Transcript astro20 telescopes - Las Positas College

More Optical Telescopes
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There are some standard reflecting telescope
designs used today
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All have the common feature of light entering a tube
and hitting a primary mirror, from which light is
reflected towards the prime focus
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One design has instruments placed at the prime focus
itself.
– Very hard to mount large intruments like spectrometers or
large cameras
– usually light is reflected by a secondary mirror to a more
convenient place to mount big measuring instruments.
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Another design, called a Newtonian telescope, has
light deflected 90° prior to prime focus by a secondary
mirror
– a very popular design for amateur telescopes
– light is usually deflected to an eyepiece
– other possibilities for instruments are cameras or
photometers (a device used to measure integrated light
levels)
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A traditional design used by professional astronomers
is the Cassegrain telescope
– light bounces from primary mirror to a secondary mirror
mounted in front of prime focus, and then reflected back
towards the primary mirror, where it exits through a hole
More Optical Telescopes
– large intruments can be mounted on the back of the
telescope
– the point beyond the mirror where the focus lies is called
the Cassegrain focus
– still a very popular design (KPNO and CTIO 4-m,
Palomar 5-m)
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Still other designs involve extra mirrors to guide light to
various measuring instruments
– one or more mirrors placed in a Cassegrain telescope
causing light to reach a focus down a tube aligned along
the north pole to large intruments in a separate room
(called a Coude’ focus)
– allows for very finely tuned instruments that can’t possibly
be mounted onto the telescope itself
– multiple reflections = higher light losses, something that
must be accounted for in instrument design
– image never moves, only rotates
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All modern telescopes can be configured to any of
these setups
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Light blocked by the secondary mirror is minimal
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The Schmidt telescope has a an unusual design
– light enters through a thin lens called a correcting plate
before reaching the primary mirror, which is spherical
– the lens bends light not exactly parallel to the axis of the
telescope so that the spherical mirror can focus large
images
More Optical Telescopes
– results in a curved image at a kind of prime focus where
a photographic plate or mosaic of detectors lies
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For those who care --- Many amateur telescopes
(Celestron and Meade) use a combined design of
Schmidt-Cassegrain
– has a correcting plate and spherical mirror with a
Cassegrain focus
– secondary mirror mounted on the back of the correcting
plate
– light can bounce back and forth multiple times between
primary and secondary
– extremely compact and portable design, but thickness of
correcting lens is too much for professional telescopes
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A variety of instruments can be used on optical
telescopes
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can be used as a giant camera to take images of
objects
– images can be recorded on film or on electronic detectors
called CCD’s (charged couple devices)
– usually done at prime or Cassegrain focus in order to
minimize light losses
– can use filters to limit which part of the EM spectrum is in
the image (how most pictures are taken)
More Optical Telescopes
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can measure levels of integrated light intensity with a
photometer
– usually done at Cassegrain focus because of ease of use
– usually performed over specific parts of EM spectrum to
determine the temperature, but can just give information
on time variations in brightness
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can measure the spectrum (usually only small pieces)
with a spectrometer
– can be done at any focus, depending on size
Telescope Size
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Size determines the light gathering power of a
telescope
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light gathered is proportional to the collecting area of
the telescope
– this refers to the refracting lens or primary mirror
– an example: if telescope #1 is 10 times the diameter of
telescope #2, telescope #1 will collect light at a rate 100
times that of telescope #2
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can also think of light gathering power in terms of time
– if telescope #1 is ten times the diameter of telescope #2,
telescope #1 will collect the same number of photons 100
times faster than telescope #2
– telescope observations are given in terms of the
exposure
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Size also determines the resolving power of the
telescope
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the ability to form distinct images of neighboring
objects is called angular resolution
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diffraction provides the lower limit for angular
resolution
– light waves entering a telescope always undergo some
degree of diffraction, which introduces fuzziness in
images
Telescope Size
– angular resolution depends both on wavelength and size
angular resolution 
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– this is also called the diffraction-limited resolution
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the human eye has an angular resolution of 0.5’
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in practice, telescopes don’t reach this limit due to
refraction in the atmosphere by turbulence (seeing)
Modern telescopes have employed new engineering
techniques
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large primary mirrors have a list of problems
– usually made of glass (pyrex) or quartz and need very
low thermal expansion properties
– hard to manufacture large pieces of glass
– only the 6-m telescope in the Caucasus, the 6.5-m
Gemini telescopes and the 8-m MMT mirrors have been
built since the Palomar 5-m in 1948
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new large optical telescopes (like Keck in Hawaii, and
HET in Texas) use segmented primary mirror designs
– hexagonal mirror segments separately controlled by
motors
– individual mirros have common focus by use of laser
sighting
– can be very expensive (Keck ~ 140 million) or very
inexpensive (HET ~ 15 million) depending on telescope
function
High Resolution Astronomy
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The useful resolution of a telescope is determined
mostly by the quality of the image transmitted by the
atmosphere
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individual turbulent motions cause a random mixture of
refractions which add a fuzziness to images
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this is called the seeing
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this is extremely dependent on local atmospheric
conditions
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for instance: the McDonald Observatory 2.7-m has a
diffraction limit of about 0.04”, yet the average
measured image is about 1-2”
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this property was one of the primary reasons for the
development of HST (no atmosphere = diffraction
limited resolution)
Image processing plays a big role in astronomy
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historically, plate film was used to record images
– hard to maintain, develop film, provide quantitative data
and store
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almost all data is recorded on CCD’s (charge coupled
devices)
– silicon wafer with a two dimensional array of elements
called pixels
High Resolution Astronomy
– when a photon hits a pixel, free electrons multiply (by the
photoelectric effect)
– after a prescribed exposure time, the number of electrons
as a function of position on the CCD are measured by a
computer and converted to intensity
– much more efficient than film
– instant digitization of data for easy storage and analysis
– can manufacture CCD’s to have peak efficiencies in
different parts of the EM spectrum
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New technology is playing a big role in image
processing
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online adjusment of the telescope mirrors to
compensate for atmospheric seeing conditions would
hopefully result in difraction limited images
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would reduce the need for expensive space-based
platforms
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this technique is called adaptive optics and is an
offshoot of the Star Wars (SDI) program of the 1980’s
– in active optics, individual actuators deform the mirror to
subtract the distortion by the atmosphere. Amount of
atmospheric distortion is measured by an artificial star
created by a laser which can only penetrate to the Na
layer of the atmosphere
– in passive systems, laser guide star measurements are
subtracted out by a computer from images after the
image is taken