1. Telescope optics Jim Burge College of Optical Sciences University of Arizona
History of telescopes
Types of telescopes
Astronomical telescope instrumentation
Modern telescopes
J. H. Burge University of Arizona

2. Telescope optical configuration
Early reflector designs: Mersenne Gregory Cassegrain 1672
Early refractors: Galileo (Lippershey) 1609 Kepler 1645
J. H. Burge University of Arizona

3. Early refractors Huygens eyepiece 1661
Refractors limited by glass quality
1800’s, improved glass, chromatic compensation
40” Yerkes refractor 1897
Increased sensitivity requires larger telescopes
J. H. Burge University of Arizona

4. King Fig 125 36-in Lick Refractor Note size of Dome/Aperture
J. H. Burge University of Arizona

5. 40” Yerkes refractor (1897)
J. H. Burge University of Arizona

6. Newtonian Telescope 1668
Optical design does not satisfy the sine condition – aberration of coma limits the field of view
J. H. Burge University of Arizona

7. Herschel Telescope 1789 Wilson Fig 1.11 Newtonian design
48” metal primary mirror
J. H. Burge University of Arizona

8. Rosse Telescope 1845 Wilson Fig 5.3 Newtonian design
72” metal primary mirror
Wilson Fig 5.3
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9. Foucault 1857 Glass mirror with silver coating
Glass is stable, metal is not.
Direct measurement of mirror surface
Wilson 5.8
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10. Mt. Wilson 100” Hooker telescope (1917)
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11. Palomar 200”
Wilson, Fig 5.16, 5.17
J. H. Burge University of Arizona

12. J. H. Burge University of Arizona

13. 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
J. H. Burge University of Arizona

14. 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
J. H. Burge University of Arizona

15. Equatorial mount Declination axis Polar axis (Note the Coude path)
Polar axis is aligned to the Earth’s axis of rotation
Polar axis
Declination axis
(Note the Coude path)
J. H. Burge University of Arizona

16. Alt – Az mounting
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17. Alt Az dome
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18. Prime focus Wide field (>1°)
Lenses correct field aberrations from primary
J. H. Burge University of Arizona

19. Schmidt Very wide field (>5°) Uses spherical symmetry
Aspheric corrector plate compensates for spherical primary mirror
J. H. Burge University of Arizona

20. LAMOST Reflective Schmidt, 5° FOV, 4-m aperture
Siderostat mirror used for pointing, also has Schmidt correction
J. H. Burge University of Arizona

21. Solid Schmidt
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22. Bouwers telescope
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23. Schmidt-Cassegrain
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24. Maksutov-Cassegrain
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25. HET, SALT
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26. Arecibo 300 m fixed spherical dish
Receiver and correction optics and moved
J. H. Burge University of Arizona

27. Couder aplanatic anastigmat
Aplanatic – coma is corrected by satisfying the sine condition
Anastigmatic – astigmatism is balanced by the two mirrors
J. H. Burge University of Arizona

28. Hubble Space Telescope
Ritchey-Chretien design
Aplanatic – coma is corrected by satisfying the sine condition
Primary mirror is not quite paraboloidal
Secondary is hyperboloid
J. H. Burge University of Arizona

29. Spitzer Telescope Ritchey-Chretien design 85 cm aperture
Cryogenic operation for low background
J. H. Burge University of Arizona

30. TMA (Three Mirror Anastigmat)
SNAP, annular FOV, 1.4 sq degrees, 2 m aperture, diffraction limited for > 1 um
1 Gpixel
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31. JWST TMA
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32. James Webb Space Telescope
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33. JWST launch/deployment
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34. 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
J. H. Burge University of Arizona

35. Instrumentation
Imagers: Resolution (arc sec) Field of view (arc min) Sensitivity
Spectrographs: Resolving power (l/Dl) Spectral range Sensitivity
J. H. Burge University of Arizona
(example from VLT 2000)

36. 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!
J. H. Burge University of Arizona

37. Revolution in data collection
CCD detectors
Many pixels (7k x 9k at Steward)
Data goes straight into the computer
QE > 90%
Read noise ~ 1 electron
Used in imagers and spectrographs
J. H. Burge University of Arizona

38. Arrays of arrays MMT f/5 focus gives 24'x24' field
36 CCDs with 2048x4608 pixels
J. H. Burge University of Arizona

39. SDSS
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40. LSST LSST Optical Layout 3.5° field of view for all-sky survey
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
J. H. Burge University of Arizona
8.36 m

