SkyView v3.5.0: Compound surveys

We announced a test release of SkyView v3.4.5 a few weeks ago, but in trying out the new version we recognized that it did not do what we wanted. In particular we had added new surveys that were the Stokes parameters for Planck data. These included that base Q and U Stokes data as well as derived quantities like the total polarized intensity, the polarization fraction and the position angle of the polarization vector.

While the data were being extracted properly, the behavior of our data when it was smoothed was not really what we expected. Basically we were computing these derived quantities right away and smoothing the derived pixels. However this emphasizes the noise in the data. What we should have been doing is smoothing the Q and U (and possibly I) data and then calculating the appropriate functions on these smoothed images.

To enable just this behavior, we added a new kind of SkyView survey: a compound survey. Compound surveys include one or more base component surveys, and a formula for combining the bases to create the ultimate result. E.g., for the total polarization, the component surveys are the Q and U surveys, and these are combined by an element that calculates sqrt(q2+u2) at each point.

When we ask for the total polarization we first compute the Q and U images for the geometry the user asked for. This includes any smoothing, or any other processing that affects the pixel value. Then we compute the final image by applying the combining element). For this final image we calculate any grid, contour or other overlays.

This is a fairly substantive change so it’s taken us a while to test it out. We also decided it meant that we’d be making this 3.5.0 and abandoning our released version of 3.4.5. We hope to release this new version in the coming week.

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