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Telescopes and Astronomical Instruments

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Title: Telescopes and Astronomical Instruments


1
Telescopes and Astronomical Instruments
  • The 2 main points of telescopes are
  • To make images with as much angular information
    as possible
  • To gather as much light as possible to allow
    study of faint things
  • Where you put the telescope can also be important
    because
  • Its better to have nice weather (or no weather)
  • Its better to have as little absorption by
    atmospheric blocks
  • Its better to have very stable (or no) air to
    minimize blurring

2
Light-Gathering Power
A telescope is like a light bucket. The amount
of light it can gather is proportional to the
area of its opening or aperture. That in turn
is proportional to the square of the diameter
L.G.P.D2 The larger the telescope, the
fainter one can see things (although detector
efficiency and exposure time also play an
important role).
3
Angular Resolution
A fundamental limit on how fine the detail a
telescope can resolve is its diffraction limit.
The resolution of a telescope is given by
A small resolution is better, as it allows more
closely spaced features to be distinguished.
4
Examples of Resolution
The real equation for resolution is
Angular units are given as degrees, arc minutes,
arc seconds. An arcsec is about the angular size
of a quarter seen 5 km away. The resolution of
your eye (which has a diameter of 2.5mm) at the
wavelength of visible light (which is 500 nm) is
therefore 2x10-3 x 5x102/2.5x10-3400arcsec6.5ar
cmin (the Moon has an angular diameter of 30
arcmin). To reach the practical limit of what
can be seen (1 arcsec) through our atmosphere,
you need a telescope with 400 times that
diameter, or 1 meter. Telescopes larger than that
only gain you light gathering power, unless you
do something to reduce the blurring by the
atmosphere. Radio telescopes, which operate at
wavelengths 10,000 times longer, will have far
worse resolution.
5
Astronomical Seeing
Its best to find a site where the air layers
above are very stable (or flow smoothly). You
also need to make sure that the telescope and
dome does not give off heat.
6
Refracting telescopes
You can use a lens to gather the light and bring
it to a focus. The magnifying power of the
telescope depends on the focal length, but mostly
on the eyepiece you use to magnify the image at
the focus. It is hard to make lenses really big.
The telescope at Chabot Space and Science Center
7
Reflecting Telescopes
It is much easier to make a large mirror, because
you can support it from behind. All large
telescopes are reflectors. You can take the light
to various foci, some of which are better for
placing heavy instruments.
One generally has a secondary mirror to take
the light to the focus this blocks some of the
light from the primary.
8
Mirrors of the World
9
The Quest for Light The Biggest Telescopes
The Keck Observatory the largest in the world
(UC, Caltech, NASA).
Twin 10-meter Telescopes On Mauna Kea
10
Segmented Mirrors
11
The European VLT (very large telescopes)
Four 8-meter mirrors on Paranal in Chile (gives
coverage of the Southern Hemisphere). Different
instruments for each telescope, but they can also
work together.
12
Active Mirrors and Adaptive Optics
You can constantly monitor the focus of a mirror
and improve the image (removes flexing and
thermal problems still seeing limited). The
alternative to segmented mirrors.
13
Observatories like to be high and dry
14
Observatories like to be high and dry
15
so they can see more of the EM spectrum
On the ground, getting above water gives you the
near infrared and sub-mm regions (good for
studying star formation and distant
galaxies). But in space, ahhh.! Not only do you
get the whole spectrum, but also no seeing
problems
Of course, it costs a LOT more
16
The Great Observatories Space Program
17
The Hubble Space Telescope
18
Astro Quiz
Which statement below is LEAST convincing as a
selling point for the Hubble Space Telescope over
the Keck telescopes (given that they are 10 times
cheaper with diameters 5 times bigger).
  • It is above all the atmospheric turbulence, so it
    can take really sharp images (better than any
    ground telescope), even though it isnt as big.
  • It is above all the atmospheric blocks, so it can
    see wavelengths we cannot access from the ground
    (both infrared and ultraviolet).
  • The sky is much darker in space, so it can see
    much fainter objects than from the ground (even
    galaxies at the edge of the Universe).

19
Other Space Telescopes
HESSI Gamma Rays
Chandra X-rays
SIRTF infrared
20
Radio Telescopes
Because radio wavelengths are much bigger, it is
easy to build radio telescopes bigger (but they
dont have better resolution).
21
The Quest for Resolution Adaptive Optics
Wouldnt it be great if you could analyse exactly
how the atmosphere is distorting the light waves
coming in, and make a correction with a flexible
mirror that was fast enough (100 times per
second) to keep up with the turbulence.
Amazingly, we can now begin to do that! The
technique is called adaptive optics. You use a
small rubber mirror near the focus (not the big
telescope mirror). You may need several hundred
little pistons to correct a large telescope.
22
Make your own correction star
Using Star Wars technology, today we are trying
to make ground-based telescopes have sharper
vision than Hubble (but only over a tiny patch of
sky), along with their superior light-gathering
power.
23
or maybe you could do even better
24
Interferometry
How about making 2 separate telescopes behave as
though they were 2 pieces of one much larger
telescope. You get the larger ones resolution,
but not its light-gathering power. The light from
the units
Must be combined in phase (preserving the
character it would have had if it came off the
same larger mirror). This is easy at radio
wavelengths, but hard at optical wavelengths.
25
The Very Large Array
26
Optical Interferometers
Thats why the new observatories come in matched
sets
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