def ps1_stack_path(proj_cell, sky_cell, band):
    """
    construct relative path for a PS1 stack image by projection cell,
    sky cell, and band.
    """
    proj_cell = str(proj_cell).zfill(4)
    sky_cell = str(sky_cell).zfill(3)
    return (
        f"/rings.v3.skycell/{proj_cell}/{sky_cell}/"
        f"rings.v3.skycell.{proj_cell}.{sky_cell}.stk.{band}.unconv.fits"
    )