Hi, This is why you must never use the magnitude given by a planetarium software for comparison star. The Tycho-2 magnitude are not in standard band. The difference is because CdC try to compute a V magnitude from the catalog Bt (11.443) and Vt (10.211). It look like Auto Star magnitude is from the GSC (very bad photometry +/- 0.5 mag) and C2A is Bt.
Other catalog have other limitation, but no one give good standard photometry on the whole sky. For reference stars you must use the AAVSO chart and comparison stars. Go to : http://aavso.org/vsp enter SU Cyg in the name field, let other field blank and because this is a bright star select Legacy B scale in the middle of the page Adjust the orientation for your instrument. Click Plot This show you a nice chart with the comparison visual magnitude you can print. Then click "Comparison Star Photometry" to get a detailed list of the comparison with all band magnitude. You will not found 2151-4816-1 in this list probably because of missing reliable photometry. Patrick
