# bcvcorrExample 3

## Compute Barycentric Velocity Correction Using Only Task Parameters

bcvcorr will compute the barycentric velocity correction and heliocentric Julian date for an observation based on time, position, and observatory information in the parameter file. The spectra parameter must be set to a null string.

The values of the needed information are set in the parameter file:
```  spectra = ""              List of input spectra
...
(ra = 4.6993444444444) Right ascension of observation
(dec = 5.6095277777778) Declination of observation
(equinox = "2000")         Coordinate equinox header keyword
...
(hjd = 0.)             Heliocentric Julian Date of observation
(gjd = 2450678.6448)   Julian Date of observation
(year = INDEF)          UT year of observation
(month = INDEF)          UT month of observation
(day = INDEF)          UT day of month of observation
(ut = INDEF)          Universal Time of observation
...
(obslong = 155.46666666667) Observatory longitude
(obslat = 19.816666666667) Observatory latitude
(obsalt = 41000.)         Observatory altitude in meters
...
```

### Results

The results are summarized to standard output as follows:
```rv> bcvcorr
List of input spectra:
test lat 19:49:00.0 , long 155:28:00.0, alt 41000.0
Julian date is 2450678.64480 at 1997-Aug-18 3:28:30.7 UT
Object at ra 4:41:57.640 dec 5:36:34.30 eq 2000
Heliocentric Julian date: 2450678.64340
gbcvel = 27.3759  ghcvel = 27.3902  geovel = -0.2008
bcv = 27.1751  hcv = 27.1894 computed
rv>
```
In this example, the date and UT are computed from the Geocentric Julian Date of the observation, gjd. Because the observatory position is given in the parameter list and there is no image from which to read it, no attempt is made to read it from the observatory database. The heliocentric Julian date is the time which the measured light from the observed object passes the sun. A correction from the center of the earth to the observatory site (geovel) is added to the geocentric barycentric (gbcvel) and heliocentric (ghcvel) velocity corrections to give the topocentric barycentric (bcv) and heliocentric (hcv corrections.

#### Last updated October 20, 2003 by Doug Minkdmink@cfa.harvard.edu

[rvsao] [bcvcorr]