1 astronomical observational techniques and instrumentation rit course number 1060-771 professor don...
TRANSCRIPT
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Astronomical Observational Techniques and Instrumentation
RIT Course Number 1060-771 Professor Don Figer
Telescopes
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Aims and outline for this lecture
• describe most important system parameters for telescopes• review telescope design forms
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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
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Telescope Parameters
• Collecting area is most important parameter– collected light scales as aperture diameter squared (A=r2)
• 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
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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
Focal length
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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
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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
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Telescope Size and SNR
• In source shot noise limited case, SNR goes as telescope diameter
teleteleshot
ii
DtFASS
S
N
S
N
S
2
• For faint sources, i.e., read noise limited cased, SNR goes as telescope diameter squared
2noise read
2 teleteleread
ii
DtFAN
S
N
S
N
S
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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
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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)
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Prime Focus
fSensor
Mirror diameter must be large to ensure thatobstruction does not cover a significant fraction of
the incoming light.
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Feature of Cassegrain Telescope
• Long Focal Length in Short Tube
Location of Equivalent Thin Lens
f
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Field of View
• Two telescopes with same diameter, different F#, and same detector have different “Fields of View”:
Small F# Large F#
large small
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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
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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
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Hale 200" TelescopePalomar Mountain, CA
http://www.astro.caltech.edu/observatories/palomar/overview.html
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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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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