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. After Servicing Mission 4 (SM4), the observatory was its highest level of capability: both the Space Telescope Imaging Spectrograph (STIS) and the Wide Field Channel of the Advanced Camera for Surveys (ACS/WFC) were repaired and new instruments, including WFC3, were installed. 

In choosing between instruments for 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 the Cosmic Origins Spectrograph (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 given 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, while the HST Primer contains additional comparison information, especially the Scientific Instruments Comparison section.