1. July 12, 20061 Optical Telescopes for Astrophysics Dummies Lance Simms MASS 7/6/06

2. July 12, 20062 The First Telescope 1608 - Jan (or Hans) Lippershey, a spectacle maker, invents the refractor telescope Rumor: his kids discovered it while playing around in his shop. Refractor - Objective is a lens ObjectiveEyepiece

3. July 12, 20063 And Then Galileo Before seeing Jupiter’s moons…. looking mad After seeing Jupiter’s moons… no more lazy eye A year Later (1609) Galileo Galilei builds a scope and looks at the Moon and discovers 4 moons around Jupiter and phases of venus

4. July 12, 20064 A Little Terminology Objective - lens or system of lenses closest to object being viewed Eyepiece - lens or system of lenses closest to eye/detector Focal length - distance of surface of lens/mirror to focal point Aperture - diameter of objective

5. July 12, 20065 Sorry, More Terminology Magnification f = focal length M= f objective / f eyepiece Field of View (FOV) Amount of sky that can be seen at one time through telescope Usually expressed in deg 2 Same FOV, different Magnification Same Magnification, different FOV

6. July 12, 20066 Kepler One-Ups Galileo Objective and Eyepiece separated by difference of focal lengths Upright Image is formed Small field of view Objective and Eyepiece separated by sum of focal lengths Inverted Image is formed Large field of view 1608 Galileo Design 1611 Kepler Design Convex eyepiece Concave eyepiece

7. July 12, 20067 They Knew it Then: The Bigger The Better Most important property of a telescope: Aperture !! Larger Objective = More light 5’’ 8’’ 14’’ Without the light magnification is useless !

8. July 12, 20068 So They Made Bigger Lenses But there was a problem: Spherical Aberration Light from edges of lens focuses at different point than light from interior of lens All lenses then were spherical lenses Descartes proposed two solutions in 1637 1. Make Lenses elliptical or hyperbolic (not realizable at time) 2. Increase the focal length while keeping same diameter to lessen aberration (means BIG telescope)

9. July 12, 20069 Result: HUGE Telescopes 1637-1722 : objectives of longer and longer focal length were made 150-200 feet long tubes were not uncommon! Largest Refractor is French - Stationary Lens - 60 m long horizontal tube - 1.25m objective lens - It was a failure

10. July 12, 200610 New Lens Design Saves the Day Alvan Clark and Sons built largest refractor lens at 40 inches 40 in. lens at Yerkes Obs. 1895 36 in. lens at Lick Obs. 1886 1720’s - Elliptical and Hyperbolic lenses finally feasible - allowed reasonably sized telescopes to be built Refractor at Yerkes Obs. Now

11. July 12, 200611 Why Stop at 40 inches? Large lenses tend to sag under their own weight -- distorts image Long mounting tubes flex under weight of lens -- bad for optical alignment Alternative: Use Mirrors. They can be supported from below Light Gravity

12. July 12, 200612 Reflector Telescopes A reflector telescope has a mirror as its objective James Gregory proposed such a telescope in 1663 but no optician could build it. He gave up, but still got a design named after him. Gregorian Telescope Concave parabolic Primary* Concave ellipsoidal Secondary - located beyond focal point of primary *Primary/Secondary/Tertiary/etc. refers to order in which light strikes surface

13. July 12, 200613 Newton’s Reflector Isaac Newton designed a reflector in 1672 in his attempt to overcome Chromatic Aberration Chromatic Aberration -Each wavelength of light is refracted at different angle -Each wavelength has different focal length -Only occurs in refraction; not reflection Newton also thought up a way to eliminate the defect by using two different lenses, but messed up an experiment and concluded that all transparent materials refract equally. Now opticians make double Achromatic lenses

14. July 12, 200614 Newton’s Reflector Newtonian Reflector Concave Spherical Primary Flat Secondary Mirror No Chromatic aberration But still Spherical aberration In 1663 John Hadley replaced the spherical mirror with a parabolic mirror, eliminating the spherical aberration

15. July 12, 200615 Other Reflectors Cassegrain Reflector Concave Parabolic Primary Convex Hyperbolic Secondary The design was conceived in about 1672 by the Frenchman Guillaume Cassegrain Little is known about him Popular twist is the Schmidt-Cassegrain - parabolic primary is replaced with spherical mirror - corrector plate is inserted to correct spherical aberration

16. July 12, 200616 More Cassegrains Ritchey-Chretien Cassegrain Concave hyperbolic primary Convex hyperobolic secondary -Design is free of 3rd order Coma and spherical aberration -Most common type used on research telescopes Coma is a an inherent property of telescopes using parabolic mirrors that causes off-axis images to have fuzzy shapes, like little comets

17. July 12, 200617 More Cassegrains? Maksutov-Cassegrain Concave spherical primary Convex spherical secondary - Spherical corrector lens plate removes first order spherical aberration - Tend to have narrower field of view than Schmidt-Cassegrains due to longer focal length -Invented by Dmitri Maksutov (1896-1964) -Does not scale very well with large aperture since meniscus corrector plate becomes prohibitively large and expensive An excellent telescope for lunar and planetary observations!

