Astronomical Observational Techniques
and Instrumentation
RIT Course Number 1060-771
Professor Don Figer
Telescopes
1
Aims and outline for this lecture
• describe most important system parameters for telescopes
• review telescope design forms
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Backyard Telescope
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Telescope System
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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=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
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Thin Lens Equation
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Refracting/Reflecting Telescopes
Focal length
Focal length
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.
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Disadvantages of Refracting
Telescopes
• Chromatic aberration: Different wavelengths
are focused at different focal lengths (prism
effect).
Difficult and expensive to
produce: All surfaces must be
perfectly shaped; glass must
be flawless; lens can only be
supported at the edges
Can be
corrected, but
not eliminated
by second lens
out of different
material.
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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:
D
A = p (D/2)2
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Telescope Size and SNR
• In source shot noise limited case, SNR goes as telescope diameter
S

N
S
N
2
i
shot


S
 S  FAtele t  Dtele
S
i
• For faint sources, i.e., read noise limited cased, SNR goes as
telescope diameter squared
S

N
S
N
2
i
noise
read


S
N read
2
 FAtele t  Dtele
i
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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.
2.
3.
4.
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)
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Prime Focus
Sensor
f
Mirror diameter must be large to ensure that
obstruction does not cover a significant fraction of
the incoming light.
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Newtonian Reflector
Sensor
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Cassegrain Telescope
Sensor
Secondary
Convex Mirror
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Feature of Cassegrain Telescope
• Long Focal Length in Short Tube
f
Location of
Equivalent Thin Lens
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Coudé or Nasmyth Telescope
Sensor
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Plate Scale
q
x
focal length
x  qF
platescale(arcseconds/mm)  206265/ F
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Field of View
• Two telescopes with same diameter, different F#, and same
detector have different “Fields of View”:
large q
Small F#
small q
Large F#
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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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http://www.cmog.org/page.cfm?page=374
Hale 200" Telescope
Palomar Mountain, CA
http://www.astro.caltech.edu/observatories/palomar/overview.html
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200" mirror (5 meters)
for Hale Telescope
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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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Keck telescopes, Mauna Kea, HI
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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
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Optical Reflecting Telescopes
Schematic
of 10-meter
Keck
telescope
(segmented
mirror)
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History and Future of Telescope Size
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Optical Telescopes: Resolution
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Optical Telescopes: Collecting Area
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Optical Telescopes: LSST
person!
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Optical Telescopes: LSST
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Optical Telescopes: Giant Magellan Telescope
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Optical Telescopes: Thirty Meter Telescope
person!
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Thirty Meter Telescope vs. Palomar
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Optical Telescopes: E-ELT (now 39m?)
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Optical/IR Telescopes: JWST
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Optical/IR Telescopes: JWST
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