Download Telescopes and Astronomical Instruments

Telescopes and Astronomical Instruments
The 2 main points of telescopes are
1) To make images with as much angular information as possible
2) To gather as much light as possible to allow study of faint things
Where you put the telescope can also be important because
1) It’s better to have nice weather (or no weather)
2) It’s better to have as little absorption by atmospheric blocks
3) It’s better to have very stable (or no) air to minimize blurring
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).
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:
resolution 

D
A small resolution is
better, as it allows
more closely spaced
features to be
distinguished.
Examples of Resolution
3
The real equation for resolution is: resolution  2 x10
 nm 
Dm 
arc sec 
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 5x102/2.5x10-3=400arcsec=6.5arcmin (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.
Astronomical “Seeing”
It’s 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.
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
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.
Mirrors of the World
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
Segmented Mirrors
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.
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.
Observatories like to be high and dry
Observatories like to be high and dry
…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…
The “Great Observatories” Space Program
The Hubble Space Telescope
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 isn’t 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).
Other Space Telescopes
HESSI : Gamma Rays
Chandra : X-rays
SIRTF : infrared
Radio Telescopes
Because radio wavelengths are
much bigger, it is easy to build
radio telescopes bigger (but
they don’t have better
resolution).
The Quest for Resolution – Adaptive Optics
Wouldn’t 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.
Make your own correction star…
Using “Star Wars” technology,
today we are trying to make groundbased telescopes have sharper vision
than Hubble (but only over a tiny
patch of sky), along with their
superior light-gathering power.
…or maybe you could do even better
Interferometry
How about making 2 separate telescopes behave as though they were
2 pieces of one much larger telescope. You get the larger one’s
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.
The “Very Large Array”
Optical
Interferometers
That’s why the new
observatories come in
matched sets…