Radiation & Photometry AS4100 Astrofisika Pengamatan Prodi Astronomi 2007/2008 B. Dermawan

Radiometry & Photometry In astronomy the term photometry covers the quite general meaning of radiometry. Noting that most of the quantities to be defined refer to some specified wavelength or frequency. Hence, the correct term would be spectrophotometry

Definitions Etendue (throughput) Transported power Monochromatic flux Reduced brightness Luminosity Polarisation Magnitudes Color indices Standard Photometry Bradt 2004

Photometry Through the Atmosphere Extinction correction must be applied even at wavelengths for which the atmosphere is almost transparent The transmitted frequency Difficult for infrared due to the considerable lack of homogeneity in the space and time distribution of water vapor In order to calculate an emitted intensity, correction for interstellar extinction is required

Calibration and Intensity Standards Determining absolute values of astrophysical quantities: 1. Fundamental because the consistency of the physical models depends on these values 2. Difficult because astronomical instruments are complex machines, observing in conditions which are often hard to reproduce

Calibration and Intensity Standards Calibration categories Energy: to measure a specific intensity (a function of angular position at a given frequency) Spectral: to establish the absolute frequency of observed spectral lines Angular: to find the absolute angular positions of a set of reference sources (astrometry) Time: to time the variations of a source (chronometry)

Calibration and Intensity Standards Radiofrequencies ( > 1 mm) Blackbody (Rayleigh-Jeans approx.): Method 1 (Earth’s surface as the blackbody source): Method 2 (linked to a noise source): Method 3: the voltage V(t) across a resistance fluctuates randomly (noise generator with  V(t)  = 0 ) Spectral calibration: direct comparison with the clocks or oscillators (electrical oscillation)

Calibration and Intensity Standards Submillimeter, infrared, and visible Absolute photometric calibration Temperature: pyrometry, lamps with ribbon filaments, uncertain in visible Emissivity  ( ) : depends on the geometry of the source and the type of material, uncertain in IR Beam geometry t( ,r, ) : geometric considerations dominate for < 5  m, there are sidelobes for mid IR and sub-mm (reduced by apodisation) Léna et al. 1996

Calibration and Intensity Standards Relative calibration Using stars or reference objects Visible: the star Vega Léna et al Near IR ( < 25  m): series of stars Far IR (20  m < < 1 mm): planetary radiation, asteroids Spectral calibration Visible: spectral lamps, lasers, atomic emission line of a gas near IR: atm. emission lines of radical OH

Calibration and Intensity Standards Ultraviolet and X-rays (0.1 nm < < 300 nm) Using thermal and synchrotron radiation sources Thermal sources Region nm: hot plasmas ( T  K), emitted intensity is very close to that of a black body if optical depth is larger than unity Region nm: thermal free-free emission of a plasma created in a pulsed discharge tube ( T = 1.5  10 6 K, N e = 2  cm -3 )

Calibration and Intensity Standards Non-thermal sources (synchrotron radiation) Emitted by electrons accelerated in a magnetic field Not only suitable in the UV and X-ray regions, but also excellent in the visible and near IR Léna et al. 1996

Calibration and Intensity Standards Gamma radiation Detectors always function in counting mode, and with two possible objectives: - an energy calibration of each photon (spectrometry), and - some measurement of the efficiency of detection (photometry) Low energies (< 2 MeV): radioactive samples High energies: particle accelerators Photometric: stable reference object of known spectrum Spectral: observing emission lines from radioactive de-excitation

Calibration and Intensity Standards Some examples of spectrophotometry The Sun Léna et al. 1996

Calibration and Intensity Standards Extragalactic radiofrequency sources Léna et al Cosmological background radiation WMAP-NASA