1 Astronomical Observational Techniques and Instrumentation RIT Course Number 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=  r 2 ) 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 Reflecting Telescope: Concave Mirror focuses light onto the focal plane Almost all modern telescopes are reflecting telescopes. Focal length

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: A =  (D/2) 2 D

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 1.Prime focus: light focused by primary mirror alone 2.Newtonian: use flat, diagonal secondary mirror to deflect light out side of tube 3.Cassegrain: use convex secondary mirror to reflect light back through hole in primary 4.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 Location of Equivalent Thin Lens f

17 Coudé or Nasmyth Telescope Sensor

18 Plate Scale focal length  x

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

20 Optical Reflecting Telescopes Concave parabolic primary mirror to collect light from source –modern mirrors for large telescopes are thin, lightweight & deformable, to optimize image quality 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" Telescope Palomar Mountain, CA

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