ASTA01H3 Lecture Notes - Focal Length

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25 Jan 2011
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Lecture 21: Telescopes
oTime machines
oWorks by gathering light bigger the telescope more light it can
oImages formed by reflection or refraction
oFocus of primary mirror is called primary focus
oReflecting mirror
oImage Formation
oLight rays of the each region of the source are parallel which
are focused into a common point
oImage is formed upside down
oTelescope Design
oBy bending light through different angles a lens can be used
to focus the light
oIn both cases the image formed is viewed with a small
magnifying lens called the eyepiece
oChromatic aberration
Differential refraction as light pass through a lens
Problem with refraction telescope:
Focal length of red, green and blue light differs
Cannot build really large ones because the lens
cannot be supported properly
oAll large modern telescope are reflectors
oImages and Detectors
Charge Coupled Devices (CCDs) are electronic devices
that store light intensity
Spatial detail is lost by the grid resolution of the CCD
oLarger telescopes are desirable
Better resolution the separation of distinct features
Gather more light
Doubling the radius of telescope gives four times the
light gathering power
oAngular resolution limit is given by
Small angular separation requires
oHigh resolution Astronomy
Atmospheric blurring: due to movement of air
Bends light effect is worst for visible light
Telescopes on mountain tops in deserts
Telescope in space
Active optics- control mirrors based on
temperature and orientation
Adaptive optics: track atmospheric changes with
laser, adjusts mirrors in real time
Radio Astronomy
Less sensitive to imprecations due to longer
wavelength can be made very large
Longer wavelength means poor angular
oCan observe 24 hours a day
oClouds dont interfere
oObservations at different frequencies get
different information
oResolution is proportional to wavelength
oCombine information with several widlet
spread radio telescopes
oResolution will be the same as that of a
dish whose diameter = largest separation
between dishes