epochPropErr( tYr, astrom22 )
The input and output astrometry parameters with associated error and correlation information are each represented by a 22-element array, with the following elements:
index gaia_source name unit description ----- ---------------- ---- ----------- 0: ra deg right ascension 1: dec deg declination 2: parallax mas parallax 3: pmra mas/yr proper motion in RA * cos(dec) 4: pmdec mas/yr proper motion in Declination 5: radial_velocity km/s barycentric radial velocity 6: ra_error mas error in right ascension 7: dec_error mas error in declination 8: parallax_error mas error in parallax 9: pmra_error mas/yr error in RA proper motion * cos(dec) 10: pmdec_error mas/yr error in Declination proper motion 11: radial_velocity_error km/s error in barycentric radial velocity 12: ra_dec_corr correlation between ra and dec 13: ra_parallax_corr correlation between ra and parallax 14: ra_pmra_corr correlation between ra and pmra 15: ra_pmdec_corr correlation between ra and pmdec 16: dec_parallax_corr correlation between dec and parallax 17: dec_pmra_corr correlation between dec and pmra 18: dec_pmdec_corr correlation between dec and pmdec 19: parallax_pmra_corr correlation between parallax and pmra 20: parallax_pmdec_corr correlation between parallax and pmdec 21: pmra_pmdec_corr correlation between pmra and pmdecNote the correlation coefficients, always in the range -1..1, are dimensionless.
Null values for parallax
, pmra
,
pmdec
and radial_velocity
are treated as if zero for the purposes of propagation.
The documentation of the equivalent function in the Gaia archive
comments "This is a reasonable choice for most stars because
those quantities would be either small (parallax and proper motion)
or irrelevant (radial velocity).
However, this is not true for stars very close to the Sun,
where appropriate values need to be taken from the literature
(e.g. average velocity field in the solar neighbourhood)."
The effect is that the output represents the best estimates available for propagated astrometry; proper motions, parallax and RV are applied if present, but if not the output values are calculated or simply copied across as if those quantities were zero.
This transformation is only applicable for radial velocities determined independently of the astrometry, such as those obtained with a spectrometer. It is not applicable for the back-transformation of data already propagated to another epoch.
This is clearly an unwieldy function to invoke, but if you are using it with the gaia_source catalogue itself, or other similar catalogues with the same column names and units, you can invoke it by just copying and pasting the example shown in this documentation.
tYr
(floating point)astrom22
(array of floating point)epochPropErr(-15.5, array(
ra,dec,parallax,pmra,pmdec,radial_velocity,
ra_error,dec_error,parallax_error,pmra_error,pmdec_error,radial_velocity_error,
ra_dec_corr,ra_parallax_corr,ra_pmra_corr,ra_pmdec_corr,
dec_parallax_corr,dec_pmra_corr,dec_pmdec_corr,
parallax_pmra_corr,parallax_pmdec_corr,
pmra_pmdec_corr))
- calculates the astrometry with all errors and correlations at
2000.0 for gaia_source values that were observed at 2015.5.double[] epochPropErr(double, double[])