Understanding Telescopes: Parameters, Design, and Powers

1. Astronomical Observational Techniques and Instrumentation RIT Course Number 1060-771 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

17. Coudé or Nasmyth Telescope Sensor

18. Plate Scale q x 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 • 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: Giant Magellan Telescope

33. Optical Telescopes: Thirty Meter Telescope person!

34. Thirty Meter Telescope vs. Palomar

35. Optical Telescopes: E-ELT (now 39m?)

36. Optical/IR Telescopes: JWST

37. Optical/IR Telescopes: JWST