Astronomical Observational Techniques and Instrumentation

1. Astronomical Observational Techniques and Instrumentation Professor Don Figer Telescopes

2. Aims and outline for this lecture • describe most important system parameters for telescopes • review telescope design forms

3. Backyard Telescope

4. Telescope System • Opto-mechanical and thermal control • Acquisition & guiding • Telemetry and sensing • Instrumentation and instrument interfaces (ports) • Software for telescope and instrument control • Technical support and maintenance • Data storage and transfer • Software pipelines for data reduction and analysis • Environment for observer and operator • Personnel management, technical and scientific leadership

5. Telescope Parameters • Collecting area is most important parameter • collected light scales as aperture diameter squared (A=pr2) • Length is a practical parameter that impacts mass and dome size • Delivered image quality (DIQ) • function of optical design aberrations • function of atmospheric properties at observing site • f/ratio determines plate scale and field of view

6. Thin Lens Equation

7. Refracting/Reflecting Telescopes Refracting Telescope: Lens focuses light onto the focal plane Focal length Reflecting Telescope: Concave Mirror focuses light onto the focal plane Focal length Almost all modern telescopes are reflecting telescopes.

8. Disadvantages of Refracting Telescopes • Chromatic aberration: Different wavelengths are focused at different focal lengths (prism effect). Can be corrected, but not eliminated by second lens out of different material. Difficult and expensive to produce: All surfaces must be perfectly shaped; glass must be flawless; lens can only be supported at the edges

9. The Powers of a Telescope:Size Does Matter 1. Light-gathering power: Depends on the surface area A of the primary lens / mirror, proportional to diameter squared: D A = p (D/2)2

10. Telescope Size and SNR • In source shot noise limited case, SNR goes as telescope diameter • For faint sources, i.e., read noise limited cased, SNR goes as telescope diameter squared

11. Reflecting Telescopes • Most modern telescopes use mirrors, they are “reflecting telescopes” • Chromatic Aberrations eliminated • Fabrication techniques continue to improve • Mirrors may be supported from behind • Mirrors may be light-weighted  Mirrors may be made much larger than refractive lenses

12. Basic Designs of Optical Reflecting Telescopes • Prime focus: light focused by primary mirror alone • Newtonian: use flat, diagonal secondary mirror to deflect light out side of tube • Cassegrain: use convex secondary mirror to reflect light back through hole in primary • Nasmyth (or Coudé) focus (coudé  French for “bend” or “elbow”): uses a tertiary mirror to redirect light to external instruments (e.g., a spectrograph)

13. Prime Focus f Sensor Mirror diameter must be large to ensure that obstruction does not cover a significant fraction of the incoming light.

14. Newtonian Reflector Sensor

15. Cassegrain Telescope Sensor Secondary Convex Mirror

16. Feature of Cassegrain Telescope • Long Focal Length in Short Tube f Location of Equivalent Thin Lens Tube

17. Coudé or Nasmyth Telescope Sensor

18. Plate Scale q x F=focal length

19. Field of View • Two telescopes with same diameter, different F#, and same detector have different “Fields of View”: large  small  Small F# Large F#

20. Concave parabolic primary mirror to collect light from source modern mirrors for large telescopes are thin, lightweight & deformable, to optimize image quality Optical Reflecting Telescopes 3.5 meter WIYN telescope mirror, Kitt Peak, Arizona

21. Thin and Light (Weight) Mirrors • Light weight Easier to point • “light-duty” mechanical systems  cheaper • Thin Glass  Less “Thermal Mass” • reaches equilibrium (“cools down” to ambient temperature) quicker

22. Hale 200" TelescopePalomar Mountain, CA http://www.cmog.org/page.cfm?page=374 http://www.astro.caltech.edu/observatories/palomar/overview.html

23. 200" mirror (5 meters)for Hale Telescope • Monolith (one piece) • Several feet thick • 10 months to cool • 7.5 years to grind (although 5 years was a pause for WWII) • Mirror weighs 20 tons • Telescope weighs 400 tons • “Equatorial” Mount • follows sky with one motion

24. Keck telescopes, Mauna Kea, HI

25. 400" mirror (10 meters) for Keck Telescope • 36 segments • 3" thick • Each segment weighs 400 kg (880 pounds) • Total weight of mirror is 14,400 kg (< 15 tons) • Telescope weighs 270 tons • “Alt-azimuth” mount (left-right, up-down motion) • follows sky with two motions + rotation

26. Optical Reflecting Telescopes Schematic of 10-meter Keck telescope (segmented mirror)

27. History and Future of Telescope Size

28. Optical Telescopes: Resolution

29. Optical Telescopes: Collecting Area

30. Optical Telescopes: LSST person!

31. Optical Telescopes: LSST

32. Optical Telescopes: LSST Fly Through

33. Optical Telescopes: Giant Magellan Telescope

34. Optical Telescopes: Thirty Meter Telescope person!

35. Thirty Meter Telescope vs. Palomar

36. Optical/IR Telescopes: JWST Deployment

37. Optical Telescopes: E-ELT

38. Optical/IR Telescopes: JWST • The James Webb Space Telescope will be a large infrared telescope with a 6.5-meter primary mirror. The telescope will be launched on an Ariane 5 rocket from French Guiana in October of 2018. • JWST will be the premier observatory of the next decade, serving thousands of astronomers worldwide. It will study every phase in the history of our Universe, ranging from the first luminous glows after the Big Bang, to the formation of solar systems capable of supporting life on planets like Earth, to the evolution of our own Solar System.

39. Optical/IR Telescopes: JWST Deployment

40. Optical/IR Telescopes: WFIRST • WFIRST is a NASA observatory designed to settle essential questions in the areas of dark energy, exoplanets, and infrared astrophysics. • The current design of the mission makes use of an existing 2.4m telescope, which is the same size as the Hubble Space Telescope. WFIRST is the top-ranked large space mission in the New Worlds, New Horizon Decadal Survey of Astronomy and Astrophysics. The Wide Field Instrument will provide a field of view of the sky that is 100 times larger than images provided by HST. The coronagraph will enable astronomers to detect and measure properties of planets in other solar systems.