Description of the CCF parameters

mask
Name of the digital mask used to measure the radial velocity by cross-correlation (operation done in pixel space using the 'e2ds' wavelength-calibrated spectrum). A mask is made of box-shaped "emission lines" corresponding to several thousand weak neutral metallic lines. The width of each box is specified by theoretical right and left wavelengths (at zero velocity) adapted to the strength of each line. The most frequently used masks are optimized for spectral types F0, G2, K0, K5, M4 and M5 and were originally designed for ELODIE and later adapted from HARPS.

ccf_offline
A value of "0" indicates a standard automatic cross-correlation covering a ± 30 km/s interval, while a value of "1" is shown if the observer used the off-line DRS facility to derive cross-correlations with other masks and/or using other velocity windows.

rv
Radial velocity measured by fitting a gaussian to the average cross-correlation function (CCF) over all orders having a satisfactory S/N ratio, and is corrected to the barycenter of the Solar System (km/s). Note that some spectra taken in 2006 and 2007 show erroneous velocities due to bad barycentric corrections (see note)

± err
estimated 1-σ error on CCF radial velocity, derived from contrast and FWHM, best for S/N < 100.

± dvrms
Estimated 1-σ radial-velocity uncertainty, based on photon noise alone, good for S/N > 100.

fwhm
Full width at half maximum of the CCF dip (km/s). This is the FWHM of the average line profile for weak neutral metallic lines. Abnormally large fwhm values coupled with low contrast indicate no meaningful correlation was achieved. Double-peaked ccf (spectroscopic binaries, chance alignments) are not handled correctly in the automatic pipeline.

span
Bisector velocity span, a measure of line asymmetry (km/s). It represents the difference in line center between the top and bottom of the line profile. Is used to detect radial velocity changes due to motions (moving star spots) over the stellar surface.

maxcpp
The maximum number of counts/pixel in the average CCF (expressed in electrons) is used to compute errors based on photon noise. Larger values indicate a better S/N ratio.

contrast
Contrast of the CCF, expressed as percent depth of dip (%). This is a measure of the strength of the average weak neutral metallic absorption line profile, relative to the local normalized continuum.

lines
Number of absorption lines used to compute the CCF. Orders with poor S/N ratio are not included in the final calculation.

view_head
Allows the user to browse through the FITS header of the cross-correlation file.

get_ccf
Download a 40-line FITS image (with 61 or more values per line depending a velocity sampling) with the CCF computed for each order. Line 40 contains the average CCF. (An accompanying ASCII file -as a FITS extension- contains the individual radial velocities derived for each order). If correlations were made with other masks than G2, the corresponding CCF are shown separately.

view_ccf
List the CCF parameters and plot the average CCF over the velocity interval explored. Additional cross-correlations done off-line using the same mask but a different velocity window are also shown.