FanCam
Near Infrared Linear Polarimetry

How Does it Work?

FanCam has a pair of identical magnesium fluoride Wollaston prisms of 18mm clear aperture as polarization analyzers. Both Wollastons are mounted in Filter Wheel 2, which is located in the cold collimated beam after Filter Wheel 1 (bandpass filters) and just after the Lyot stop pupil.


Reference: "Optical Materials for near infrared Wollaston prisms"
Oliva, E., Gennari, S., Vanzi, L., Caruso, A., & Ciofini, M. 1997, A&AS, 123, 170

Commercial presentations on Wollaston prisms:

The prisms were paid for under the AAS Small Research Grant project "A Polarimetry Module for the Fan Mountain Near Infrared Camera," so papers based on FanCam polarimetry should include the following text in their acknowledgements:
This research was supported in part by NASA through the American Astronomical Society's Small Research Grant Program.

The beam displacement direction of prism P1 is nearly N-S, so for each point source in the telescope field of view it produces a pair of images separated by about 35 arcsec (about 70 pixels) in the N-S direction. One image of the pair is linearly polarized with electric vector N-S (position angle 0° in the equatorial system), and the other image is linearly polarized with electric vector E-W (90°). Prism P2 is oriented differently by 45°, producing a similar double image with electric vectors at position angles 45° and 135° when it is placed in the beam.

Filter Wheel 2 is manually controlled by turning a knurled brass knob near the edge of the camera back to positions which may be read from a mechanical decade counter to a precision of about ±0.2 index units, corresponding to about ±0.15°.

  • P1 position index for FW2: 376.8 (0-90°)
  • P2 position index for FW2: 301.2 (45-135°)

Each double image contains two orthogonally polarized components of the detected flux which may be used to calulate one of the two normalized linear Stokes parameters, q and u. The P1 frame gives q and the P2 frame gives u. The degree of polarization p and the position angle may then be calculated.

Since the two flux components necessary to determine each normalized Stokes parameter are recorded simultaneously on the same exposure, seeing and transparency variations affect both components equally and only degrade the measurement if the image quality or the number of detected photons is reduced. The quantities p and are linearly independent combinations of q and u, so good measurements are possible even when observing conditions are not perfectly photometric.

REFERENCES

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Last modified: April 10, 2006

David McDavid