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
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.
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 http://www.astro.caltech.edu/observatories/palomar/overview.html http://www.cmog.org/page.cfm?page=374
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
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