For some while we have been looking at how to put the data from the Wilkinson Microwave Anistropy Probe (WMAP) into SkyView. The WMAP data are central to our current understanding of cosmology. However WMAP images are generally stored in a the HEALPix format. HEALPix is a recursive projection where each pixel has equal area, and pixels are arranged in rings of constant latitude. This is illustrated in the image below
Pixels are successively refined with each pixel dividing into four subpixels. The pixels are normally stored as a linear array of pixels using one of two standard orders — not as an image.
Since HEALPix didn’t seem to be a normal projection, we’d held off supporting it. The original Gorski paper does suggest that you can represent the HEALPix data in a projection plane. However the HEALPix pixels are diamonds, not rectangles — and that doesn’t fit in with how we store images.
Recently we realized that by thinking just slightly outside the box, we can get around this and treat HEALPix as a standard projection. Although the HEALPix pixels normally look like:
in the plane where the equator runs horizontally, we’re not restricted to that. If we use a projection plane rotated by 45 degrees then those same HEALPix pixels look like
In this frame the pixels are perfectly normal, so SkyView can treat HEALPix just like any other projection. SkyView doesn’t care that the equator isn’t horizontal!
In practice we still do some special handling of the HEALPix data. To use our existing code we would have to reformat HEALPix-formatted FITS files as more standard images in the rotated HEALPix frame. Instead we choose to use the original format and use a new HealPixImage class. This makes a HEALPix file look like a regular image to the rest of SkyView. We’re in the process of testing the WMAP data and the HEALPix projection but they should be showing up in the released version of SkyView shortly.
[Added:] A paper by Mark Calabretta and B.F. Roukema discusses this in much more detail and in a more general context of HEALPix like projections. I’ve read this paper in the past, so it’s not unlikely that it’s ultimately where the I got the idea as well.