41. Spectrographs
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42. Echelle spectrograph
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43. Cross-dispersed Echelle spectrographs
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44. Multiple object spectrographs
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45. Fiber coupled spectrographs
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46. Long slit spectrograph
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47. Integral field spectroscopy
Gives spatial variation of spectrum
Usually uses some “image slicer” to feed a spectrograph, multiplexing spatial and spectral information
J. H. Burge University of Arizona
(2 x 2.4 arcmin field from HDF)

48. 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
J. H. Burge University of Arizona

49. Image slicer using mirrors
J. H. Burge University of Arizona

50. Optical telescopes
Astronomy is observational as opposed to experimental science.
What we learn depends on sensitivity of instruments, i. e. telescopes.
In period of unprecedented growth in power of optical telescopes.
At turn of century, largest telescope was refractor with 1 m diameter lens. At beginning of C, reflectors became design of choice. First half of 20th century saw steady increase in sensitivity, thanks to Hale.
After 200”, stagnant 40 yr--will discuss why.
Suddenly in 90s saw explosion in light-gathering power with telescopes of m diam, and looking forward to 12 m LBT soon.
10 m Keck tels with segmented mirrors; seven 8 m tels using thin, flexible mirrors; m telescopes using HS mirrors produced on campus, our LBT soon to come.
J. H. Burge University of Arizona

51. Multiple Mirror Telescope
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52. MMT
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53. MMT at the top of Mt Hopkins
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54. The road to the top
J. H. Burge University of Arizona

55. Large Binocular Telescope
Two of these 8.4 m honeycomb sandwich mirrors will go in LBT.
Partners are the 3 AZ universities, Italy, Germany, Ohio State, and Research Corporation.
Drawing of the LBT showing the two 8.4 meter mirrors on a common mount. It will be the world’s 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 Telescope scheduled to open with first mirror in 2002, both mirrors in 2004.
J. H. Burge University of Arizona

56. Honeycomb sandwich mirrors
Third breakthrough, pioneered here by Roger Angel, uses new technology to make mirrors with traditional values of stiffness and light weight.
Honeycomb sandwich mirrors made on UA campus are 8 times stiffer and 2/3 the weight of thin solid mirrors.
Honeycomb sandwich is 2-D version of I beam, with most of mass at top and bottom where it buys most stiffness, and little in between.
Overall thickness is 900 mm, but mirror is 80% hollow. Front and back plates are 25 mm thick; honeycomb ribs only 12 mm.
Thin glass sections also makes mirror easy to cool or heat. With ventilation of internal structure, glass tracks outside air temperature with delay of only 40 min.
List projects.
Maximize stiffness:weight — 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 world’s largest.
J. H. Burge University of Arizona

57. Casting process (1)
Complex manufacturing process produces world’s 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.
New technology is in manufacturing process, casting and polishing the mirror.
J. H. Burge University of Arizona

58. 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.
J. H. Burge University of Arizona

59. Keck Telescopes Twin 10 meter telescopes on Hawaii’s Mauna Kea.
Built by U California and Cal Tech.
Commissioned 1992, 1996.
First new tels on line are Keck Tels.
Innovation pioneered by Jerry Nelson was to make each 10 m mirror out of 36 segments.
Segments small enough they can be supported easily and thin enough that their temperature follows air temp.
Design relies on actuators and electronic to hold accurate shape over 10 m diam.
Bold design 20 yr ago when mechanics, electronics, computers were only marginally up to the task. Project caught technology wave at right time.
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.
J. H. Burge University of Arizona

60. 1.8-m segments
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61. Thin solid mirrors ESO’s Very Large Telescope (4x8 m in Chile)
Gemini Telescopes (8 m in Hawaii and Chile)
Subaru Telescope (8 m in Hawaii)
Several international projects use different innovation that relies on new technology: thin flexible mirrors with active optics.
Active optics, pioneered by Ray Wilson at ESO, involves measuring shape of wavefront continually--minute by minute--and adjusting shape of mirror with actuators.
What it buys you is ability to use big mirror without letting weight increase.
List projects, dimensions, actuators.
Mirrors mm thick; require active optics to hold shape.
Wavefront sensor monitors shape; ~150 active supports bend mirror.
J. H. Burge University of Arizona

62. 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 honeycomb Segments 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
J. H. Burge University of Arizona