Midterm Review. AST443 Stanimir Metchev

Transcription

1 Midterm Review AST443 Stanimir Metchev

2 Administrative Homework 2: problems 4.4, 5.1, 5.3, 5.4 of W&J due date extended until Friday, Oct 23 drop off in my office before 6pm Midterm: Monday, Oct 26 8:20 9:20pm in ESS 450 closed book no cheat sheet no need to memorize obscure constants or formulae 2

3 Midterm Review Coordinates and time Detection of light: telescopes, detectors Radiation: specific intensity, flux density, etc optical depth, extinction, reddening differential refraction Magnitudes and photometry: apparent, absolute; distance modulus CMDs and CCDs photometry, PSF fitting Statistics testing correlations and hypotheses non-parametric vs. parametric tests one-tailed vs. two-tailed tests one-sample, two-sample tests 3

4 Celestial Coordinates horizon coordinates altitude/elevation (a), azimuth (A) zenith, zenith angle (z) observer s latitude angle PZ 4

5 Celestial Coordinates equatorial coordinates right ascension R.A., α declination DEC, δ cf., Earth s longitude, latitude meridian, hour angle (HA) 5

6 Celestial Coordinates ecliptic coordinates ecliptic longitude (λ) ecliptic latitude (β) vernal equinox (ϒ) Earth passes through ecliptic on Mar 20/21 origin of equatorial and ecliptic longitude Earth s axial tilt: ε = º a.k.a., obliquity of the ecliptic 6

7 Celestial Coordinates galactic coordinates galactic longitude (l; letter ell ) l = 0 approx. toward galactic center (GC) definition l NCP = 123º (B1950.0) galactic latitude (b) NGP definition (B1950.0) α = 12h 49m δ = 27.4º Sagittarius A l = 359º b = 0º

8 Coordinate Transformations equatorial ecliptic cos" cos# = cos$ cos% cos" sin# = cos$ sin%cos& ' sin$ sin& sin" = cos$ sin%sin& + sin$ cos& cos$ sin% = cos" sin# cos& + sin" sin& sin$ = sin" cos& ' cos" sin# sin& equatorial horizontal cosasin A = "cos# sinha cosacos A = sin# cos$ " cos# coshasin$ sina = sin# sin$ + cos# coshacos$ cos# sinha = "cosasin A sin# = sinasin$ + cosacos Acos$ φ observer s latitude 8

9 Equatorial Coordinate Systems FK4 precise positions and motions of 3522 stars adopted in 1976 B FK5 more accurate positions fainter stars J ICRS (International Celestial Reference System) extremely accurate (± 0.5 milli-arcsec) 250 extragalactic radio sources negligible proper motions J

10 Astronomical Time sidereal time determined w.r.t. stars local sidereal time (LST) R.A. of meridian HA of vernal equinox sidereal day: 23h 56m 4.1s object s hour angle HA = LST α 10

11 Astronomical Time sidereal time determined w.r.t. stars local sidereal time (LST) R.A. of meridian HA of vernal equinox sidereal day: 23h 56m 4.1s object s hour angle HA = LST α solar time solar day is 3 min 56 sec longer than sidereal day 11

12 Astronomical Time universal time UT0: determined from celestial objects corrected to duration of mean solar day HA of the mean Sun at Greenwich (a.k.a., GMT) UT1: corrected from UT0 for Earth s polar motion 1 day = s, but duration of 1 s is variable UTC: atomic timescale that approximates UT1 kept within 0.9 sec of UT1 with leap seconds international standard for civil time set to agree with UT1 in

13 Astronomical Time tropical year measured between successive passages of the Sun through the vernal equinox 1 yr = mean solar days mean sidereal year Earth: 50.3 /yr precession in direction opposite of solar motion days Julian calendar leap days every 4th year; 1 yr = days t 0 = noon on Jan 1st, 4713 BC Gregorian calendar no leap day in century years not divisible by 400 (e.g., 1900) 1 yr = days 13

14 Coordinate Epochs Coordinates are given at B or J epochs Besselian years (on Gregorian calendar; tropical years) Julian years (Julian calendar) Gregorian calendar is irregular complex for precise measurements over long time periods Julian epoch: Julian date: JD = 0 at noon on Jan 1, 4713 BC J = (JD ) / J defined at JD January 1, 2000, noon 14

15 Focusing focal length (f L ), focal plane object size (α, s) in the focal plane s = f L tan α f L α plate/pixel scale P = α/s = 1/f L Lick observatory 3m f L = 15.2m, P = 14 /mm 15

