Before we study stars we need to take time out to look at the telescope, the main tool that astronomers use.
Up till Galileo, about 1600
Discovery of nebulae, galaxies up till about 1880
Astronomy dominated by study of motions of planets, stars.
Up till till about 1950. Serious physics now possible, using spectra, energy measurements.
Up till about 2000, by which time most of the gaps had been filled.. Part of this was due to the Space program.
Dominated by automated surveys?
Small telescopes, and also binoculars, usually use lenses to focus light. We call such a telescope a refracting telescope; (refraction is the property of glass that makes it bend light).
It
is the first, large lens that is the important one. In some telescopes the
lens plays the role of a camera lens, in focusing light on a photographic plate,
or a digital detector or a spectrograph. Large telescopes can't usually be
looked through.
Reflecting
telescope uses a concave mirror to focus the light. The advantages are
that you only have one surface to polish, the glass can be supported all over
except at the edge, and you don't lose light as it passes through the glass. The
disadvantage is that the focus is in the way of the beam, so you have to find a
way of getting the light out of the telescope using another mirror. All
the telescopes on Mauna Kea are reflecting telescopes.
An important difference between astronomical telescopes and binoculars is that binoculars are mainly used to produce magnified images, whereas astronomical telescopes are mainly used to collect more light. Nobody ever quotes the "magnification" of an astronomical telescope.
The largest single mirrors are about 8 meters (25 feet across),s:
Gemini Telescope on Mauna Kea is jointly operated by US, UK. Canada, Australia, Argentina, Brazil, Chile.
Keck
telescopes on Mauna Kea, are are the largest in the world, with10 meters
diameter mirrors made of 36 smaller mirrors
No optical viewing by professionals
Cameras are now almost all electronic rather than film.
Astronomical cameras have many more pixels than ordinary digital cameras, but
only measure one wavelength at a time. If you want a color image you take
two or three photographs with different wavelengths and combine them in a
computer.
More than half of all observations by professional astronomers are
spectroscopy. We have elaborate instruments that split the radiation
from stars into its spectrum lines, so that we can measure things like
composition, Doppler shifts.etc.
Major telescopes are so expensive that they are only built in the very best locations.
Among the requirements:
The best two sites in the world are Hawaii and Chile. Both need to be developed because you need to cover both the northern and southern hemispheres.
Mauna
Kea Observatory12 telescopes on the summit (+ 1 lower down)
13 countries collaborating.
Big industry on Big Island: 600 jobs, $140 million per year
Remember how the transparency of the sky changes with wavelength.
Radio
waves travel through the Earth's atmosphere, so we can build radio telescopes on
the ground. This one is 300 feet in diameter in West Virginia.
The laws of physics limit the detail we can see using any telescope.
The smallest angle that can be discerned depends on the ratio
(wavelength/diameter). For best detail we need small wavelength and large
diameter. Unfortunately radio waves have large wavelength, so that the smallest
angle is large and we don't see details. 
The
way round this is to build a number of smaller antennas and link them together
electrically and using computers. This is the Very Large Array in New Mexico,
which is about 20 miles across. We call this an interferometer or
an array.
On
Mauna Kea there is a "submillimeter array" that works at a wavelength
of around 0.3 mm. These are the shortest radio wavelengths used in
astronomy, and only Mauna Kea has the atmosphere good enough for it to
work.
Radio telescopes are particularly good at studying gas between the stars, and powerful objects like black holes and supernovae.
![opt_assy[1].jpg (60594 bytes)](images/opt_assy[1].jpg)
Hubble
Space Telescope was primarily designed to overcome the effects of
"seeing", or blurring of images due to the Earth's atmosphere Nowadays
we can use special optical techniques to improve the images from Mauna Kea to
the same quality as the HST. Mauna Kea is cheaper, and has much bigger
telescopes and newer technology, but Hubble has produced some fantastic images.
It has the additional advantage that it can work at ultraviolet wavelengths that are blocked by the Earth's atmosphere. Many atoms have most of their spectrum lines in the ultraviolet, so Hubble contains a UV spectrograph.
![cxro_art_m[1].jpg (30216 bytes)](images/cxro_art_m[1].jpg)
All
has to be done from Space. More than a dozen X-ray observatories have been
launched by US, Russia, Japan and European countries
Major observatory is NASA's Chandra observatory
One of the big advantages of infrared astronomy is that infrared waves penetrate the dust clouds in space and allow us to see things that emit no light.
Some wavelengths can be done from Mauna Kea, but others are absorbed by water vapor in the Earth's atmosphere. We can do more by putting a telescope in an airplane. NASA has a project called SOFIA (Stratospheric Observatory for Infrared Astronomy) that has a 3-meter diameter telescope inside a Boeing 747.
![2000AC0017[1].gif (63402 bytes)](images/2000AC0017[1].gif)
![1998TA0011_sm[1].gif (14713 bytes)](images/1998TA0011_sm[1].gif)
A major problem for infrared astronomy is that that the telescope and mirror itself radiated infrared radiation. It's like looking through a white-hot telescope. You can cut down this "noise" by cooling the telescope to a few degrees above absolute zero. But this would cause the oxygen and nitrogen in the Earth's atmosphere to condense as a liquid on the mirror. The only way to avoid this is to put the telescope in space. There are advantages and disadvantages of space versus airplane versus ground
Image of the Milky Way galaxy based on a satellite-based infrared map.
The third of the major observatories is called the Spitzer observatory and is dovoted to infrared studies.
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