LSF_ROTATE:

     Create a 1-d convolution kernel to broaden a spectrum from a rotating star

     Can be used to derive the broadening effect (line spread function; LSF)
     due to  rotation on a synthetic stellar spectrum.     Assumes constant
     limb darkening across the disk.

     lsf = LSF_ROTATE(deltav, vsini, EPSILON=, VELGRID=)

    deltaV - numeric scalar giving the step increment (in km/s) in the output
             rotation kernel.
    Vsini - the rotational velocity projected  along the line of sight (km/s)

    LSF - The convolution kernel vector for the specified rotational velocity.
          The  number of points in LSF will be always be odd (the kernel is
          symmetric) and equal to  either ceil(2*Vsini/deltav) or
          ceil(2*Vsini/deltav) +1 (whichever number is odd).    LSF will
          always be of type FLOAT.
          To actually compute the broadening. the spectrum should be convolved
          with the rotational LSF.

    Epsilon - numeric scalar giving the limb-darkening coefficient,
          default = 0.6 which is typical for  photospheric lines.    The
          specific intensity I at any angle theta from the specific intensity
          Icen at the center of the disk is given by:
          I = Icen*(1-epsilon*(1-cos(theta))

     Velgrid - Vector with the same number of elements as LSF

    (1) Plot the LSF for a star rotating at 90 km/s in both velocity space and
        for a central wavelength of 4300 A.    Compute the LSF every 3 km/s
       IDL> lsf = lsf_rotate(3,90,velgrid=vel)      ;LSF will contain 61 pts
       IDL> plot,vel,lsf                    ;Plot the LSF in velocity space
       IDL> wgrid = 4300*(1+vel/3e5)       ;Speed of light = 3e5 km/s
       IDL> oplot,wgrid,lsf                ;Plot in wavelength space

    Adapted from rotin3.f in the SYNSPEC software of Hubeny & Lanz
        http://tlusty.gsfc.nasa.gov/index.html.    Also see Eq. 17.12 in
    "The Observation and Analysis of Stellar Photospheres" by D. Gray (1992)

    Written,   W. Landsman                November 2001

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