Astronomical Observational Techniques and Instrumentation RIT Course Number 1060-771 Professor Don Figer Telescopes

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

Backyard Telescope

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

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.

Thin Lens Equation

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.

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

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

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

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

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)

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

Newtonian Reflector Sensor

Cassegrain Telescope Sensor Secondary Convex Mirror

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

Coudé or Nasmyth Telescope Sensor

Plate Scale q x focal length

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

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

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

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

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

Keck telescopes, Mauna Kea, HI

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

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

History and Future of Telescope Size

Optical Telescopes: Resolution

Optical Telescopes: Collecting Area ORM is now the Gran Canary Telescope (GTC).

Optical Telescopes: LSST person!

Optical Telescopes: LSST

Optical Telescopes: Giant Magellan Telescope

Optical Telescopes: Thirty Meter Telescope person!

Thirty Meter Telescope vs. Palomar

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

Optical/IR Telescopes: JWST

Optical/IR Telescopes: JWST