16 Energy and Focal Ratio Specific intensity: Planck law [erg s 1 cm 2 Hz 1 sterad 1 ] or [Jy sterad 1 ] Integrated apparent brightness E p (d / f L ) 2 : energy per unit detector area focal ratio: R f L / d fast (< f/3) vs. slow optics (>f/10) fast data collection vs. larger magnification magnification = f L / f camera I(",T) = 2h" 3 c 2 1 e h" kt #1 16

17 Optical Telescope Architectures Also: Schmidt-Cassegrain spherical primary (sph. aberration), corrector plate; cheap for large FOV no coma or astigmatism; severe field distortion Ritchey-Chrétien modified Cassegrain with hyperbolic primary and hyperbolic convex secondary no coma; but astigmatism, some field distortion 17

18 Imaging through a Turbulent Atmosphere: Seeing FWHM of seeing disk θ seeing <1.0 at a good site r 0 : Fried parameter θ seeing = 1.2 λ/r 0 r 0 λ 6/5 (cos z) 3/5 θ seeing λ 1/5 t 0 : coherence time t 0 = r 0 / v wind v wind ~ several m/s t 0 is tens of milli-sec 18

19 Basic Concept of a Semi-Conductor Detector electron-hole pair generation doping: n-type (electrons) p-type (holes) creates additional energy levels within band gap increases conductivity silicon (Si) band gap: E g = 1.12 ev cut-off wavelength λ c = 1.13µm " c = hc = 1.24µm E g E g (ev) free-electron energy: 4 ev (3000Å) 1 photon -> 1 electron 19

20 Basic Concept of a CCD Pixel: A P-N Photo Diode depleted region low conductivity can support an E field net positive charge (higher charge density near top) additional E-field applied subsequently generated electrons get trapped in potential well near top 20

21 Charge Transfer 21

22 Front- vs. Back-Illumination 22

23 Read Noise electrons / pix / read sources A/D conversion not perfectly repeatable spurious electrons from electronics (e.g., from amplifier heating) alleviated through cooling nowadays: <3 10 electrons 23

24 Dark Current electrons / pixel / second source thermal noise at non-zero detector temperature higher at room temperature (~10,000) at cryogenic temperatures LN2, 100 C e /pix/s 24

25 Detector Calibration bias frames non-zero bias voltage 0s integrations dark frames equal to science integrations flat field frames QE of detector pixels is non-uniform in 2-D QE is dependent on observing wavelength bad pixels 25

26 Sky and Telescope Calibration sky background images photometric calibration: airmass curve, filter transmission sky transmission spectrum of a star of a known spectral energy distribution astrometric calibration binary with a known orbit star-rich field with precisely known positions: HST observations of globular clusters point-spread function calibration nearby bright star (for on-axis calibration) star-rich field (2-d information on PSF) 26

27 Aperture Photometry object flux = total counts sky counts estimation of background N pix, bkg > 3 N pix, src use r bkg >> FWHM, whenever possible enclosed energy P(r) curve of growth optimum aperture radius r SNR(r) first increases, then decreases with r Fig. 5.7 of Howell dependent on PSF FWHM and source brightness 27

28 source: Kitt Peak National Observatory 28

29 Radiation specific intensity I ν de = I ν dt da dν dω [erg s 1 cm 2 Hz 1 sterad 1 ] or [Jy sterad 1 ] 1 Jy = erg s 1 cm 2 Hz 1 = W m 2 Hz 1 surface brightness of extended sources (independent of distance) spectral flux density S ν S ν = I ν dω [erg s 1 cm 2 Hz 1 ] or [Jy] or [W m 2 Hz 1 ] point sources, integrated light from extended sources flux density F F = S ν dν [erg s 1 cm 2 ] or [W m 2 ] power P P = F da = de / dt [erg s 1 ] or [W] received power: integrated over telescope area luminosity: integrated over area of star conversion to photon counts energy of N photons: Nhν 29

30 Blackbody Radiation (Lecture 4) Planck law specific intensity I(",T) = 2h" 3 c 2 1 e h" kt #1 Wien displacement law T λ max = 0.29 K cm Stefan-Boltzmann law F = σ T 4 energy flux density [erg s 1 cm 2 ] Stellar luminosity power [erg s 1 ] Inverse-square law L(r) = L * / r 2 " = 2# 5 k 4 15c 2 h 3 = 5.67 $10%5 erg cm 2 s 1 K 4 L * = 4"R 2 4 * #T eff 30

