def convert_plate_pix(plate_scale, pix_size):
    """
    Plate scale is calculated with the equation:
    P = 206265 / (D*f/#)
    206265 is the amount of arcsecs in a radian.
    D is the diameter of the telescope
    f/# is the f-number: Focal length/Diameter
    ( http://www-supernova.lbl.gov/~sed/telescope/obsguide/platescale.html )
    
    For a telescope of 20 cm, and focal length of 50 cm, the plate scale is 412.53 arcsec/mm
    Parameters
    ----------
    plate_scale : float
        Must be in arcsec/mm.
    pix_size : float
        Must be in mm/pixel.

    Returns
    -------
    convert_factor : float
        How large a sky area a single pixel width covers.
    """
    convert_factor = plate_scale * pix_size # [arcsec per pix] = [arcsec/mm] * [mm/pix] 
    return convert_factor