Lecture 14: Non-Optical Telescopes. Resolving Power. When light enters a telescope, it is bent slightly:

Transcription

1 Lecture 14: Non-Optical Telescopes When light enters a telescope, it is bent slightly: Wave fronts Light rays D The angle of bending limits the resolution of the telescope This depends on the aperture of the telescope, D=2 x R Resolving Power 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 λ( µ m) a(") = 0.25 D( m) 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 1

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 = 10 8 Angstroms (radio) D = 43 meters 5 λ(a) a(") = a = 1 D(m) Resolving Power Examples: l= 4 mm = 4 x 10 7 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! a(") = λ(a) D(m) 2

3 Optical Telescopes We have learned that larger telescopes Collect more light Have 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 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 3

4 Atmospheric Blurring We can defeat atmospheric blurring in several ways: 1. Place the telescope high up in the atmosphere New GMU Observatory 4

5 Atmospheric Blurring We can defeat atmospheric blurring in another way: 2. Place the telescope in space 5

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 6

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8 Multiwavelength Astronomy The Milky Way at many wavelengths 8

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 9

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 10

11 Compton Gamma-Ray Observatory Focusing X-rays 11

12 Infrared astronomy using SIRTF 12

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! 13

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 14

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 can be achieved the best in all of astronomy Radio Interferometry The angular resolution of the radio interferometer depends on the baseline L according to the formula λ(cm) a(") = 2.5 L(km) 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 15

16 resolution of 1 (separation of radio lobes is 100 kiloparsecs) Radio Interferometry resolution of 1 (length is 74 parsecs) resolution of (130 light-days!) length is 3 milliarcseconds Radio morphology of Cygnus A (distance = 170 Megaparsecs) 16