Radial Velocity Correction¶
Be careful with the fact that the date/hours recorded in the HDF file can be different than the real hours when the cube has been taken at least because there is only one date recorded in the HDF5 file but your data may have been taken over multiple nights at different hours. Is is possible to get a more precise radial velocity correction from the exact moment when each individual image of the data cube has been recorded.
[1]:
import orcs.process
[2]:
cube = orcs.process.SpectralCube('/home/thomas/M31_SN3.merged.cm1.1.0.hdf5')
master.03e73|INFO| Cube is level 3
master.03e73|INFO| shape: (2048, 2064, 840)
master.03e73|INFO| wavenumber calibration: True
master.03e73|INFO| flux calibration: True
master.03e73|INFO| wcs calibration: True
Print default barycentric radial velocity correction¶
[3]:
corr = cube.get_radial_velocity_correction(kind='barycentric')
print('Barycentric velocity correction: {:.2f} km/s'.format(corr))
master.03e73|INFO| Observation date: 2016-08-25T00:00:00.000 = 2457625.5 Julian days
master.03e73|INFO| Observatory location: LAT 19.825231999999996 deg |LON -155.46876099999997 deg |ALT 4214.999999999996 m
master.03e73|INFO| Observed Target: 00h42m44.3572s +41d16m07.8713s
Barycentric velocity correction: 20.13 km/s
Use more precise dates¶
Here we have created a file (abell426.dates.list) which records all the dates at which the individual files have been taken (as supplied by the keyword DATE of each individual FITS file).
~/data/1867004o.fits 2016-01-10T05:06:02
~/data/1867005o.fits 2016-01-10T05:06:32
~/data/1867006o.fits 2016-01-10T05:07:03
~/data/1867007o.fits 2016-01-10T05:07:33
~/data/1867008o.fits 2016-01-10T05:08:06
~/data/1867009o.fits 2016-01-10T05:08:36
~/data/1867010o.fits 2016-01-10T05:09:06
~/data/1867011o.fits 2016-01-10T05:09:36
~/data/1867012o.fits 2016-01-10T05:10:08
~/data/1867013o.fits 2016-01-10T05:10:39
~/data/1867014o.fits 2016-01-10T05:11:09
~/data/1867015o.fits 2016-01-10T05:11:40
~/data/1867016o.fits 2016-01-10T05:12:11
~/data/1867017o.fits 2016-01-10T05:12:42
...
[5]:
# The results of the radial velocity correction for
# each individual file of the initial inteferometric
# cube are recorded here
fout = open('abell426.barycorr.list', 'w')
# we loop in the dates file
with open('abell426.dates.list', 'r') as f:
for line in f:
date = line.split()[1]
corr = cube.get_radial_velocity_correction(
kind='barycentric', date=date, silent=True)
fout.write('{} {}\n'.format(date, corr))
print(date, corr)
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output¶
File abell426.barycorr.list has been created with the previous code.
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...
[6]:
# Now, we can then display the output file,
# i.e. the barycentric velocity correction
import astropy.time
dates = list() ; corrs = list()
with open('abell426.barycorr.list', 'r') as f:
for line in f:
line = line.split()
dates.append(line[0])
corrs.append(float(line[1]))
dates = astropy.time.Time(dates, format='isot', scale='utc')
import pylab as pl
pl.plot_date(dates.plot_date, corrs, ls='None', marker='+', c='orange')
pl.grid()
pl.xlabel('Date (UTC)')
pl.ylabel('Barycentric velocity correction (in km/s)')
master.03e73|WARNING| /home/thomas/miniconda2/envs/orb3/lib/python3.7/site-packages/pandas/plotting/_matplotlib/converter.py:103: FutureWarning: Using an implicitly registered datetime converter for a matplotlib plotting method. The converter was registered by pandas on import. Future versions of pandas will require you to explicitly register matplotlib converters.
To register the converters:
>>> from pandas.plotting import register_matplotlib_converters
>>> register_matplotlib_converters()
warnings.warn(msg, FutureWarning)
[6]:
Text(0, 0.5, 'Barycentric velocity correction (in km/s)')
How to use the computed correction¶
In general, for km/s precision, just add the barycentric velocity to the measured velocity. For higher precision you can refer to http://docs.astropy.org/en/stable/api/astropy.coordinates.SkyCoord.html#astropy.coordinates.SkyCoord.radial_velocity_correction