pixsample: Samples from a HEALPix pixel data file
pixsample samples data at the sky position
represented by each row from an all-sky map contained
in a HEALPix-format pixel data file.
Such files are actually tables (usually in FITS format)
in which the row number corresponds to
a HEALPix pixel index, and the pixel values are cell contents;
one or more columns may be present containing values for one or more
all-sky maps. The result of this command is to add a column to
the input table representing the pixel data at the position of
each input row for each of the data columns in the
This command does not attempt to convert between coordinate systems except as instructed, so it is important to know what coordinate system the HEALPix file is in, and ensure that the coordinates supplied from the input table match this. You may need to examine the documentation or headers of the HEALPix file in question to find out. See the Examples section for some examples.
There is a choice of how the sampling is done; the simplest way is just to use the value of the pixel covering the indicated position. An alternative is to average over a disc of given radius (perhaps a function of the input row). Other options (e.g. max/min) could easily be added.
Although HEALPix is not a common format for storing image data in general, it is used for storing a number of important all-sky data sets such as the WMAP results and Schlegel dust maps. The NASA LAMBDA (Legacy Archive for Microwave Background Data Analysis) archive has a number of maps in a suitable format, including foreground data like predicted reddening as well as CMB maps.