1. M4 1997 The Proposal Points to keep in mind: 1. M4 was to be built from scratch as a polarimeter. 2. Science was the primary driver for the project. 3. Science goals and technical capabilities had to agree.

2. Primary Science Goals What is the magnetic field structure in the ISM of the Milky Way? What role do magnetic fields play in the star formation process? What magnetic field structures exist in the infrared cirrus? What is the global magnetic field structure in M31?

11. To Move or Not to Move No moving parts is good. Rotating spacecraft near the Earth is a problem. One simple moving part is not impossible, but it is expensive (several million extra). Not rotating the spacecraft relieves the thermal shielding requirements and improves the observing efficiency somewat. Rotating a waveplate greatly improves data reduction.

13. What M4 Could Not Do Resolve the magnetic field geometry in dense cloud cores. Survey the entire galactic plane. Measure magnetic field strength. Work longward of 100 microns.

14. Complimentary with SOFIA SOFIA will have 12x the spatial resolution but it will be less sensitive than M4. SOFIA will have to ‘chop’, so it is limited to bright, compact regions, M4 is not. SOFIA will be unable to connect the MC core to the general ISM, M4 will. M4 will provide targets for SOFIA.

15. The Task Ahead Define the science. Confine the science goals by technical reality. Articulate the resulting science case as best as possible.