre 1900 visual observations
• Film used for imaging and spectroscopy, followed
  up with scanning densitometer for data processing
• 48 Palomar Schmidt used 14 plates
• Single point detectors used for photometry
• Photodiodes for visible light
• Photoconductors for IR
• Photomultipliers for photon counting

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Instrumentation
Imagers Resolution (arc sec) Field of view
(arc min) Sensitivity
Spectrographs Resolving power (l/Dl) Spectral
range Sensitivity
(example from VLT 2000)
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Imaging

• Desire good sampling
• Wide field of view (many pixels)
• Low noise
• High QE
• Use filters to select BW
• Use shutters to control exposure
• The optical systems that give good images over
  wide fields are difficult!

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Revolution in data collection
CCD detectors Many pixels (7k x 9k at
Steward) Data goes straight into the computer QE
gt 90 Read noise 1 electron Used in imagers and
spectrographs
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Arrays of arrays
MMT f/5 focus gives 24'x24' field 36 CCDs with
2048x4608 pixels
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SDSS
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LSST
200 4k x 4k detectors
3.5 field of view for all-sky survey Primary and
Tertiary mirrors to be made at UA on the same
substrate
3.4 m
Secondary
64 cm
Focal Plane
Filters
Field Flattening Lens
LSST Optical Layout
6.28 m
Tertiary
4.96 m
Primary
8.36 m
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Spectrographs
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Echelle spectrograph
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Cross-dispersed Echelle spectrographs
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Multiple object spectrographs
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Fiber coupled spectrographs
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Long slit spectrograph
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Integral field spectroscopy
Gives spatial variation of spectrum Usually uses
some image slicer to feed a spectrograph,
multiplexing spatial and spectral information
(2 x 2.4 arcmin field from HDF)
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Image slicer using fibers
Implemented in VIMOS
Telescope images onto area array of 80 x 80
lenslets, coupled to fibers
Fibers feed spectrograph with a linear array of
lenslets, coupled to fibers
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Image slicer using mirrors
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Optical telescopes
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Multiple Mirror Telescope
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MMT
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MMT at the top of Mt Hopkins
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The road to the top
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Large Binocular Telescope
Drawing of the LBT showing the two 8.4 meter
mirrors on a common mount. It will be the worlds
most powerful telescope with collecting area
equivalent to a 12 meter telescope and the
angular resolution of a 23 meter telescope (4
milli-arcsecond).
LBT enclosure on Mt. Graham in December 1999.
Telescope scheduled to open with first mirror in
2002, both mirrors in 2004.
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Honeycomb sandwich mirrors
Maximize stiffnessweight 2D version of
I-beam. Optimum thermal response ventilation
reduces time constant to 40 min. Used in MMT,
Magellan (2), LBT. 8.4 meter LBT mirrors are
worlds largest.
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Casting process (1)
Complex manufacturing process produces worlds
largest mirrors with almost ideal
properties. Mold consists of ceramic fiber boxes
inside silicon carbide tub 1600 hexagonal boxes
will form cavities in mirror. Ceramic fiber
maintains strength at 1200ºC, does not react
chemically with glass, and can be removed without
applying high stress to glass. Each box is
machined to precise dimensions and bolted in
place.
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Casting process (2)
Borosilicate glass is purchased as irregular 10
pound blocks with pristine fracture surfaces.
Melts together seamlessly. 20 tons of glass are
placed on top of mold. Furnace is closed, heated
to 1200ºC while spinning at 7 rpm to form
paraboloid. After melting, mirror cools for 3
months to minimize stress. Mirror is lifted from
furnace and ceramic fiber boxes are removed with
high-pressure water.
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Keck Telescopes
Twin 10 meter telescopes on Hawaiis Mauna
Kea. Built by U California and Cal
Tech. Commissioned 1992, 1996.
Primary mirror 36 hexagonal segments, 1.8 meter
diameter Each segment positioned by 3 actuators
to form continuous paraboloid. Edge sensors
(capacitors), interferometer and image analyzer
provide feedback.
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1.8-m segments
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Thin solid mirrors
ESOs Very Large Telescope (4x8 m in
Chile) Gemini Telescopes (8 m in Hawaii and
Chile) Subaru Telescope (8 m in Hawaii)
Mirrors 175-200 mm thick require active optics
to hold shape. Wavefront sensor monitors shape
150 active supports bend mirror.
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GMT Design

• 36 meters high
• 25.3 meters across
• Alt-Az structure
• 1000 tons moving mass
• Primary mirror (f/0.7)
• 7 segments 8.4 meters each
• Cast borosilicate honeycombSegments position
  controlled to 10 µm
• 3.2-m segmented secondary mirror
• corrects for PM position errors
• deformable mirror for adaptive optics
• Instruments mount below primary at the Gregorian
  focus