Telescopes:

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Telescopes
Seeing The Universe
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Telescopes our eyes on the Universe

• Definition
• a telescope is a device for collecting and
  bringing to a focus electromagnetic radiation

Astronomer
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Reflecting telescope light captured by a
curved mirror
Radio telescope light captured by curved metal
mirror
Refracting telescope light captured by a
glass lens
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Refracting Telescopes

• Most common design due to Johannes Kepler
• Two convex lenses
• Objective collects light eyepiece magnifies
  image
• Distance (D) between objective and eyepiece is
  the sum of focal lengths
• D Fobjective Feyepiece
• Magnification (M) is ratio of focal lengths

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Refracting telescope
lab 1
Feyepiece
Fobjective
starlight
Detector
starlight
eyepiece
Focal point
Objective lens
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The Fraunhofer 22 cm Refractor (Berlin
Observatory)
Johann Galle
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The 40-inch Yerkes Telescope University of
Chicago Worlds largest refracting
telescope constructed 1897
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Reflecting Telescopes

• Original construction due to Isaac Newton (1670)
• Curved mirror objective gathers light eyepiece
  to magnify image
• Distance between objective and eyepiece and
  magnification are the same as for the refracting
  telescope

Most modern large telescopes are reflectors. The
problem with refractors is that lenses are
heavy - which makes for engineering
problems, and good quality, large lenses are
very difficult to make
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Reflecting telescope - Newtonian
Detector
eyepiece
Feyepiece
starlight
Secondary mirror
starlight
Curved mirror (objective)
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5-m Hale telescope at Mount Palomar in California
Completed in 1948
Edwin Hubble in the observers cage at the prime
focus
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8.1-m Gemini South Telescope (Cerro Pachon,
Chile)
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Baker-Nunn Schmidt Camera Telescope lens
corrector combined with mirror objective Universi
ty of Calgary
Used as a wide field of view survey
camera. Looking for near-Earth asteroids
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Brightness
Time ?
Bright Perseid meteor captured with U of C
Baker-Nunn telescope, on August 12th, 2004
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Viewing the Universe

• When it comes to telescopes SIZE matters

By increasing the size of the objective
(diameter) Can detect fainter stars Can see
finer detail (resolution) Can gather more
information more rapidly but, can encounter
expensive engineering problems (e.g., pointing
and control)
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M31- the Andromeda galaxy
Light Grasp
The light gathering power (LGP) of a telescope is
related to the area of the objective. For an
objective of diameter D LGP constant x D2
LGP 4X
The larger the objective the greater the LGP and
the brighter the image for a given exposure time
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The eye versus Palomar
?

• Diameters are
• human eye D 0.5 cm
• 5-m Palomar telescope D 500 cm

Or, the LGP of the 5-m Mount Palomar telescope
is 1 million times greater than that of the human
eye
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Galileo versus Gemini
How much has the LGP of telescopes Improved in
the past 400 years?
? 16-mm diameter
Galileo Galilei (1564 1642)
8.1-m diameter ?
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M31- the Andromeda galaxy
Light Grasp
The light gathering power (LGP) of a telescope is
related to the area of the objective. For an
objective of diameter D LGP constant x D2
LGP 4X
The larger the objective the greater the LGP and
the brighter the image for a given exposure time
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Exposure time

• Telescope
• Primary function is to gather EM radiation
• Amount of EM radiation gathered is measured by
  the flux received
• The greater the area of the objective, the
  greater the flux received per unit time
• Flux gathered LGP constant x objective area
  (A)
• Time to gather a given quantity (flux) of EM
  radiation is the exposure time
• Exposure time constant / flux gathered
  constant / A
• The greater the flux gathered the shorter the
  exposure time
• Hence, the greater the LGP (area A of objective)
  the shorter the exposure time to produce an image

? OOTETK
? OOTETK
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Example

• A 0.76-m telescope can collect enough light to
  make an image of a galaxy in 1-hour. What is the
  exposure time to image the same galaxy with a
  4.5-m telescope?

Note Area p R2 p (D/2)2
So, exposure time is T4.5 1.7 minutes
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Lets be resolved

• Definition
• Resolution (R) the telescopes ability to
  distinguish between two point sources

For a telescope with an objective of diameter D(m)
arc seconds
1/3600 of a degree Also 1 arc minute
1/60th of a degree
Where l is the wavelength in meters
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Mizar (and Alcor) Single star to the eye. Twin
star to camera
Two objects less than R arc seconds apart on the
sky will be seen as a single object in the
telescope the smaller the resolution the
greater the detail the telescope can see
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R 1
R 10
R 5
R 1
Image detail improves with decreasing R
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• Ground based telescopes
• The resolution is actually set by atmospheric
  blurring (a phenomenon called seeing)
• Small pockets of different density air refract
  starlight as it moves through through the
  atmosphere
• For better seeing place telescope on top of a
  tall mountain to get above lower atmosphere
• Typical limit is 1 arc sec. at the best
  mountain sites
• Other option
• Go into space to improve resolution limit
• i.e no atmospheric interference at all

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HST image
Ground-based image of R136 (star cluster in
the Large Magellanic Cloud) Blurring due to
Earths atmosphere
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• Also go into space because
• Earths atmosphere is a good absorber of
• Selective Infrared wavelengths
• Microwaves
• UV radiation
• X-ray and g-ray
• Radio waves on the other hand can be observed
  come rain or shine and the Earths atmosphere is
  completely transparent

Can only study from spacecraft in Earth orbit
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Windows through the Earths atmosphere
Optical window
Total absorption at UV, X-ray and g-ray
wavelengths
Radio Window
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Telescopes - summary

• Two basic design features
• LGP light gathering power
• Varies as D2
• Exposure time varies as 1 / D2
• Resolution ability to distinguish detail
• The smaller the resolution the finer the detail
  seen
• Varies as l / D, where l is the wavelength of EM
  radiation being observed
• Ground based optical telescopes resolution set
  by atmosphere to 1 arc second