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# Copyright (c) 2016 Weitian LI <liweitianux@live.com>
# MIT license
"""
FITS utilities
--------------
read_fits_healpix:
Read the HEALPix map from a FITS file or a BinTableHDU to 1D array
in *RING* ordering.
write_fits_healpix:
Write the HEALPix map to a FITS file with proper header as well
as the user-provided header.
"""
from datetime import datetime, timezone
import numpy as np
from astropy.io import fits
import healpy as hp
# Column formats for FITS binary table
# Reference:
# http://docs.astropy.org/en/stable/io/fits/usage/table.html#column-creation
FITS_COLUMN_FORMATS = {
np.dtype("bool"): "L",
np.dtype("uint8"): "B",
np.dtype("int16"): "I",
np.dtype("int32"): "J",
np.dtype("int64"): "K",
np.dtype("float32"): "E",
np.dtype("float64"): "D",
np.dtype("complex64"): "C",
np.dtype("complex128"): "M",
}
def read_fits_healpix(filename):
"""Read the HEALPix map from a FITS file or a BinTableHDU to 1D array
in *RING* ordering.
Parameters
----------
filename : str or `~astropy.io.fits.BinTableHDU`
Filename of the HEALPix FITS file,
or an `~astropy.io.fits.BinTableHDU` instance.
Returns
-------
data : 1D `~numpy.ndarray`
HEALPix data in *RING* ordering with same dtype as input
header : `~astropy.io.fits.Header`
Header of the input FITS file
NOTE
----
This function wraps on `healpy.read_map()`, but set the data type of
data array to its original value as in FITS file, as well as return
FITS header as `~astropy.io.fits.Header` instance.
"""
if isinstance(filename, fits.BinTableHDU):
hdu = filename
else:
# Read the first extended table
hdu = fits.open(filename)[1]
# Hack to ignore the dtype byteorder, use native endianness
dtype = np.dtype(hdu.data.field(0).dtype.type)
header = hdu.header
data = hp.read_map(hdu, nest=False, verbose=False)
return (data.astype(dtype), header)
def write_fits_healpix(filename, hpmap, header=None, clobber=False,
checksum=False):
"""Write the HEALPix map to a FITS file with proper header as well
as the user-provided header.
This function currently only support one style of HEALPix with the
following specification:
- Only one column: I (intensity)
- ORDERING: RING
- COORDSYS: G (Galactic)
- OBJECT: FULLSKY
- INDXSCHM: IMPLICIT
Parameters
----------
filename : str
Filename of the output file to write the HEALPix map data
hpmap : 1D `~numpy.ndarray`
1D array containing the HEALPix map data, and the ordering
scheme should be "RING";
The data type is preserved in the output FITS file.
header : `~astropy.io.fits.Header`, optional
Extra header to be appended to the output FITS
clobber : bool, optional
Whether overwrite the existing file?
checksum : bool, optional
Whether calculate the checksum for the output file, which is
recorded as the "CHECKSUM" header keyword.
NOTE
----
- This function is intended to replace the most common case of
`healpy.write_map()`, which still uses some deprecated functions of
`numpy` and `astropy`, meanwhile, it interface/arguments is not very
handy.
- This function (currently) only implement the very basic feature of
the `healpy.write_map()`.
"""
hpmap = np.asarray(hpmap)
if hpmap.ndim != 1:
raise ValueError("Invalid HEALPix data: only support 1D array")
# Hack to ignore the dtype byteorder, use native endianness
dtype = np.dtype(hpmap.dtype.type)
hpmap = hpmap.astype(dtype)
#
npix = hpmap.size
nside = int((npix / 12) ** 0.5)
colfmt = FITS_COLUMN_FORMATS.get(hpmap.dtype)
if hpmap.size > 1024:
hpmap = hpmap.reshape(int(hpmap.size/1024), 1024)
colfmt = "1024" + colfmt
#
hdr = fits.Header()
# set HEALPix parameters
hdr["PIXTYPE"] = ("HEALPIX", "HEALPix pixelization")
hdr["ORDERING"] = ("RING",
"Pixel ordering scheme, either RING or NESTED")
hdr["COORDSYS"] = ("G", "Ecliptic, Galactic or Celestial (equatorial)")
hdr["NSIDE"] = (nside, "HEALPix resolution parameter")
hdr["NPIX"] = (npix, "Total number of pixels")
hdr["FIRSTPIX"] = (0, "First pixel # (0 based)")
hdr["LASTPIX"] = (npix-1, "Last pixel # (0 based)")
hdr["INDXSCHM"] = ("IMPLICIT", "Indexing: IMPLICIT or EXPLICIT")
hdr["OBJECT"] = ("FULLSKY", "Sky coverage, either FULLSKY or PARTIAL")
#
hdr["EXTNAME"] = ("HEALPIX", "Name of the binary table extension")
hdr["CREATOR"] = (__name__, "File creator")
hdr["DATE"] = (datetime.now(timezone.utc).astimezone().isoformat(),
"File creation date")
# Merge user-provided header
# NOTE: use the `.extend()` method instead of `.update()` method
if header is not None:
hdr.extend(fits.Header(header))
#
hdu = fits.BinTableHDU.from_columns([
fits.Column(name="I", array=hpmap, format=colfmt)
], header=hdr)
hdu.writeto(filename, clobber=clobber, checksum=checksum)
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