TPF interferometer architecture drivers and implementations

In this paper, the scientific drivers for the interferometer design are first briefly tabulated and explored, with a view to constraining the possible solution space. This is done by linking the experimental drivers to affected aspects of the instrumental design. In particular, the high angular resolution requirement (of order 50 mas) leads primarily to the well understood constraint on the minimum acceptable baseline length (of order 40 m), while the high stellar rejection requirement leads to an increase in the number of apertures, either directly, to provide high-order ing, or indirectly, through the need to provide a phase modulation capability. The requirement that large numbers of stars be observable leads primarily to the need for high sensitivity, i.e., large cryogenically-cooled telescope apertures (and to an increase in the angular resolution requirement). The desire to observe the nearest stars of all leads primarily to refinements in the overall configuration, such as the ability to provide shorter baseline lengths in some fashion. Of course, all of the requirements which call for more than two telescopes result in the need for multiple levels of beam combination, and so in increased beam combiner complexity as well. In exploring the numerous goals and derived instrumental requirements, the main conclusion here is that reasonable solutions exists in all areas necessary to effectively carry out an interferometric TPF mission. In particularly, maximum structure lengths remain within the reach of current and planned rocket fairing constraints, at least one of the beam combiners called for by the various pupil configurations under consideration is readily viable, and it is in fact possible to observe stars at a range of distances, including the nearest ones, with a suitable four telescope pupil configuration. It is thus possible to conclude that a three to four element interferometer with baseline lengths consistent with realistic spacecraft envelopes is, in fact, a viable approach for the TPF mission to pursue.