FITS output and plotting¶

outputs.HEALPixFormat writes a primary HDU followed by one or more binary table HDUs. The important metadata are:

Keyword	Meaning
`SOFTWARE`, `VERSION`	HERMES and its Git description
`PIXTYPE`	`HEALPIX`
`ORDERING`	`RING`
`COORDSYS`	Galactic coordinates (`G`)
`NSIDE`, `FIRSTPIX`, `LASTPIX`	map geometry
`BAD_DATA`	sentinel for masked or unavailable pixels
`OBJECT`	`FULLSKY` or `PARTIAL`
`PROCESS`	integrator description
`ENERGY` or `FREQ`	map parameter in SI base units
`TUNIT1`	physical unit of the value column

Safe overwriting¶

CFITSIO uses a leading ! to replace a file:

output = hermes.outputs.HEALPixFormat("!map.fits.gz")
skymap.save(output)

Use it only after an explicit overwrite decision. The example suite refuses to replace products unless --overwrite is passed.

Inspect a file with Astropy¶

from astropy.io import fits

with fits.open("map.fits.gz", memmap=False) as hdus:
    print(hdus[0].header["VERSION"])
    table = hdus[1]
    print(table.header["NSIDE"], table.header["ORDERING"])
    print(table.header["ENERGY"], table.header["TUNIT1"])
    values = table.data.field(0)

Validate headers before interpreting numbers. In particular, do not assume the energy unit, coordinate system, or pixel ordering from the filename.

Unit conversion for plots¶

HERMES quantities are stored in the units declared by TUNIT1. The maintained gamma-ray plotting examples convert the default differential intensity from per square metre to per square centimetre before displaying E² I(E); the FITS data themselves are not changed.

Avoid hard-coded plot labels. Derive them from the validated product metadata or perform and document an explicit conversion.