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=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
The photo shows the Yerkes 40 inch refractor. Note that the floor has been raised to nearly the highest level. If one is not careful, it is possible to drive the telescope into the ground.

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. 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
ORM is now the Gran Canary Telescope (GTC).

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