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33.3 Heliocentric Radial Velocity Calculation

FOS output wavelengths (.c0 files) are placed on the wavelength system of the dispersion coefficients in the CCS6 reference table. As described in Chapter 31 those coefficients contain an "internal/external" correction to account for the different illumination patterns produced on the dispersers by internal and external beams. However, no corrections for spacecraft motions of any kind are applied to calfos-calibrated wavelengths.

The procedure described here calculates the heliocentric motion of the Earth at the time of observation. This procedure does not correct for the small effect (at FOS dispersions) of HST orbital motion.

The IRAF task rvcorrect is used to compute radial velocity corrections. rvcorrect is in the IRAF astutil package and it calls on task observatory, which is also found in astutil. The following steps are an example of its use for an HST dataset where the HST velocity around the Earth is ignored (maximum residual: +/- ~7.8 km/s):

Set parameters to their default values.

          cl> unlearn rvcorrect

Use the default observatory task parameters except the following, which must be set to prevent rvcorrect from calculating an unwanted diurnal velocity correction based on the observatory position with respect to the Earth's center.

Figure 33.1: Observatory pset Values



as> lpar observatory

      command = "list"           Command (set|list|images)

              obsid = "obspars"       Observatory to set, list, or image default

       images =                 List of images

     override =                 Observatory identification

     (verbose = no)             Verbose output?\n

 (observatory = "obspars")      Observatory identification

        (name = " ")            Observatory name

   (longitude = 0.)             Observatory longitude (degrees)

    (latitude = 0.)             Observatory latitude (degrees)

    (altitude = -6378204.8)     Observatory altitude (meters)

    (timezone = 0.)             Observatory time zone

Create an ASCII file (named rvcor.in in the following example) with a row entry for each spectrum. Each row will contain: UT, RA, DEC, EPOCH, and Observed Velocity (more examples are provided in the help file for the task). Here is an example of file rvcor.in:

93 01 21 18:50:07 0.2974443291667E+03 0.4896100000000E+02  2000 0

93 01 21 20:05:09 0.2974443291667E+03 0.4896100000000E+02  2000 0

Run the rvcorrect task:

          cl>rvcorrect f=rvcor.in > outputfile

The above example produced the following output file. VHELIO contains the (very small, in this case) correction for the heliocentric velocity of the telescope with respect to the target.

                HJD              VOBS      VHELIO    VLSR    VDIURNAL      VLUNAR      VANNUAL    VSOLAR

2449009.81098          0.00        2.38      2.72      0.000            0.001        2.381        0.342

2449009.88335          0.00        2.35      2.69      0.000            0.001        2.349        0.342

Read the rvcorrect task help carefully, particularly to insure that you keep the signs of the velocities in the appropriate frame.

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