Astronomical Techniques I

Transcription

1 Astronomical Techniques I Lecture 4 Yogesh Wadadekar Jan-Feb 2015 IUCAA-NCRA Grad School 1 / 21

2 Coma or comatic aberration - inherent to parabolic telescopes show video IUCAA-NCRA Grad School 2 / 21

3 Schmidt camera IUCAA-NCRA Grad School 3 / 21

4 Schmidt Cassegrain IUCAA-NCRA Grad School 4 / 21

5 Schmidt Cassegrain IUCAA-NCRA Grad School 5 / 21

6 Ritchey Chretien hyperboloid primary and secondary, no coma or spherical aberration! IUCAA-NCRA Grad School 6 / 21

7 Almost all large telescopes are RC two 10 m telescopes of the Keck Observatory four 8.2 m telescopes comprising the Very Large Telescope in Chile two 8 m telescopes comprising the Gemini Observatory 10.4 m Gran Telescopio Canarias 8.2 m Subaru telescope at Mauna Kea Observatory 3.5 m Herschel Space Observatory orbiting telescope 2.5 m Sloan Digital Sky Survey telescope (modified design) 2.4 m Hubble Space Telescope 85 cm Spitzer Space Telescope 30 m TMT IUCAA-NCRA Grad School 7 / 21

8 Altazimuth mount IUCAA-NCRA Grad School 8 / 21

9 Dobsonian mount IUCAA-NCRA Grad School 9 / 21

10 Equatorial mount IUCAA-NCRA Grad School 10 / 21

11 Most modern telescopes use altazimuth mounts IUCAA-NCRA Grad School 11 / 21

12 Fundamental limit on resolution - diffraction (Airy disk) sin α n = m n λ/d IUCAA-NCRA Grad School 12 / 21

13 Rayleigh criterion 1.22λ/D IUCAA-NCRA Grad School 13 / 21

14 Diffraction in astronomy Why do some stars show spikes? Where are the rings? Why are bright stars larger than faint stars? IUCAA-NCRA Grad School 14 / 21

15 Apparent magnitude - flux measure m 1 m 2 = 2.5 log 10 (f 1 /f 2 ) logarithmic IUCAA-NCRA Grad School 15 / 21

16 Apparent magnitude - flux measure m 1 m 2 = 2.5 log 10 (f 1 /f 2 ) logarithmic base 100 1/5 IUCAA-NCRA Grad School 15 / 21

17 Apparent magnitude - flux measure m 1 m 2 = 2.5 log 10 (f 1 /f 2 ) logarithmic base 100 1/5 inverted - brighter objects have lower magnitude IUCAA-NCRA Grad School 15 / 21

18 Apparent magnitude - flux measure m 1 m 2 = 2.5 log 10 (f 1 /f 2 ) logarithmic base 100 1/5 inverted - brighter objects have lower magnitude measurements relative IUCAA-NCRA Grad School 15 / 21

19 Absolute magnitude - luminosity measure M = m 5((log 10 D L ) 1) for cosmological distances, D L is the luminosity distance. IUCAA-NCRA Grad School 16 / 21

20 Review questions The two stars of a double star system are separated by 1.5 arcsec. Could these be resolved with a 10 cm diameter telescope? Is there any location on Earth where a telescope with an altazimuth mount will function just as well as an equatorially mounted telescope for that location? IUCAA-NCRA Grad School 17 / 21

21 Effects of the atmosphere on incoming radiation refraction and dispersion- changes apparent direction of a star IUCAA-NCRA Grad School 18 / 21

22 Effects of the atmosphere on incoming radiation refraction and dispersion- changes apparent direction of a star thermal emission - affects infrared/mm observations IUCAA-NCRA Grad School 18 / 21

23 Effects of the atmosphere on incoming radiation refraction and dispersion- changes apparent direction of a star thermal emission - affects infrared/mm observations Atmospheric scattering - prevents day and affects night observing IUCAA-NCRA Grad School 18 / 21

24 Effects of the atmosphere on incoming radiation refraction and dispersion- changes apparent direction of a star thermal emission - affects infrared/mm observations Atmospheric scattering - prevents day and affects night observing atmospheric turbulence affects wavefronts IUCAA-NCRA Grad School 18 / 21

25 Refraction and dispersion Refractive index η of air at sea level at 15 degrees Celsius: where λ is in microns. (η 1) 10 8 = λ 2 40 λ 2 changes position, important in high resolution (eg. adaptive optics) observation because η = f (λ), refraction corrections depend on zenith angle and λ (called chromatic refraction correction. IUCAA-NCRA Grad School 19 / 21

26 Green flash IUCAA-NCRA Grad School 20 / 21

27 Temperature/Pressure/Density in the lower atmosphere IUCAA-NCRA Grad School 21 / 21