def get_psf_sigma(exposure):
    """
    Get sigma of point-spread function of HSC patch.

    Parameters
    ----------
    exposure : lsst.afw.image.ExposureF
        Exposure object. 

    Returns
    -------
    sigma : float
        Approximate standard deviation of the PSF
        of the image in this exposure.
    """
    psf = exposure.getPsf()
    sigma = psf.computeShape().getDeterminantRadius()
    return sigma