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.