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#!/usr/bin/env python3
#
# Copyright (c) 2017 Weitian LI <weitian@aaronly.me>
# MIT license
#
"""
Convert the image (e.g., by WSClean) from units [Jy/beam] to [K]
by taking into account the telescope's beam size.
"""
import os
import sys
import argparse
import numpy as np
from astropy.io import fits
import astropy.units as au
def open_image(infile):
"""
Open the FITS image and return its header and data, but requiring
the input image has only ONE frequency.
The input FITS image may have following dimensions:
* NAXIS=2: [Y, X]
* NAXIS=3: [FREQ=1, Y, X]
* NAXIS=4: [STOKES, FREQ=1, Y, X]
"""
with fits.open(infile) as f:
header = f[0].header
data = f[0].data
if ((data.ndim == 3 and data.shape[0] != 1) or
(data.ndim == 4 and data.shape[1] != 1)):
# NAXIS=3: [FREQ!=1, Y, X]
# NAXIS=4: [STOKES, FREQ!=1, Y, X]
raise ValueError("input file '{0}' has invalid dimensions: {1}".format(
infile, data.shape))
print("Read in FITS image from: %s" % infile)
return (header, data)
def main():
parser = argparse.ArgumentParser(
description="Convert image unit from [Jy/beam] to [K]")
parser.add_argument("-C", "--clobber", dest="clobber",
action="store_true",
help="overwrite existing output file")
parser.add_argument("-f", "--frequency", dest="frequency",
help="frequency [MHz] of the input image (NOTE: " +
"required if cannot get frequency from file header)")
parser.add_argument("infile",
help="input FITS image file (NOTE: only single " +
"frequency supported)")
parser.add_argument("outfile",
help="output filename of the converted image")
args = parser.parse_args()
header, data = open_image(args.infile)
bunit = header["BUNIT"]
if bunit.upper() == "JY/BEAM":
unit = "Jy"
elif bunit.upper() == "MJY/BEAM":
unit = "mJy"
else:
raise ValueError("input image has wrong unit: %s" % bunit)
print("Data unit: %s/beam" % unit)
if args.frequency:
freq = args.frequency # [MHz]
else:
try:
freq = header["FREQ"] # [MHz]
except KeyError:
if header.get("CTYPE3", "").upper() == "FREQ":
freq = header["CRVAL3"] / 1e6 # [MHz]
else:
raise ValueError("--frequency required")
print("Frequency: %.2f [MHz]" % freq)
# beam size
bmajor = header["BMAJ"] * 3600 # [arcsec]
bminor = header["BMIN"] * 3600 # [arcsec]
beam = np.pi * bmajor*bminor / (4*np.log(2)) # [arcsec^2]
print("Beam area: %.2f [arcsec^2] (major: %.2f, minor: %.2f)" %
(beam, bmajor, bminor))
equiv = au.brightness_temperature(beam*au.arcsec**2, freq*au.MHz)
jybeam2k = au.Unit(unit).to(au.K, equivalencies=equiv)
print("Conversion factor [%s/beam] -> [K]: %g" % (unit, jybeam2k))
header["BUNIT"] = ("K", "Kelvin; converted from [%s/beam]" % unit)
header["JyBeam2K"] = (jybeam2k,
"[%s/beam] -> [K] conversion factor" % unit)
header["FREQ"] = (freq, "[MHz] frequency")
header.add_history(" ".join(sys.argv))
data = data * jybeam2k
if os.path.exists(args.outfile):
if args.clobber:
os.remove(args.outfile)
else:
raise OSError("output file already existed: %s" % args.outfile)
hdu = fits.PrimaryHDU(data=data, header=header)
hdu.writeto(args.outfile)
print("Converted image wrote to: %s" % args.outfile)
if __name__ == "__main__":
main()
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