Frequency and related quantities

Freq info required/desirable at *Registry Resource metadata level or as UCDs - plus the relevant errors and uncertainties.

• Frequency metadata - *coverage
  • *Band - some examples, based on Hanisch et al.ResourceServiceMetadataV6 (.doc also available)
    • *Radio (nu <\= 3000 GHz) (e.g.)
      Sub-bands (also known as filters espec. in IR/Optical) (some examples, might want to set different boundaries)
      • 1 - 3 GHz (incl. L-band)
      • 3 - 7 GHz (incl. C-band)
      • 20 - 25 GHz (incl K-band)
    • *IR (e.g.)
      Sub-bands (some examples, might want to set different boundaries)
      • 2 - 2.5 um (incl (a different) K-band)
    • *optical (e.g.)
      Sub-bands (e.g.)
      • R-band (incl. COUS_R, JOHNSON_R ...)

• Frequency metadata - Units
  • The commonest units of measurement vary:
    • longer wavelength data in frequency (nu, in Hz),
    • IR/optical/UV data in wavelength (lambda, in m),
    • shorter wavelength data in energy (E, in eV)
  • These can in principle be converted (NB these values are approximate, use accurate ones e.g. from http://physics.nist.gov/cuu/Constants) by using
    • c = nu lambda
    • E = h nu
    and this level of accuracy is good enough for the Resource Registry - it is suggested this is all in *Hz. Note the non-linearity of some conversions and therefore of finding the central frequency from a wavelength range, propagating errors etc. However for evaluating queries needing high accuracy, the response of the detector and the source emission may vary across the bandpass, which can be very wide (e.g. 1 - 10 keV approx for some CHANDRA data), and/or the conversion may not be known accurately.

• Frequency metadata - *observational parameters
  • Observing bandwidth (within *Band or Sub-band) (max, min, nominal centre freq.)
  • Bandpass shape (NB some sub-bands e.g. Johnson I have implied bandpass shape? - or may be look-up table)
  • Continuum
    (Usually emission from a plasma (thermal or non-thermal) or dust/planets, in some circumstances can be approximated by a black body. Does not have sharp lines due to well-defined energy transitions in the emitting/absorbing medium, but can have dips, bumps and edges due to e.g. absorption by intervening clouds of cool dusty gas which preferentially scatter or absord a range of frequencies.)
    • Number of channels within band (as some continuum observations are made up of separate channels which may or may not be merged).
  • Spectral
    (Absorption or emission at well-defined frequencies due to quantum transitions between electronic, vibrational or rotational states. Can sometimes appear as broad as continuum features due to solid-state transitions or to superposition of gas transitions at many frequencies or velocities along the line of sight.)
    • *Spectral resolution
      • Continuum bands as above
      • Low resolution e.g. delta_nu/nu in range 10^-5 - 10^-4 - exact value of channel width and No. chans
      • High resolution e.g. delta_nu/nu < 10^-5 - exact value of channel width and No. chans

• Frequency metadata - derived quantities
  • Continuum properties (despite some names! - to avoid confusion these should not generally be referred to as spectral properties.)
    • Colour - ratio of continuum flux density in one sub-band to another, (often expressed as difference in magnitudes, e.g. B-V)
    • Colour correction - adjustment to flux density or colour based on assumptions about source spectral index and/or instrumental behaviour, may be function of both frequency and flux density.
    • SpectralIndex
    • SpectralEnergyDistribution
    • Extinction or reddening (e.g. A_v) - in IR, optical or UV, interstellar material (dust) scatters radiation, higher frequencies in this range are most affected. For objects at a known distance seen through a known environment the flux density can be corrected for extinction; conversely it can be used to estimate distance. This also affects spectral lines.
    • Line ratio - ratio(s) of pair(s) lines, can give information on excitation conditions, abundance ratios etc.
  • *Spectral line properties
    • *Doppler Velocity
      • Conventions
        • Reference point incl.
          • LSR (Local standard of rest - kinematic usually) - probably should be default
          • Heliocentric (Sun-centred)
          • Barycentric (Earth-centred)
      • Velocity Calculation conventions (see Greisen et al 2002 Representations of spectral coordinates in FITS)
        • Relativistic
        • Frequency-based
        • Wavelength-based (diverges at high redshift)
    • Physical conditions
      • Line ratio - ratio(s) of pair(s) lines, can give information on excitation conditions, abundance ratios etc.
      • First moment or difference between observed and rest frequency - gives Doppler velocity of emitting/absorbing gas relative to observer
      • Second moment or velocity dispersion or line width - gives range of velocities within emitting/absorbing gas
      • Line shape - can be thermal (Gaussian), pressure-broadened (Lorenzian) etc.
      • Line structure - especially (sub)-mm - superposition of e.g. rotational fine structure on vibrational transition.

-- AnitaRichards - 23 Apr 2003