IMAGING WITH STIS ON THE HUBBLE-SPACE-TELESCOPE - ASTRONOMY AT V=30

In 1997 February, the second Space Shuttle servicing mission to the Hu bble Observatory installed the Space Telescope Imaging Spectrograph (S TIS). This new instrument greatly enhances the spectroscopic capabilit ies of the Hubble Space Telescope by providing a long-slit format and CCD detector technology. STIS can also be used as an imager, providing an alternative to the Wide Field Planetary Camera 2. The optical filt er set of STIS is limited and does not contain standard bandpasses, bu t we show here that this does not preclude useful two-color broadband photometry. Because the STIS bandpasses reach similar to 1.5 mag deepe r than WFPC2, the STIS photometric system may be preferable for many a pplications where a faint limiting magnitude and fine spatial resoluti on are overriding considerations. The two optical wideband choices on STIS are a clear aperture and a longpass (lambda > 5500 Angstrom) filt er. We define an effective shortpass filter from the difference of the se, making two-color photometry possible with STlS. We present prelimi nary transformations between the STIS system and Kron-Cousins BVRI ban dpasses, showing that these transformations are very well behaved over almost all temperatures, luminosities, and abundances for normal star s. In an 8-orbit cycle, STIS will be able to reach signal-to-noise of similar to 5-10 at V = 30.0 in its clear and longpass imaging modes, a significant increase in the power of HST to address a number of funda mental issues out of reach of current instrumentation capabilities on the ground or in space.