18. July 12, 200618 Enough with the Cassegrains! Dall-Kirkham Cassegrain Concave parabolic primary Convex spherical secondary -Under corrected primary removes first order spherical aberration of the spherical secondary -Large coma makes its usable field of much smaller than true Cassegrain -Developed in 1930s by Horace Dull of Luton, England That about covers Cassegrains…except for minor tweaks

19. July 12, 200619 A Comparison of Points Point Spread Function (PSF) The irradiance distribution resulting from a single point source (e.g. a star) in object space Simulated PSF for LSST telescope

20. July 12, 200620 Large Mirrors = Large Mount William Herschel’s 40 foot long, 4 foot mirror telescope in Slough, England 1789 It took 2 assistants to point while he observed They had speaking tubes to communicate Example of Alt-Az Mount Herschel didn’t like using it; he preferred his 20 footer 1) Up/Down -- Altitude 2) Left/Right -- Azimuth Alt-Az Mount: 2 axes

21. July 12, 200621 Bigger Mirrors Better Mounts Mirrors continued to get bigger and optical quality improved Equatorial Mount introduced Equatorial Mount: 2 axes 1) Right Ascension - celestial longitude 2) Declination - celestial latitude Turning one knob follows a star! 1 2

22. July 12, 200622 And How to Keep it Dry? CFD simulation showing turbulence generated by 3m/s wind Turbulence is the enemy! Put it in a Dome! - protects telescope from elements, bird droppings - care must be taken to avoid large temperature gradients/turbulence Dome of SOAR telescope

23. July 12, 200623 The Big Guns: Gemini Twins Primary Mirror Outside Diameter: 8.10 metres Central Cassegrain Hole: 1.18 metres Thickness: 20 cm/7.87 inches Optical Configuration: Ritchey-Chretien Cassegrain Optical Surface: Concave, hyperboloid Secondary Mirror: Diameter: 1.023 metres/3.36 feet. Central Hole Diameter: 0.168 metres Optical Surface: Convex, hyperboloid Gemini South: (above) Location: Cerro Pachon, Chile Elevation: 2700 meters Gemini North: (background) Location: Mauna Kea in Hawaii Elevation: 4200 meters

24. July 12, 200624 The Keck Telescopes Location: Mauna Kea Primary Mirrors: 10 m, 36 hexagonal concave hyperbolic segments Optical Design: Ritchey-Chretien Cassegrain Na Laser Guide Star Adaptive Optics 8 Stories high Both telescopes can be used together as an optical interferometer 85 m baseline gives 0.005’’ resolution at 2 microns Alt-Az Mount

25. July 12, 200625 Sloan Digital Sky Survey (SDSS) Location: Apache Point Obs. Sacramento Peak, NM Primary: 2.5 meter Secondary: 1.08 meter Design: Gascoigne-Ritchey Cassegrain COSMIC MAP With its wide field, SDSS will map 1/4 of the sky Small Scope, Large Field of View ~ 3 deg 2 of sky in one image

26. July 12, 200626 Wider Fields Wanted: LSST Still in the works… Large Synoptic Survey Telescope Location: Cerro Pachon,Chile Elevation: 2700 meters Primary: 8.4 meters concave Secondary: 3.4 meters convex Tertiary: 5.0 meters concave FOV: 10 deg 2 Design: Paul-Baker 3 element.5 deg.25 deg 2

27. July 12, 200627 And Who Could Forget HST? Hubble Space Telescope Telescope style: Ritchey-Chretien Cassegrain Diameter: 2.4 m (94 in) Collecting area: approx. 4.3 m² (46 ft²) Effective focal length: 57.6 m (189 ft) Bigger is better on earth, but location trumps size Above atmosphere Below it

28. July 12, 200628 And Right in Our Backyard Stanford Student Observatory Scope Telescope style: Cassegrain/Newtonian* Diameter: 0.61m (24 in) *Secondary mirror is convertible to accommodate Cassegrain and Newtonian foci Capable of “research” science ! We’ll be using it soon

29. July 12, 200629 In Honor of the Soccer Champs GRAZIE