NASA Sensors and Instruments Technology

Micro-Precision Interferometer (MPI) Testbed

First Light

The Micro-Precision Interferometer (MPI) Testbed is a full scale (7m x 6m x 6m) ground model of a future flight interferometer. It is intended to provide high fidelity emulation of on orbit interferometer behavior, especially as to the transmission of flight like mechanical disturbances through the lightweight truss structure to sensitive optical elements. The purpose of the MPI Testbed is to demonstrate three fundamental capabilities which, taken together, provide high confidence in interferometer technology flight readiness:

1. that stabilization of optical elements to nanometer and nanoradian levels in the presence of expected on orbit disturbances is feasible;
2. that dynamical response of the system at sub-micron levels can be accurately modeled and simulated, thus validating analytical tools for flight application; and
3. that the various mechanical, optical, and control subsystems that comprise a spacebased optical interferometer "play" together in an integrated and stable fashion.

The build phase of the MPI Testbed was completed this past June. The CSI team has now achieved first operating capability. On 26 August 1994 the delicate process of quieting the testbed, acquiring the laboratory pseudo-star, and proceeding to closed-loop fine tracking was accomplished. This initial effort saw a painstaking day of " tweaking" the extremely sensitive testbed culminate in two brief five minute periods of stellar fringe tracking - 57 nanometers RMS fringe stability was achieved in the lab ambient disturbance environment.

MPI " first light" comes almost exactly three years after the start of the testbed build (it was delayed approximately one year by funding shorfalls). Although a push button acquisition and track capability is not yet in place, an *existence proof* of end-to-end interferometer operation on a free-free flexible structure representative of a first generation space system is at hand.

Planned next:

• achieve "turnkey" operation
• establish sub 10 nanometers noise floor
• introduce representative spacecraft disturbances
• complete testbed analytical model
• develop test plan to exercise spectrum of operating conditions
• perform tests & acquire data
• validate (we hope) analytical model
• learn, reassess, target open issues, resume testing

All this is planned for FY '95. After years of planning, designing, and building we should see, over the next 12 months, a tremendous increase in our understanding of how spacebased optical interferometry will be done.

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