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ASTR2050 Spring 2005
Lecture 10am 1 February 2005
Please turn in your homework now!
In this class we will cover Telescopes:
• What does a telescope do?
• Kinds of optical telescopes
• Photography and light detection
• Observing outside the visible
1
This is a telescope:
Hale Telescope (Mt Palomar)
See Kutner Figure 4.7a
2
So is this:
VLA (Kutner Fig.4.30)
3
These are telescopes, too:
Chandra
Hubble Space Telescope
4
What does a telescope do?
Telescopes perform two main functions:
1. Light gathering: The “aperture” of a telescope is
much larger than the pupil of the eye. Detectors
allow it to “integrate” over a longer time.
2. Angular resolution: Waves “diffract” when they
pass through an opening, and make images
“fuzzy”. Diffraction is smaller for large openings.
5
Light gathering power
Diameter of the pupil of your eye: 5mm or so
Diameter of the “aperture” of a telescope: BIGGER!
Relative light gathering power is just
the area of the (circular) aperture.
Area = !(D/2)
2
Relative LGP = (DTel /DEye)
2
!m = 2.5 log10(LGP)
6
Example 4.1 in Kutner
How many magnitudes fainter can the eye
observe using a 1m diameter telescope?
!
"
2
2
!m = 2.5 log10 (1m) /(5mm) = 11.5
So, if your naked eye can see down to magnitude 6,
this telescope lets you see down to magnitude 17.5
7
Angular Resolution
First discuss “diffraction”: General phenomenon for waves
passing through some opening
8
Example: Water waves
Go to “ripple tank”
Java applet
9
Similar for “circular aperture”
10
So, back to “angular resolution”...
How close together can two objects be and still be “resolved”?
11
The Rayleigh Criterion
Images “resolved”
when the peaks
are separated by
the distance
between minima
for a single image.
i.e. “resolution”
!
≈
D
12
Kinds of optical telescopes
• Refracting telescopes
- Magnification
• Reflecting telescopes
- Newtonian, Cassegrain, and Coude
focal configurations
• The Hubble Space Telescope
13
Refracting telescopes
See Also Kutner Fig.4.5
fOb j
magni f ication =
fEye
fEye
14
fOb j
Yerkes Observatory
Refracting Telescope
40 Inch Diameter Objective
It is the largest refracting
telescope in the world!
It is hard to make a large lens
that is optically “perfect”, and
to keep it that way!
15
Another problem with refracting
telescopes: Chromatic aberration
It can be corrected to some extent, but not perfectly.
16
Reflecting telescopes
Fix problems
encountered with
refracting
telescopes
17
The Keck 10m Telescope
The (two) largest telescopes in the world
18
The Hubble Space Telescope
Why put a telescope in space?
1. Absence of atmospheric
distortion
2. Near UV and IR
3. Unlimited seeing time
19
Atmospheric Distortion
Demonstration
of “speckling”
20
When the Hubble Telescope
got its “eyeseight” corrected
Will the HST ever get serviced again??
21
Photography and light detection
Photography remains
a useful “true color”
technique.
Nowadays, CCD’s are
used instead of film.
22
Observing outside the visible
Fundamental principles of “light” gathering
and angular resolution are the same!
But, there are
some special
tricks, for
example, the
VLA makes use
of radio wave
interferometry:
23
X-ray and Gamma Ray astronomy
Chandra is a spacecraft
The Auger project
24