Wave fronts Light rays D - Information Technology Services

1

•When light enters a telescope, it is bent slightly:

D

Light rays

Wave fronts

•The angle of bending limits the resolution of the telescope

•This depends on the aperture of the telescope, D=2 x R

Lecture 14: Non-Optical Telescopes

•here, the wavelength is measured in micrometers and the aperture is measured in meters

•This angle is the theoretical limit of resolution for the telescope

•The angle of bending limits the resolution of the telescope

•This depends on the aperture of the telescope, D=2 x R

•The bending angle is given by

)()(25.0)("

mDma µλ×=

Resolving Power

2

Resolving Power•Examples:

l= 4,000 Angstroms (blue light)

D = 1 meter

a = 0.1” (less than the atmospheric limit of 1”)

l= 4,000 Angstroms (blue light)

D = 5 meters (Hale telescope)

a = 0.02” (less than the atmospheric limit of 1”)

l= 1 cm = 108 Angstroms (radio)

D = 43 meters

a = 1’ )m()A(105.2)(" 5

Da λ××= −

Resolving Power•Examples:

l= 4 mm = 4 x 107 Angstroms (radio)

D = 100 meters

a = 10” (best we can do using one radio telescope)

l= 5,000 Angstroms (green light)

D = 2.4 meters (Hubble Space Telescope)

a = 0.05” (less than the atmospheric limit of 1”)

This resolution can be achieved

using the Hubble, since the telescope

is located above the atmosphere!)m()A(105.2)(" 5

Da λ××= −

3

Optical Telescopes•We have learned that larger telescopes

Collect more lightHave better angular resolution

•Using high magnification in a small telescope results in dim, fuzzy images because you are exceeding the theoretical limit for the telescope

•You shouldn’t go over about 100-200 times magnification in a small telescope

•Low-budget reflector telescopes offer a better value than refractors (mirrors are cheaper to make than lenses)

Atmospheric Blurring•The theoretical limit of resolution for the 5 meter Hale telescope on Palomar mountain is 0.02”

•This cannot be achieved in practice due to the turbulent motion of the Earth’s atmosphere

•The turbulent motions limit the angular resolution to about 1”

•The resulting observational conditions are called the “seeing”

4

Atmospheric Blurring•We can defeat atmospheric blurring in several ways:

1. Place the telescope high up in the atmosphere

New GMU Observatory

5

Atmospheric Blurring•We can defeat atmospheric blurring in another way:

2. Place the telescope in space

6

Atmospheric Blurring•We can defeat atmospheric blurring in another way:

3. Use adaptive optics to adjust the mirror in real time,

guided by lasers…

7

8

Multiwavelength Astronomy

The Milky Way at many wavelengths

9

Atmospheric Blockage•Radiation propagates forever unless it is absorbed by something along the way

•The Earth’s atmosphere is opaque to most radiation

Atmospheric Blockage•Radio waves are reflected by the ionosphere•Microwaves and infrared radiation are absorbed by water molecules in Earth’s atmosphere

•X-ray, ultraviolet, and gamma-ray radiation is blocked by the ozone layer (these are harmful forms of high-energy radiation)

•Hence, due to atmospheric blockage, certain wavelengths can be observed from space only

High-Energy Astronomy•X-ray and gamma-ray astronomy can be performed only from space due to atmospheric blockage

•High-energy emission tells us about very hot environments

10

High-Energy Astronomy•Using x-ray and gamma-ray astronomy, we can study violent processes occurring near compact objects like black holes, neutron stars, and white dwarf stars

High-Energy Astronomy

High-Energy Astronomy

11

Compton Gamma-Ray Observatory

Focusing X-rays

12

Infrared astronomy using SIRTF

13

Advantages of Radio Astronomy•Radio telescopes are almost completely unaffected by atmospheric blurring

•This is because because radio waves have longer wavelengths than optical waves

•Weather doesn’t matter much either

•“Seeing” is almost always at the theoretical limit

•Observations can be made even in daylight!

14

Radio Interferometry•The angular resolution of radio telescopes can be improved by using them in groups called interferometers

•The radio waves reaching telescope 1 take longer to arrive than those reaching telescope 2

15

Radio Interferometry

The Very Large Array (VLA) in New Mexico

Radio Interferometry•Information from two or more telescopes can be combined using interferometry

•The resulting angular resolution is equal to that of a single telescope with diameter equal to the spacing between the telescopes (the baseline L)

•With widely-spaced telescopes, resolution of 0.001” can be achieved – the best in all of astronomy

where the wavelength is measured in cm and the baseline is measured in kilometers

•This angle is the theoretical limit of resolution for the telescope

•Hence, for a baseline of about 10,000 km (Earth’s radius), we can achieve an angular resolution of less than one milli-arcsecond!

•Note: we do not receive all the radiation that we would if we had a single dish the size of the baseline

•The angular resolution of the radio interferometer depends on the baseline L according to the formula

Radio Interferometry

)km()cm(5.2)("

La λ×=

16

Radio Interferometry

resolution of 0.00015”

(130 light-days!)

resolution of 1”(separation of radio lobes

is 100 kiloparsecs)

Radio morphology of Cygnus A (distance = 170 Megaparsecs)

resolution of 1”(length is 74 parsecs)

length is 3 milliarcseconds