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Photometry & Virtual Observatory Gijs Verdoes Kleijn Kapteyn Institute, room 147 [email protected] 050-3638326 Concepts discussed • The light path • Photometric calibration – Standard systems – Calibration procedures • Photometric calibration & VO • In other words: physics of interaction over light path; calibration: quantifying interactions; sharing your photometry Goal: physics via Spectral Energy Distribution (SED) Stellar SEDs •What is required spectral resolution (?/d?) to get physics? •Example: temperature of blackbody can be obtained from relative intensity at two wavelengths •Spectral resolution ? ? Efficiency? •broad-band spectroscopy =photometry Jargon and conventions • • • • • Flux (e.g., erg/s/cm^2, W/m^2) Flux density (e.g., erg/s/cm^2/Hz or /Ang) m(agnitude)=-2.5log10(flux/flux0) m: Apparent magnitude M: Absolute Magnitude= apparent magnitude at 10pc • Color: e.g., blue-red (B-R) Example: stellar colors Hertzsprung Russell Diagram B star M star Hertzsprung -Russell diagram=life of star (gal=n_stars ) 1 Richards etal AJ, 123, 2945 Higher z QSOs QSOs A stars stars • Discovery: 1930s • Extinction=(Mie) scattering+absorption by dust particles • Net effect: reddening k(?)~1/? Atmosphere: obscures+shines counts • +Continuum+Line Emission Days from new moon Sky Brightness U B V R I z 0 22 .0 22 .7 21 .8 20 .9 19 .9 18 .8 3 21 .5 22 .4 21 .7 20 .8 19 .9 18 .8 7 19 .9 21 .6 21 .4 20 .6 19 .7 18 .6 10 18 .5 20 .7 20 .7 20 .3 19 .5 18 .3 14 17 .0 19 .5 20 .0 19 .9 19 .2 18 .1 http://webast.ast.obs-mip.fr/hyperz/hyperz_manual1/node10.html Galactic ISM: Interstellar extinction “Maltreatment” of photons filters Intergalactic Medium • Time/location-variability: Earth atmosphere, telescopes, filters, detectors. • How to compare results with this variability? http://www.sc.eso.org/~isaviane/photometry/Optical%20 Photometry_files/v3_document.htm http://www.journals.uchicago.edu/AJ/journal/issues/v123n6/ 201557/201557.html Example Quasar colors Atmosphere: obscures+shines • - Extinction by dust, aerosols, molecules Atmospheric extinction • Extinction per unit atmosphere is time/location dependent (haze, clouds, dust) • Proportional to airmass~1/cosz ~1/cosz (nm) 2 Telescope Filters Passbands,transmission curves • Mirrors • Lenses • Filter widths ? ?/? Subaru telescope primary mirror – Narrow <0.02 – Intermediate0.02-0.1 – Wide >0.1 • Filter materials: – Glass: red (IR) leaks – gelatin films – Interference Commonly used filter sets Detector effects: Quantum efficiency (nm) Detector effects: pixel to pixel variation quantum efficiency: flatfield Detector effects: fringing Fringing= variation in background light •Origin: Interference of nightsky lines within CCD •More pronounced in red part spectrum •Only affects “background” light 3 •due to internal scattering of light in instrument •Affects both source and background light “Maltreatment” of photons filters Intergalactic Medium • Time/location-variability: Earth atmosphere, telescopes, filters, detectors. • How to compare results with this variability? Solution: relative measurements • Measure relative to flux I0 of reference object: m-m0 = -2.5 log10 ( I/I0) – i.e., measure (I/I0) instead of I: constants cancel • Unitless system • m0 = -2.5 log 10 ( I0/I0) = 0 by definition • I0 proportional to flux, but can have arbitrary units: – m=-2.5log 10 (countrate ) +zeropoint Photometric standard systems • Goal: putting mags on common scale • Standard system= http://www.sc.eso.org/~isaviane/photometry/Optical%20 Photometry_files/v3_document.htm Detector effects: illumination variation What one observes • Effects of ism, atmosphere, telescope, filter and detector QE and flatfielding are multiplicative gains: – Iobs = I*g ISM(a,d) * g atm(k,z0) * g tel*gfilt1* g det1(x,y) – I0,obs = I0*g ISM(a 0,d 0) * g atm(k,z) * gtel1*gfilt1* g det1(x 0,y0) • Neglected fringing and illumination correction: discussed in werkcollege • For telescope2,filter2,detector2: – Iobs = I*g ISM(a,d)*g atm(k,z)*gtel2*gfilt2*gdet2(x,y) – I0,obs = I0*g ISM(a 0,d 0)*g atm(k,z0)*gtel2*gfilt2*gdet2(x 0,y0) Integrating up-link and down-link: Determining