SkyView V3.4.5: New Surveys and Projection

We’re announcing a test release of SkyView V3.4.5. The new version includes the DRAO 22 MHz survey of most of the sky. This is now the lowest energy survey in SkyView. See Rogers et al, 1999 for details. In addition to missing much of the southern hemisphere the area around a few of the brightest radio sources has been excised since the survey was primarily intended to study diffuse emission.

We’ve also updated the Planck survey data and added the ability to get various polarization measures for most of the Planck frequencies. For all but the two highest energy bands users can get the total intensity, I, the Stokes polarization parameters, Q and U, the total polarized intensity, PI, the fractional polarization, PI/I, and the position angle, PA, of the polarization. Note that the position angle is currently with respect to Galactic coordinates even if you sample the survey in equatorial coordinates — we’re working on updates that will transform the angles appropriately. Future versions of SkyView should have new ways to display these data, e.g., to show positional angle overlays over other maps.

Finally, we also added the Mollweide (Mol) projection as one of SkyView‘s supported projections. This is a fixed projection similar to the Hammer-Aitoff projection in that it is an equal area projection that projects the entire sky into an ellipse. However, unlike the Aitoff, lines of constant latitude are projected into horizontal lines in the projection. Apparently this makes it popular in the CMB community.

Update: In testing out all of the new Planck image planes, we finally had to confront the known issue of the slow generation of Planck images — we’d mentioned this when we first added Planck data. Our test images are usually just a small fraction of the sky, but because of the way Planck data are stored, a single HEALPix file, SkyView was reading in the entire image. We’ve added a new feature to this release where a HEALPix image can be split into tiles and only the tiles that are needed for a given request will be read. We’re currently splitting the HEALPix image into 48 tiles. For a typical small image the new approach can be 10-20 times faster than reading the entire survey.

At least from the perspective of our software this is a bit different than the tiling used in many of our other surveys. We usually have separate FITS files covering regions typically with some overlap between the images. Internally in SkyView each of these files is treated as a separate Image object. SkyView mosaics these images together. In this new feature, there is only a single Image covering the entire sky but the data from that image is segmented and we read a segment only at the first request for data from it.

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