Hubble Space Telescope
New APT ETC Features and Improvements
(for ETC version delivered on November 25, 2003)

1. For point sources, the calculations can be done at any user-selected circular aperture size. The apertures can also be defined in terms of percentage of encircled energy.
2. For point sources, the ETC also gives an estimate of the "optimal S/N" (i.e. PSF-fitting method).
3. A new thermal model has been implemented.
4. The Backgrounds and extinction laws have been homogenized with those used by the ETCs of other HST instruments.
5. It offers more choices for the spectral energy distributions, including the possibility to add emission lines.
6. It offers more choices for the backgrounds, with the possibility of normalizing zodiacal and earth-shine fluxes independently at any desired value.
7. The Backgrounds and extinction laws have been homogenized with those used by the ETCs of other HST instruments.
8. The calculations are more accurate, because the fraction of encircled energy is taken at the effective wavelength (instead of the pivot wavelength as in CGI version).

Further information can be obtained from the help link connected to the APT ETC user interface.