Monday March 22nd 1999
Announcements:

Lecture notes:
Class 28.

"Optical telescopes - Penn State is building a giant optical telescope"

Starting a new section of the course.  Telescopes and cosmology.

For next 2 lectures we'll discuss telescopes and then will start on cosmology.

Telescopes.  So the main way that astronomers learn about the Universe is by observing electromagnetic radiation with telescopes.

Since astronomers cannot experiment with celestial sources but only observe electromagnetic radiation from them they constantly strive to improve their telescopes.

Another important point is that when we speak of a telescope it won't always mean an optical (visible light) telescope.  Huge
range of electromagnetic radiation requires many different telescopes to study universe effectively.

e.g. radio - radio dishes
X-ray - "Geiger counters" on satellites because of atmosphere.

However visible light telescopes have been very important historically and today I'll focus on them.

They can be built on Earth since the atmosphere is mostly transparent to optical light.  However as we'll discuss
atmosphere does cause some problems so optical telescopes often built on high mountains to eliminate as much of the atmosphere as possible.

Functions of a telescope.  Two main goals
Gather as much light (photons) as possible from a faint celestial object.
Resolve a celestial object to see fine details of it -- often limited by atmosphere "seeing"

There are two main types of optical telescopes.

Refractors focus light with a lens.
Reflectors focus light with a curved (concave) mirror.

Refractor - light rays come in parallel.
    Light is bent by lens glass
    Objective lens - main lens
    Aperture - area from which light collected
    focal length - length from lens to focus point.
    Gives inverted image.

Reflector - light rays come in parallel
    Light is reflected by main mirror
    Objective mirror = main mirror
    Aperture
    Role of secondary mirror and location of observer.

Now astronomers today need to build big telescopes to get better light gathering power - use reflectors and not refractors.

Reflectors are better because
Less expensive - only have to polish one surface and don't needpure flawless glass as for refractor lens.
Less sagging - can support mirror on back surface.  For refractor can only support lens at its edges.
Refractors have a problem called chromatic aberration - see book.

Now astronomers spend great effort trying to build bigger and bigger telescopes.  Why?  Bigger telescopes give better light
gathering power.  In fact we often describe telescopes primarily by their diameter.

Collecting photons with a  telescope is like gathering raindrops in a bucket.  Want biggest bucket.  Diameter of primary/objective mirror determines collecting area.

Now you have learned equations.

Diameter = 2 X radius
Area = pi (radius) squared
so Area = pi/4 D squared

photon collecting area depends on square of diameter - can collect more light from distant objects and see deeper into
universe.
example:  Palomar 5 M telescope in California
                Keck 10 M telescope in Hawaii

Keck has twice the diameter so 4X the collecting area.

Also bigger telescopes in principle give better angular resolution.  But in practice this is limited by atmosphere
"seeing" to about 1 arcsec.

Earth's atmosphere has turbulent air patterns and these make image dance and blur - like twinkling star.

Now the biggest optical telescopes today are very impressive in their own right - kind of like the pyramids of Egypt and other
wonders of the world.  Built in exotic places.

Amazing amount of work goes into building them - for example the Keck telescope took about 5 years and $120 million to build.

Segmented mirror technology.  Now there are 2 Keck telescopes.

Penn State's answer to Keck.  The Hobby - Eberly Telescope. Located in West Texas.  Conceived by Professor Ramsey and
Professor Weedman.  Goal was to design a telescope as big as and as good as Keck in many ways but for much less cost.  About 13 million.

Construction started in March 1994.  Full science operations should begin soon.