Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 14

2.3 New versions of STIS Exposure Time Calculators

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As part of the Astronomer's Proposal Tool (APT) project, new Java-based versions of the exposure time calculators (ETCs) have been developed for all HST instruments. The switch to the APT ETCs was made in order to standardize the calculators for all the instruments, both to simplify development and support, and to provide a more consistent interface. Users will find that the new ETCs provide more realistic calculations and improved cross-instrument consistency.

The new STIS APT ETCs are available from our website at:

http://www.stsci.edu/hst/stis/software/etc

Differences and improvements with respect to CGI ETCs

Most results obtained from the latest versions of the APT ETCs should be close to those calculated by the older Common Gateway Interface (CGI) ETCs. There are, however, some notable differences and enhancements.

• The new sky background model of Giavalisco, Sahu, & Bohlin (2002), discussed in WFC3 ISR 2002-12, has been implemented for all HST instruments.
• More realistic curves of encircled energy as a function of wavelength have been adopted. This is especially important for the CCD at IR wavelengths, where in-chip scattering puts a large fraction of the detected light in an extended halo.
• Calculations for extended and emission-line sources are significantly more accurate, and relax a number of approximations made in older versions.
• The full complement of supported apertures for each configuration is now included.
• Imaging peakup calculations for point-sources are now included in the Target Acquisition ETC.
• Bruzual synthetic spectra are offered in addition to Kurucz models as stellar templates.
• A selectable extraction radius for imaging of point-sources has been implemented.
• Rectangular (in addition to square) CCD binning is now available.
• Additional background specification options are offered.
• Total counts are now given for resolution elements rather than lambda-pixels.
• The extended source lambda-pixel is now 2 pixels high.
• Many additional parameters, particularly for backgrounds and noise, are listed in the APT output page. The time to saturation for CCD images is now also included.

Significant changes since the Phase I version of the APT Spectroscopic ETC

A preliminary version of the new Java-based APT ETCs was available for GO use during the Phase I of Cycle 13. However, a warning was also given indicating that this version had not been exhaustively tested. Indeed, subsequent testing did turn up a few issues with the Spectroscopic ETC which have since been corrected.

• ETC calculations for extended sources are now correct. Results with the earlier APT version may have underestimated extended source count rates by as much as a factor of three when observations are done using narrow slits.
• An error in the reported count rates for G230LB and G230MB modes has been fixed. Note that this error did not affect the reported S/N ratio.
• The results for the full output table are now given per resolution element, rather than per lambda-pixel.
• The extraction box heights used for calculating dark and sky counts for point-sources have been changed back to the smaller values (3 or 5 pixels) that they had in the old CGI ETC. The earlier APT release had changed the extraction box sizes to the larger values used in the calstis pipeline. However, when reducing observational data where the S/N is limited by the dark or sky counts, the user can easily exclude this extra background by recalibrating the data with a smaller-than-default extraction box. Adopting the smaller box sizes gives a more realistic estimate of the exposure time required to achieve a given S/N ratio in these cases. In the near future we plan to implement an option to allow the user to specify the extraction box size in the ETC.

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