Documentation on velsw

Task: velsw
Purpose: Change the `velocity' axis of an image.
Categories: map manipulation

        VELSW is a Miriad task which changes the units on the `velocity'
        axis. As velocity and frequency are related by a Doppler shift
        formula, it is possible to switch an axis between being labelled
        in frequency or velocity. VELSW can switch the axis description
        between frequency (labelled FREQ-) and velocity, using either
        the `radio convention' (labelled VELO-) or `optical convention'
        (labelled FELO-) for the formula relating velocity 
        and frequency.

        Assuming that the data were measured with equal frequency
        increments, some approximation is involved assigning a velocity
        axis increment for the optical convention. In this case,
        the increment stored is correct at the reference pixel.

Key: in
        The name of the input image data set. No default.

Key: axis
        This determines what the labelling on the `velocity' axis will
        be changed to. Possible values are "frequency", "optical" (for
        velocity using the optical convention) or "radio" (for velocity
        using the radio convention). The default is "frequency" if the
        `velocity' axis is given as a velocity, and "radio" otherwise.