STIS ETC Update
The STIS Exposure Time Calculators have been re-engineered, and they were made available on 4 August 1999 to help GOs prepare their Phase 1 proposals for Cycle 9. These ETCs offer a number of new features, which are described in the on-line help documentation. Briefly the new features are:
- The Spectroscopic ETC now supports the addition of up to 3 user-defined emission lines to the input spectrum. Narrow emission lines, when added on the Web form, do not suffer from the undersampling problems that are not otherwise properly handled if the feature widths are comparable to or narrower than the spectral resolution. As well, the profiles of added emission lines will exhibit a more reasonable behaviour (i.e., flatten out) when widening the slit on extended targets.
- The support for emission lines permits a more correct treatment of the 3 strongest Geo-coronal lines (although the behavior for slitless mode is still not modelled correctly when the slit width is large (>2 arcsec) as explained in the help-pages.
- To better model pure emission sources in the Spectroscopic ETC, a "No Continuum" option has been added to the input form. This must be used only when emission lines are explicitly added, and the reference wavelength (for the S/N calculation) has been set to one of the added emission features (otherwise you will be presented with an error message).
- Better error trapping & reporting. The new ETCs are much better equipped to detect error conditions (such as invalid combinations of parameters) and report the problem to the user on the output Web page.
- The Imaging and Target-Acquisition ETCs now compute more accurately the contributions to the background from the detector dark rate, readnoise, and the sky background. This contribution had previously assumed a constant size (per detector) for the aperture that encloses 80% of the flux from a point-source. The new ETCs compute the size of the aperture on the fly, based upon measurements of PSFs obtained from in-flight data.
- All the ETCs have been updated with the latest values for the detector gain, readnoise, and dark current values.
- The new input forms offer a consistent look & feel with the rest of the STIS Web site. This includes the navigation links to the ETC help documentation and the ETC FAQ lists.
The new ETCs have been quite extensively tested, and the results are known to agree well with the ETCs used for Cycle 8. Users who have been preparing their Cycle 9 proposals with the old ETC do not have to recalculate in general, except when the source is an extended source with narrow emission lines, or in the low S/N cases where the no. of pixels used in the PSF has a significant contribution on the S/N calculations. Please note that the old (Cycle-8) ETCs have been disabled for public use, and are no longer supported.