def calculate_exposure_time(input, webapp=False, **kwargs):
    """
    This is a function to do the 'reverse' exposure time calculation where given a desired
    signal-to-noise ratio we calculate the optimal exposureSpecification and return a Report
    on the results.

    Parameters
    ----------
    input: dict
        Dictionary containing the information required to perform the calculation.
    psf_library : psf_library.PSFLibrary instance
        Library of PSF files (e.g. produced by webbpsf) to be used in the calculation

    Returns
    -------
    report.Report instance
    """
    raise NotImplementedError(
        'Reverse ETC calculations are not yet implemented.')
