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10.2.3 Polarization while Chopping
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For Cycle 4, HAWC+ polarization observations may only be performed using the C2N (NMC) observing mode (see Section 10.2.2 above). In this mode, four standard C2N (NMC) observations are taken, one at each of four angles of the HWP (relative angles 0, 22.5, 45, and 67.5 degrees). This is followed by dithering, where the HWP cycle is repeated again for a total of four dither positions. We currently estimate an additional overhead of 90% efficiency associated with moving the HWP between positions. This has been incorporated into polarization sensitivities in Figure 10-3. The minimum time for a single polarization C2N observation with dithering is ~20 minutes.
As in the case of TOTAL INTENSITY C2N, one must take care to avoid chopping into regions of bright, extended flux. Additionally one must consider the polarization state of that reference flux, in both percent polarization and angle. Typically, neither of these values will be known for HAWC+ observations (although users may want to consult the latest Planck data release). This "polarized reference" beam will produce additional systematic uncertainties in the data. In the case where the source and reference beam have the same polarization level, the systematic polarization uncertainty is linearly proportional to the reference-to-source intensity ratio. For further discussion, see Schleuning et al. (1997) and Novak et al. (1997).