31 Blackbody Radiation (Lecture 4) T eff, Sun = 5777 K T λ max = 0.29 K cm 31

32 Magnitudes (Lecture 4) apparent magnitude: m = 2.5 lg F/F 0 m increases for fainter objects! m = 0 for Vega; m ~ 6 mag for faintest naked-eye stars faintest galaxies seen with Hubble: m 30 mag times fainter than faintest naked-eye stars dependent on observing wavelength m V, m B, m J, or simply V (550 nm), B (445 nm), J (1220 nm), etc bolometric magnitude (or luminosity): m bol (or L bol ) normalized over all wavelengths 32

33 Magnitudes and Colors magnitude differences: relative brightness color (Lecture 4) V 1 V 2 = 2.5 lg F V1 /F V2 m = 5 mag approx. equivalent to F 1 /F 2 = 100 B V = 2.5 (lg F B /F V lg F B,Vega /F V,Vega ) 33

34 Extinction and Optical Depth (Lecture 4) Light passing through a medium can be: transmitted, absorbed, scattered extinction at frequency ν over distance s dl ν (s) = κ ν ρ L ν ds = L dτ ν L ν = L ν,0 e τ = L ν,0 e κρs =L ν,0 e s/l A ν = 2.5 lg (F ν,0 /F ν ) = 2.5 lg(e)τ ν = 0.43τ ν mag medium opacity κ ν [cm 2 g 1 ], density ρ [g cm 3 ] optical depth τ ν = κ ν ρs [unitless] photon mean free path: l ν = (κ ν ρ) 1 = s/τ ν [cm] A V = m V m V,0 reddening between two frequencies (ν1, ν2) E ν1,ν2 = m ν1 m ν2 (m ν1 m ν2 ) 0 [mag] (m ν1 m ν2 ) 0 is the intrinsic color of the star 34

35 Interstellar Extinction Law extinction is highest at ~100 nm = 0.1 µm unimportant for >10 µm 35

36 Interstellar Extinction Law A V / E(B V) = 3.1 A V / E(J K) = 5.8 A V / E(V K) = 1.13 A λ / E(J K) = 2.4 λ 1.75 (0.9 < λ < 6µm) A V 0.6 r / (1000 ly) mag b < 2º (galactic latitude) A V 0.18 / sin b mag b > 10º N H / A V 1.8 x atoms cm 2 mag 1 atoms of neutral hydrogen (H I) 36

37 Atmospheric Extinction 37

38 Photometric Bands: Visible 38

39 Photometric Bands: Near- Infrared 39

40 Extinction and Reddening: CMD Legend: arrow: A V = 5 mag extinction solid line: main sequence dotted line: substellar models crosses: known brown dwarfs solid points: brown dwarf candidates A V = 5 mag Metchev et al. (2003) 40

41 Extinction and Reddening: CCD Legend: arrow: A V = 5 mag extinction solid line: main sequence + giants dotted line: substellar models crosses: known brown dwarfs solid points: brown dwarf candidates A V = 5 mag Metchev et al. (2003) 41

42 OBAFGKM + LT higher ionization potential species 42

43 Statistics: Basic Concepts Binomial, Poisson, Gaussian distributions calculating means and variances Central Limit Theorem: Let X 1, X 2, X 3,, X n be a sequence of n independent and identically distributed random variables each having finite expectation µ > 0 and variance σ 2 > 0. As n increases, the distribution of the sample average approaches the normal distribution with a mean µ and variance σ 2 / n irrespective of the shape of the original distribution. 43

44 p.d.f. of sum of 2 random variables sampled from p(x) (i.e., autoconvolution of p(x)) A bizarre p.d.f. p(x) with µ = 0, σ 2 = 1 p.d.f. of sum of 3 random variables sampled from p(x) p.d.f. of sum of 4 random variables sampled from p(x) source: wikipedia 44

45 Statistics: Basic Concepts probability density function (p.d.f.) density of probability at each point probability of a random variable falling within a given interval is the integral over the interval 45

46 Hubble (1929) 46

47 Student s t Distribution k = d.o.f. source: wikipedia 47

48 F Distribution d1, d2 = d.o.f. source: wikipedia 48

49 χ 2 Distribution k = d.o.f. f (χ 2,k) χ 2 source: wikipedia 49

50 χ 2 Distribution probability that measured χ 2 or higher occurs by chance under H 0 source: wikipedia 50