gains translate into procedurized observations Atmosphere – telescope+filter+detector • Natural system= – Your telescope+filter+detector Telescope+filter+QE • Convert your measurements as if observed with standard system • Example standard systems: – – – – Johnson-Cousins Sloan Stroemgren Walraven Flatfielding 4 from Design-> deliver •• Scientific Scientific requirements requirements -- SRD SRD –– Science Science goals goals (e.g., (e.g., determine determine temperature temperature of of stars stars out out to to 10kpc) 10kpc) •• User User requirements requirements -- URD URD –– Shalls: Shalls: what what photometric photometric accuracy accuracy is is needed needed for for science science Reflecting on design->deliver slides from previous lectures…. •• Architectural Architectural design design -- ADD ADD –– Designing Designing aa data data model model to to capture capture the the physics physics of of photometric photometric calibration calibration •• Detailed Detailed design design – – DDD DDD –– Working Working out out the the details details and and writing writing the the code code •• •• •• New approaches new balances Quantify Quantify Build Build Qualify Qualify – – unit unit tests tests 1- calibration plan integrated up-link /down link Anarchy ç àcoordinated Freedom ß à fixed system Standard Standard data data products products ç ç àuser àuser tuned tuned products products Data releases ß à user defined hunting DESIGN 5 Essential STEPS: 2 -Procedurizing 3 Data Model • Procedurizing – – Data Data taking taking at at telescope telescope for for both both science science and and calibration calibration data data -- Templates Templates •• Observing Observing Modes: Modes: — —Stare Stare — —Jitter Jitter — —Dither Dither — —SSO SSO Sanity Sanity checks checks •• Observing Observing Strategies: Strategies: — —Stan Stan — —Deep Deep — —Freq Freq — —Mosaic Mosaic – – Full Full integration integration with with data data reduction reduction – – DesignDesign- ADD ADD – – Data Data model model (classes) (classes) defined defined for for data data reduction reduction and and calibration calibration – – View View pipeline pipeline as as an an administrative administrative problem problem Quality Quality control control Image Image pipeline pipeline Calibration Calibration procedures procedures Source Source pipeline pipeline 5 4 Integrated archive and Large Data Volume •• Handling Handling of of the the data data is is non-trivial non-trivial – – Pipeline Pipeline data data reduction reduction – – Calibration Calibration with with very very limited limited resources resources – – Things Things change change in in time: time: –Physical –Physical changes changes (atmosphere, (atmosphere, various various gains) gains) –Code –Code (new (new methods, methods, bugs) bugs) –Human –Human insight insight in in changes changes – – Working Working with with source source lists lists Photometric calibration and the VO • Now you have your result and you want to share it…..=VO • Describing photometry universally: UCDs – Properties measurement: aperture….. – Value and error Science can only be archive based Photometric calibration & VO UCDs for photometry • • • • • • • • • • • • • • • • • • • • E | phot E | phot.antennaTemp Q | phot.calib C | phot.color Q | phot.color.excess Q | phot.color.reddFree E | phot.count E | phot.fluence E | phot.flux Q | phot.flux.bol E | phot.flux.density E | phot.flux.density.sb E | phot.flux.sb E | phot.limbDark E | phot.mag Q | phot.mag.bc Q | phot.mag.bol Q | phot.mag.distMod E | phot.mag.reddFree E | phot.mag.sb How to compare magnitudes of extended sources… | Photometry | Antenna temperature | Photometric calibration | Color index or magnitude difference | Color excess | Dereddened, reddening-free color | Flux expressed in counts | Fluence | Photon flux | Bolometric flux | Flux density (per wl/freq/energy interval) | Flux density surface brightness | Flux surface brightness | Limb-darkening coefficients | Photometric magnitude | Bolometric correction | Bolometric magnitude | Distance modulus | Dereddened magnitude | Surface brightness in magnitude units NED: http://nedwww.ipac.caltech.edu 6