NICMOS contains optics which enable polarimetric imaging with high
spatial resolution and high sensitivity to linearly polarized light from 0.8 to 2.1 microns. The filter wheels of NIC1 and NIC2 each contain three polarizing filters (sandwiched with band-pass filters) with unique polarizing efficiencies and position angle offsets. The design specified that the position angle of the primary axis of each polarizer (as projected onto the detector) be offset by 120o
from its neighbor, and that the polarizers have identical efficiencies. While this clean concept was not strictly achieved in NICMOS, the reduction techniques described in the HST Data Handbook permit accurate polarimetry using both cameras over their full fields of view. A description on NICMOS polarimetry can also be found in Hines, Schmidt, and Schneider (2000, PASP, 112,983).
The spectral coverage is fixed for each camera, and the polarizers
cannot be crossed with other optical elements. For NIC1 the polarizers cover the wavelength range 0.8 to 1.3 microns (short wavelength), and for NIC2 the coverage is 1.9 to 2.1 microns (long wavelength). Observations in all three polarizers will provide the mechanism for calculating the degree of polarization and position angle at each pixel. To properly reduce polarimetry data obtained with NICMOS, a new algorithm different from that needed for ideal polarizers has been developed (Hines, D.C. et al. The Polarimetric Capabilities of the NICMOS, in the 1997 HST Calibration Workshop with a New Generation of Instruments, ed. Casertano et al, 1997; Sparks, W.B. et al, 1999, PASP, 111, 1298).
Combined with calibration measurements of polarized and unpolarized stars, this algorithm enables accurate imaging polarimetry to roughly 1% (in percentage polarization) over the entire field of view in both cameras (Mazzuca, NIMOCS ISR-98-017
; Mazzuca, NICMOS ISR-99-004
; Batcheldor et al. 2009, PASP, 121, 153).
The three polarizers in NIC1 are called POL0S, POL120S and
POL240S, and in NIC2 are called POL0L, POL120L, and POL240L, where the suffix 0, 120 and 240 indicates the design specifications for the position angle of the polarizer’s primary axis (in degrees). A summary of the characteristics of the NIC1 and NIC2 polarizers are given in Table 5.1
below. The final column lists Pixel fraction which is the fraction of total energy of the PSF contained in one pixel, assuming the source to be centered on that pixel.
In each Camera the three polarizers were designed to be identical and to
have the position angle of the primary axis of each polarizer offset by 120o
from its neighbor. In practice, this was not completely achieved and:
below lists for each polarizer the position angle of the primary axis and the filter efficiency (throughput of the filter only).
As with the imaging filters, sensitivity plots for the two sets of
polarizers for both extended and point sources are shown in , which also contains throughput curves (convolved with the HST and NICMOS optics and the detector’s response) for the polarizers. To work out how many integrations are needed to get the desired S/N, the observer can use the Exposure Time Calculator available on the WWW (see Chapter 1
or Chapter 9
). To get the total exposure time required for a polarimetric observation the final answer must be multiplied by three to account for the fact that all three polarizers must be used to get a measurement.
For the long wavelength polarizers in NIC2, thermal background must
be considered (see Chapter 4
for a description of the thermal background seen by NICMOS and for related observing strategies).
For a polarized source, the intensity measured by the detector depends
on the orientation of the spacecraft relative to the source in the sky. The range of intensities is given by the Exposure Time Calculator value multiplied by
, where p
is the fractional polarization of the source and εk
is the polarizer efficiency.
Multiple ghost images are present in NIC1 and NIC2 polarimetry data,
though the NIC2 ghosts are much fainter than in the NIC1. The location of ghosts in each polarizer appears constant on the detector relative to the position of the target (i.e. independent of telescope or object orientation). For example, the NIC1 ghosts are offset between POL0S and POL240S, which produces a very highly polarized signal (100%) in percentage polarization. This allows them to be easily distinguished from real polarized signal. While all emission in the POL0S and POL240S frames will produce ghosts, experience with real data shows that the effect is most important for strong point sources.
shows an example of the ghosts in NIC1 POL0S, POL240S, and the percentage of polarization. These ghosts will typically be seen as regions of 100% polarization (seen as white blobs)
Observers should always use a dither pattern to help alleviate residual
image artifacts, cosmic rays, and image persistence, as well as to improve sampling. The best choice for the number and size of the dithers depends on the amount of time available and the goals of the project, but a minimum of four positions will allow optimal sampling and median filtering.
One strong recommendation is to execute a four position pattern
separately for each polarizing filter with N+1/2 pixel offsets, where N ≥
10–50 depending on the structure of the object and the field of view that the observer wants to maintain. N=10 alleviates most persistence problems from point sources, and the additional 1/2 pixel ensures good sampling. The reason for executing a pattern separately for each polarizer is to remove latent images. By the time the pattern completes and starts for the next polarizer, the latent image from the previous polarizer is essentially gone. The same observing process should be applied to each polarizer observation (e.g. POL120L and POL240L). This strategy will result in a minimum of 12 images with which to construct the linear Stokes parameters (I
). For more information on dithering, see .
Exposure times should be set such that the source does not drive the
arrays into saturation, and only one exposure should be attempted per dither position because the long decay time for persistence. If more integration time is needed to achieve the desired S/N, the entire dither pattern for each polarizer should be repeated. For the best results, the observing sequence should be POL0*, POL120*, POL240*, then repeat POL0*, POL120*, POL240*, etc.
The raw polarimetric images obtained through each polarizer are
routinely processed by the first stage of the pipeline like any other exposure.
Because the errors for percentage polarization follow a Rice
distribution, precise polarimetry requires measurements such that
, where p
is the percentage polarization and σp
its standard deviation (Simmons and Stewart, 1985, A&A, 142, 100.). Therefore, uncertainties 0.5–3% (per pixel) imply that objects should have minimum polarizations of at least 2–12% per pixel.
Binning the Stokes parameters before forming the percentage
and the position angles reduces the uncertainties by ~
, where N
is the number of pixels in the bin. Uncertainties as low as 0.2% in NIC2 should be achievable with bright objects.
The non-optimum polarizer orientations and efficiencies cause the
uncertainty in polarization to be a function of the position angle of the electric vector of the incoming light. For observations with low signal-to-noise ratios (per polarizer image), and targets with lower polarizations, the difference between the signals in the images from the three polarizers becomes dominated by (photon) noise rather than analyzed polarization signal. Therefore, observations that place important incoming electric vectors at approximately 45°
in the NICMOS aperture reference frame should be avoided in NIC1. No such restriction is necessary for NIC2.
shows the fractional signal measured in each NICMOS polarizer as a function of incident electric position angle (PA) for 20% polarized light. The lower curves are the differences in fractional signal between images taken with successive polarizers. The vertical dashed lines in the left panel (NIC1) represent the position angles of the incoming electric vector where these differences are all small, and thus produce the largest uncertainties in the measured polarization.
The decision chart given in Figure 5.8
below helps guide the proposer through the selection process to construct a polarimetry observation.