STScI Logo

Hubble Space Telescope
NICMOS Units Conversion Help

Nicmos Units Conversion Program HELP

The NICMOS Units Conversion Program is a tool for converting fluxes of astronomical sources from units which are widely used in Astronomy (e.g., magnitude, ergs/sec/cm^2/A, etc.) into Jansky (Jy), and vice versa.

The Jy, or, for the case of extended sources, Jy/arcsec^2, is the flux unit adopted in the NICMOS Handbook and used by the NICMOS Exposure Time Calculator. For general reference:

1 Jy = 1.0E-26 W/m^2/Hz = 1.0E-23 erg/s/cm^2/Hz

The details of the FORTRAN program which handles the unit conversion are explained in the Instrument Science Report NICMOS-014, written by Chris Skinner. The report is available under the Documentation section of the NICMOS WWW Page. Modifications to the FORTRAN program were added by Daniela Calzetti (02-July-1996 and 09-April-1997) to handle power-law spectra and AB mags.

Inputs

The user has to provide the following inputs on the click-on menu:

  1. Input Units: one of the many available options.
  2. Output Units: also one of the options.
  3. Input Flux or Magnitude Value: the input value of the flux or magnitude.
  4. Input Wavelength: the wavelength at which the input flux is given. If the Input Unit is a magnitude, the value of the wavelength does not need to be specified; the field can be given any value, but NOT left blank. However, if the Input Unit is AB magnitude, the input wavelength need to be specified.
  5. Output Wavelength: the wavelength at which the output flux is desired. PLEASE NOTE: IF THE INPUT AND OUTPUT WAVELENGTHS DIFFER BY MORE THAN 5%, THIS PROGRAM EXTRAPOLATES ASSUMING ONE OF THE THREE SPECTRAL SHAPES SUPPLIED BELOW. If the Output Unit is a magnitude, the value of the wavelength does not need to be specified; the field can be given any value, but NOT left blank. However, if the Input Unit is AB magnitude, the output wavelength need to be specified.
  6. Blackbody Spectrum: if the user wants to use a blackbody spectrum to extrapolate fluxes, then the Blackbody Temperature, in Kelvin, should be supplied, in addition to select the blackbody option.
  7. Power-Law Spectrum: if the user wants to use a power-law spectrum to extrapolate fluxes, then the Spectral Index should be supplied, in addition to select one of the two power-law options. NOTE! the convention for the FIRST power-law index is:
    Flux(Frequency) = A * Frequency(Spectral Index)
    and the convention for the SECOND power-law index is:
    Flux(Frequency) = B * Wavelength(Spectral Index)

AB Magnitude

The AB magnitude system is defined as (Oke, J.B. & Gunn, J. 1983, ApJ, 266, 713):

AB = -2.5 * log [Flux(frequency)] - 48.57

where the Flux(frequency) is in units of ergs/cm2/s/Hz.

Magnitude Zero-Points

Unit conversion between magnitude and fluxes, and vice versa, requires information on the magnitude zero-point value and on the bandpass central wavelength. The zero-point magnitudes used in this program are from the CIT system, or, in the case of the L1 band (commonly known as L' band), from the UKIRT system, and are listed below:

  1. magnitude U: zero-flux=1810 Jy; central wavelength=0.36 microns;
  2. magnitude B: zero-flux=4260 Jy; central wavelength=0.44 microns;
  3. magnitude V: zero-flux=3540 Jy; central wavelength=0.55 microns;
  4. magnitude R: zero-flux=2870 Jy; central wavelength=0.70 microns;
  5. magnitude I: zero-flux=2250 Jy; central wavelength=0.90 microns;
  6. magnitude J: zero-flux=1670 Jy; central wavelength=1.25 microns;
  7. magnitude H: zero-flux= 980 Jy; central wavelength=1.65 microns;
  8. magnitude K: zero-flux= 620 Jy; central wavelength=2.20 microns;
  9. magnitude L: zero-flux= 280 Jy; central wavelength=3.40 microns;
  10. magnitude L1: zero-flux= 252 Jy; central wavelength=3.74 microns;
  11. magnitude M: zero-flux= 150 Jy; central wavelength=4.80 microns;
  12. magnitude N: zero-flux= 37 Jy; central wavelength=10.10 microns;
  13. magnitude Q: zero-flux= 10 Jy; central wavelength=20.00 microns.