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Using the HEALPix Fortran 90 facilities
Default File Names and Directories: for some applications, the HEALPix facilities
require some precalculated input files describing the pixel window
function and quadrature ring weight (shipped as Healpix/data/pixel_window_n????.fits and Healpix/data/weight_ring_n?????.fits respectively).
By default, files with the very same name will be looked for into: the current directory (.), the parent directory (..),
./data, ../data, $HEALPIX and $HEALPIX/data where $HEALPIX is
a system variable defined as the full path to the HEALPix package
(see the installation documentation).
The user has the possibility to change both the name of those files
and their location.
Double/Single Precision mode: several facilities offer the option of switching at run time
the precision of the internal variables and arrays and of the I/O data from single to double precision
floating point reals. The following points should be noted: 
Facilities running in double precision mode can read indifferently single and double
precision data files (and the same is true for single precision facilities). On
the other hand, a double (resp. single) precision facility will only output double
(single) precision files.
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Since the internal calculations sensitive to numerical round-off error
(like the spherical harmonics recurrence) are
always performed in double precision, switching to double precision mode
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will have a limited impact on the output accuracy if the input file contains only
single precision data,
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is recommended if the input file contains double precision data, and the precision of the output is critical
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will not alter the execution speed, but
it will almost double the memory consumption of the facility,
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will obviously double the size of the output file(s).
Eric Hivon
2010-06-18
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