Optical Telescopes for Astrophysics Dummies

1. Optical Telescopes for Astrophysics Dummies Lance Simms MASS 7/6/06

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

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

4. 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. Sorry, More Terminology Same Magnification, different FOV Magnification f = focal length M= fobjective/ feyepiece Field of View (FOV) Amount of sky that can be seen at one time through telescope Usually expressed in deg2 Same FOV, different Magnification

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

7. 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. 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. 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. New Lens Design Saves the Day 1720’s- Elliptical and Hyperbolic lenses finally feasible - allowed reasonably sized telescopes to be built 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 Refractor at Yerkes Obs. Now

11. 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. Reflector Telescopes • 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. A reflector telescope has a mirror as its objective 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. 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. Newton’s Reflector Newtonian Reflector Concave Spherical Primary Flat Secondary Mirror No Chromatic aberration But still Spherical aberration In1663 John Hadley replaced the spherical mirror with a parabolic mirror, eliminating the spherical aberration

15. 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. 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. 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. 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. 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. 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 Alt-Az Mount: 2 axes 1) Up/Down -- Altitude 2) Left/Right -- Azimuth

21. Bigger Mirrors Better Mounts • Mirrors continued to get bigger andoptical 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. And How to Keep it Dry? 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 CFD simulation showing turbulence generated by 3m/s wind • Turbulence is the enemy!

23. The Big Guns: Gemini Twins 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 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 Gemini North: (background) Location: Mauna Kea in Hawaii Elevation: 4200 meters

24. The Keck Telescopes Alt-Az Mount 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

25. 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 deg2 of sky in one image

26. Wider Fields Wanted: LSST Still in the works… Large Synoptic Survey Telescope .25 deg2 Location: Cerro Pachon,Chile Elevation: 2700 meters .5 deg Primary: 8.4 meters concave Secondary: 3.4 meters convex Tertiary: 5.0 meters concave FOV: 10 deg2 Design: Paul-Baker 3 element

27. And Who Could Forget HST? Bigger is better on earth, but location trumps size 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) Above atmosphere Below it

28. 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. In Honor of the Soccer Champs GRAZIE