bcvcorr Example 2

Compute Barycentric Velocity Correction Using Tabulated Observatory

bcvcorr will compute the barycentric velocity correction and heliocentric Julian date for an observation based on position and observatory information from a table.

bcvcorr Task Parameters

The names of the keywords to be used are set in the parameter file:

  (keyra = "RA")            Right ascension header keyword
  (keydec = "DEC")          Declination header keyword
  (keyeqnx = "EPOCH")       Coordinate equinox header keyword
...
  (keydate = "DATE-OBS")    Header keyword for date of observation (dd/mm/yy)
  (keystart = "UTOPEN")     Header keyword for UT at start of observation
  (keymid = "UTMID")        Header keyword for UT in middle of observation
  (keyend = "UT")           Header keyword for UT at end of observation
  (keyexp = "EXPOSURE")     Header keyword for duration of observation in sec
...
  (obsname = "flwo")        Observatory name--file to read from image heade

Tabulated Observatory Position

The observatory specified by the parameter obsname, as it is not file, is looked up in the file noao$lib/obsdb.dat. Its entry gives the position needed for this computation:

observatory = "flwo"
        name = "Whipple Observatory"
        longitude = 110:52:39
        latitude = 31:40:51.4
        altitude = 2320
        timezone = 7

Spectrum Image Header

Here is the relevant portion of the image header:

...
    RA      = '09:36:48.0'
    HA      = '00:53:18.9'
    DEC     = '16:59:00.0'
    EPOCH   =                1950.
    UT      = '04:32:26'
    ST      = '10:32:24.6'
    DATE-OBS= '13/04/91'
...
    EXPOSURE=                3600.
...

Results

The results are summarized to standard output as follows:

ar> bcvcorr
List of input spectra (22408): 
22408: 4096 x 1 x 1 1-D image
UT: 4:32:26.0, UTOPEN: INDEF, UTMID: INDEF, EXP: 3600
4/13/1991 4:02:26.0 UT
RA: 9:36:48.0, Dec: 16:59:00.0  1950.0
Mt. Hopkins 60-inch lat 31:40:51.4 , long 110:52:39.0, alt 2320.0
Julian date is 2448359.68919 at 4/13/1991 4:02:26.0 UT
Object at ra 9:36:48.000 dec 16:59:00.00 eq 1950
Heliocentric Julian date: 2448359.69197
gbcvel = -25.7858  ghcvel = -25.7864  geovel = -0.0875
bcv = -25.8732  hcv = -25.8739 computed
bcv = INDEF  hcv = -25.8721 from file
BCVCORR: writing BCV to 22408
ar>

As only the exposure time and the UT of the end of the observation are given, the midtime of the event is computed by subtracting half of the exposure time from the end time. This time is given on the Julian Date line. 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. The computed corrections can be compared to those already in the header (keywords BCV and HCV) which are given on the last line.

Resulting Header

One line is added to the header:

     BCV     =            -25.87322

Last updated October 20, 2003 by Doug Mink dmink@cfa.harvard.edu

[rvsao] [bcvcorr]