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Polarimetry

 Cameras NIC1, and NIC2, each contain 3 polarizers, whose principal axes of transmission are separated by 120 degrees. Observations in all three polarizers will provide the Stokes parameters of linearly polarized light. The spectral coverage is fixed for each camera, and the polarizers cannot be crossed with other optical elements. For Camera 1, the polarizers cover the wavelength range 0.8 to 1.3 microns, and for Camera 2, 1.9 to 2.1 microns.

Instrumental Polarization

Since there are a number of internal reflections in NICMOS prior to reaching the polarizers there will be instrumental polarization, estimated to be 1 to 2 percent. It should, however, be quite stable. Measurements of polarized and unpolarized standard stars will be obtained as part of the Cycle 7 calibration program and these will be used to measure the instrumental polarization and the zero position angle for each polarizer (remember the actual zero point will depend on your spacecraft orient).

Theory

The raw polarimetric images obtained through the three polarizers will be routinely processed by the first stage of the pipeline like any other exposure. The resulting images will have been placed onto a common intensity scale by correcting for the relative transmissions of each of the polarizers. If we define the intensity and statistical uncertainties (including read-noise) obtained in the 3 polarizers after processing by the pipeline to be I0, I120 and I240 and 0, 120, 240 respectively, then we may obtain the total intensity I from:

and the Stokes parameters Q and U:

The statistical uncertainties are obtained by straightforward propagation of errors:

The Stokes parameters can then be combined to yield the polarized intensity, Ip:

and the degree, P, and position angle of polarization, , using:

Polarimetry Decision Chart

 The decision chart given in Figure 5.5 leads you through the selection process to construct a polarimetry observation.

Figure 5.5: Polarimetry Decision Chart

Polarimetric Sensitivity

As with the imaging filters, sensitivity information for the two sets of polarizers may be obtained from the Exposure Time Calculator. It gives the same information as was described in Chapter 4: namely a sensitivity curve, plotted as flux against time for a constant S/N ratio and an exposure exclusion curve for both point and extended sources. To use these, look up the integration time required for your source flux on the sensitivity curve for the signal to noise you want (see Chapter 12 if you need to convert the units). Then go to the associated exclusion curve and check that you are not in the shaded areas. If you are, adjust your integration time appropriately until you are in the clear area. If you are to the left of the vertical dashed line then you must use bright object mode. Work out how many integrations you need to get your desired S/N. To get the total exposure time required for a polarimetric observation multiply your final answer by 3 to account for the fact that you need to use 3 polarizers to get a measurement. Note that the transmission curves are for a 100% polarized source while all the sensitivity information here is calculated for a single polarizer image, assuming an unpolarized source.

The polarizers have yet to be extensively tested or calibrated -- this will be accomplished as part of the Cycle 7 calibration program. Current estimates are than single observations will have ~1% uncertainties implying 3-5% accuracy for polarization measurements.

In NIC1 the POL120 filter only has 48% transmission while the POL0 filter has 98%. Observers may wish to consider using POL0 at multiple spacecraft roll angles rather than POL120. Further, there is some indication of ghost images in the polarizers.

Further information on the performance of the polarization capability should become available in August 1997 and will be posted on the STScI NICMOS WWW page as soon as possible.

Camera 1, Polarizers

The polarizers consist of 3 identical elements at relative angles of 0, 120, and 240 degrees.

Central (microns)

Mean (microns)

Peak (microns)

FWHM (microns)

Range (microns)

Principal Tr percent

Pixel fraction
1.0450

 1.0384

 1.0245

 0.4750

 0.8-1.3

 77.60

 0.048

Figure 5.6: Throughput of Short Wavelength Polarizers

Camera 2, Polarizers

 The polarizers consist of 3 identical elements at relative angles of 0, 120, and 240 degrees. Thermal background is important.

Central (microns)

Mean (microns)

Peak (microns)

FWHM (microns)

Range (microns)

Principal Tr

Pixel Fraction
1.9943

 1.9946

 1.9100

 0.2025

 1.9-2.1

 96.67

 0.33

Figure 5.7: Throughput of Long Wavelength Polarizers


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