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

Telescope optical configuration Early reflector designs: Mersenne 1636 Gregory 1663 Cassegrain 1672 Early refractors: Galileo (Lippershey) 1609 Kepler 1645 J. H. Burge University of Arizona

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

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

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

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

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

Rosse Telescope 1845 Wilson Fig 5.3 Newtonian design 72” metal primary mirror Wilson Fig 5.3 J. H. Burge University of Arizona

Foucault 1857 Glass mirror with silver coating Glass is stable, metal is not. Direct measurement of mirror surface Wilson 5.8 J. H. Burge University of Arizona

Mt. Wilson 100” Hooker telescope (1917) J. H. Burge University of Arizona

Palomar 200” Wilson, Fig 5.16, 5.17 J. H. Burge University of Arizona

J. H. Burge University of Arizona

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

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

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

Alt – Az mounting J. H. Burge University of Arizona

Alt Az dome J. H. Burge University of Arizona

Prime focus Wide field (>1°) Lenses correct field aberrations from primary J. H. Burge University of Arizona

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

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

Solid Schmidt J. H. Burge University of Arizona

Bouwers telescope J. H. Burge University of Arizona

Schmidt-Cassegrain J. H. Burge University of Arizona

Maksutov-Cassegrain J. H. Burge University of Arizona

HET, SALT J. H. Burge University of Arizona

Arecibo 300 m fixed spherical dish Receiver and correction optics and moved J. H. Burge University of Arizona

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

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

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

TMA (Three Mirror Anastigmat) SNAP, annular FOV, 1.4 sq degrees, 2 m aperture, diffraction limited for > 1 um 1 Gpixel J. H. Burge University of Arizona

JWST TMA J. H. Burge University of Arizona

James Webb Space Telescope J. H. Burge University of Arizona

JWST launch/deployment J. H. Burge University of Arizona

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

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)

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

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

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

SDSS J. H. Burge University of Arizona

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

Spectrographs J. H. Burge University of Arizona

Echelle spectrograph J. H. Burge University of Arizona

Cross-dispersed Echelle spectrographs J. H. Burge University of Arizona

Multiple object spectrographs J. H. Burge University of Arizona

Fiber coupled spectrographs J. H. Burge University of Arizona

Long slit spectrograph J. H. Burge University of Arizona

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)

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

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

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 6.5-10 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; 3 6.5 m telescopes using HS mirrors produced on campus, our LBT soon to come. J. H. Burge University of Arizona

Multiple Mirror Telescope J. H. Burge University of Arizona

MMT J. H. Burge University of Arizona

MMT at the top of Mt Hopkins J. H. Burge University of Arizona

The road to the top J. H. Burge University of Arizona

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 1999. Telescope scheduled to open with first mirror in 2002, both mirrors in 2004. J. H. Burge University of Arizona

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

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

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

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

1.8-m segments J. H. Burge University of Arizona

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 175-200 mm thick; require active optics to hold shape. Wavefront sensor monitors shape; ~150 active supports bend mirror. J. H. Burge University of Arizona

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