NIMPOL: An imaging polarimeter for the mid-infrared.

Craig H. Smith, Toby J.T. Moore, David K. Aitken, Takuya Fujiyoshi, PASA, 14 (2), in press.

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At mid-infrared wavelengths, information about magnetic fields can be obtained from polarimetric observations, particularly in regions like the Galactic Centre (e.g. Smith, Aitken & Roche, 1988: Aitken et al., 1991), Orion (e.g. Aitken, Smith & Roche, 1989: Gonatas et al., 1990) and other star forming regions (e.g. Aitken, Smith, Roche & Wright, 1990: Aitken, Wright, Smith & Roche, 1993). Magnetic fields are thought to play a significant dynamical role in the process of star formation, and mid-infrared polarimetry is one of the few techniques which can determine the magnetic field structure in these heavily obscured sources. From the birth of stars in the interstellar medium to active galactic nuclei, magnetic fields often produce dynamically important and sometimes dominating effects. From previous studies that we have undertaken it has been found that the polarization (and hence magnetic fields) structures are complex, varying on extremely small spatial scales, and yet are present in large scale structures.

The best way to study these structures is with an imaging polarimeter, combining sub-arcsecond imaging/polarimetry with a wide field. We have constructed such an instrument, using a focal plane array (FPA) detector. The instrument was successfully commissioned at the 3.9m Anglo-Australian Telescope (AAT) in July 1993. Of course, all instruments need an acronym and we have named our instrument NIMPOL (for N-band IMaging POLarimeter). This is the first and only mid-infrared imaging polarimeter in the world.

NIMPOL is the first stage of our instrumentation development program, and we are currently constructing a long-slit spectrometer (MIRAS for Mid-IR Array Spectro-polarimeter), utilising the same detector, readout and control system. Currently the planned commissioning date for the long-slit spectrometer is late 1996.

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