Download Document

Telescopes
How big is yours?
5
Goals
•
•
•
•
Telescopes
Angular Sizes
“Seeing”
Magnitudes
5
5
Radio
g-ray
UV
Optical and infrared
X-ray
5
What is the Purpose of a Telescope?
1. Increase the amount of light we see.
Sensitivity is proportional to Collecting Area.
S = constant times D2
If D increases, then S increases by D2
•
If your telescope is 3 times bigger than mine, then your
telescope can see 32 = 9 times fainter objects than mine.
•
Can you read a book at night? What’s the faintest
star you can see with your naked eye?
5
Sensitivity
5
What is the Purpose of a Telescope?
2. Increase the detail (resolution) we see.
Resolution is inversely proportional to Telescope Diameter.
 = constant times 1/D
 Diffraction Limit
If D increases then  decreases by the same amount.
•
If your telescope is 3 times bigger than mine then you can see 3
times smaller angles (3 times smaller objects or detail).
•
Can you read a street sign a block away? Can you see
the binary star in the Big Dipper with your naked eye?
5
Resolution
5
Angles
• The sky is 360 arc degrees around.
• 60 arcminutes = 1 arc degree
– The Full Moon is about half an arc degree =
30 arcminutes.
• 60 arcseconds = 1 arcminute
– Mars is about 2 arcminutes now.
• 1000 milliarcsecond = 1 arcsecond
– Polaris is 46 milliarcseconds in diameter
– An astronaut on the Moon is 2 milliarcseconds tall!
5
5
Angular Size
• Angular size: How big does something look as
viewed from the Earth?
– During a solar eclipse, the Moon looks big enough to
cover the Sun.
• The Sun is a million times larger than the Earth.
• The Moon is a fourth the size of the Earth.
• The distance from the Earth determines their
ANGULAR SIZE.
5
Types of Telescopes
5
Refractor
5
optical
Yerkes 40-inch refractor
5
Reflector
5
Hooker 100-inch reflector
optical
5
Keck twin 10meter reflector
optical/IR
5
Arecibo 300-foot reflector
radio
5
Interferometry
• Combine the light from two or more telescopes to
simulate the RESOLUTION of one giant
telescope.
NPOI - optical
VLA - radio
5
Atmospheric Seeing
5
Beat the Seeing
• Seeing degrades resolution
• Can put a telescope in space
(Hubble Space Telescope)
– Expensive!
• Can make interferometers
(NPOI)
– Complicated!
• Adaptive optics can recover
resolution
5
Hubble Space Telescope
Space
– 1.0m
Ground- HST
- KPNO
4.0m – Copyright NOAO/AURA/NSF
5
Resolution and Seeing
HST
Neptune with the Palomar 200-inch reflector and HST
5
Seeing and Magnification
• Larger than a few inches, a telescope’s resolution
stops getting better due to seeing.
• Don’t be fooled by advertisements claiming huge
magnification increases!
– “Amazing 500X magnification!”
• But sensitivity ALWAYS increases with bigger
telescopes.
5
Magnitude
Scale
• The SMALLER the
number the BRIGHTER
the star!
• Every difference of 5
magnitudes is a 100X
difference in
BRIGHTNESS.
• Every difference of 1
magnitude = 2.5X
brightness
5
Magnitude vs. Brightness
Mag.
Difference
1
Factors of 2.5
2.51 = 2.5
Brightness
Diff.
2.5
2
2.52 = 2.5 X 2.5
6.3
3
2.53 = 2.5 X 2.5 X 2.5
16
4
2.54 = 2.5 X 2.5 X 2.5 X 2.5
40
5
2.55 = 2.5 X 2.5 X 2.5 X 2.5 X 2.5
100
6
2.56 = 2.5 X 2.5 X 2.5 X 2.5 X 2.5 X 2.5
250
5
But what does it
look like from the
back?
5