3.1 Overview

This chapter addresses the general questions that arise when observers choose between proposing to use WFC3 or one or more of the other imaging instruments that are available on HST. Following SM4, the observatory is at its highest level of capability: new instruments have been installed, and both STIS and ACS/WFC have been repaired.

In choosing between instruments for their particular projects, observers should carefully evaluate the capabilities of WFC3 and compare them to those of the other HST instruments, in the context of their own scientific goals. There is some intentional redundancy between WFC3, ACS, STIS, and COS to provide a degree of protection against degradation or failure of the instruments. However, the instruments largely complement one another, and therefore it is likely that one instrument will be preferred for any science program. Therefore, observers do need to give careful consideration to instrument capabilities in order to optimize their observations. They should refer to the HST Call for Proposals for any policy issues, and the HST Primer for additional comparison information.