Normalizing the Source Flux

Regardless of whether you use your own input spectrum, a standard spectrum or a modeled spectrum you may wish to renormalize the source's brightness. Sources can be normalized either to a specified magnitude within a given band or to a specified flux density (erg cm^-2 s^-1 Å^-1) value at a given wavelength. When normalizing to flux at a given wavelength, it is necessary to choose a wavelength for which the source flux is non-zero, but it is not necessary to choose a wavelength within the pass band of the instrument's observation mode.

Total flux or Vega magnitudes are used for point sources. Flux or Vega magnitudes per square arc second are used for extended/diffuse sources and the brightness is considered to be uniform. For instruments that have global count rate limits (MAMA detectors, for example), the radius of the source in arc seconds should be specified. This allows the ETC to estimate the global count rate and to properly check the global count rate limit.

When using a user supplied or HST calibrated spectrum, it is not necessary to renormalize. Select the "Do not renormalize" option on the form, in which case the input file will be assumed to have the correct units (erg cm^-2 s^-1 Å^-1) for point sources and [erg cm^-2 sec^-1 Å^-1 arcsec^-2] for diffuse sources.

The bands supported by the APT ETC can be found below

Johnson: U, B, V, R, I, J, K

Bessell: H, J, K

Cousins: R, I

Sloan: G, R, I, Z

NICMOS: F110W, F160W