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Session 9 - SOFIA and IR Instrumentation.
Display session, Wednesday, January 07
Exhibit Hall,
We present design considerations for a new mid-IR (5-25\mum) imaging polarimeter, TNTCAM II. Built around a 256x256 Si:As BIB array from Boeing, as an imager the camera will be unparalelled by any instrument currently in use at these wavelengths. Access to this instrument is planned as part of the funding agreement under the NSF Major Research Infrastructure grant supporting its development.
This camera can contribute to the understanding of YSOs and evolved stars, obtaining high resolution mid-IR observations of dusty environments immediately surrounding these objects. In ordinary imaging mode mosaics of extended objects can be made in 2x2 arcmin intervals. In polarimetry mode, assuming adequate grain alignment timescales, magnetic fields in YSOs can be probed by dust emission from hot cores. The camera can better constrain grain alignment scenarios in young stellar environments.
Emission (rather than scattering) dominates signal at mid-IR wavelengths, allowing determination of grain alignment in a source. This enables distinction between models explaining near-IR polarization seen in dust shells surrounding late red giants, i.e. scattering from asymmetric shells or aligned grains.
There are no mid-IR array polarimeters in operation. In polarimetry mode, TNTCAM II will be sensitive to linear polarizations as small as 0.2%. We have chosen the simple approach of imaging one state at a time and modulating polarization at a frequency high enough to remove atmospheric and system noise fluctuations. Dewar design and the optical system are discussed, including the pros and cons of rotating waveplates or an Abbe-Konig "K"-mirror to modulate the polarization, and the use of a rotating window assembly allowing on-the-fly f-ratio adjustment and observation across the entire 5-25\mum band. We acknowledge support under NSF grant AST-9724506 to the University of Denver.
