Photometry data accuracy from AAVSO Variable Star Database

Mike, I've asked Arne to comment on this topic as he is much more versed in the differences between the databases. From what I recall, the SIMBAD values might be in a different photometric filter set on some stars. I've noticed similar problems with infrared photometry on bright stars which often use the 2MASS values in an older filter set. This type of data discrepancy has to be my biggest complaint while learning astronomy. Brian

Hi Mike, 68 UMa is an interesting star; the Tycho2 converted magnitude is 6.33 with a (B-V) of 1.50. The Bright Star Catalog (5th edition) lists it as 6.43; WVBR also gives it 6.43. Somebody is wrong or the star is variable at this level. The VSD HIP 58302 magnitudes also come from WVBR. This is given in VSP as GSC 03836-00816, so I'd assume it is a possible variable, yet I can't find it immediately in VSX. Let me see if I can't trace down these inconsistencies in the next day or so (I'm at a meeting right now). In general, look at the photometric errors when using stars from VSD. For example, HIP 58302 is shown with errors of 0.141mag for B and 0.100mag for V, indicating that there was a good chance for the quoted values to be different. Arne

Hi Arne, Thanks for the reply. Looking at the photometry list for 68 UMa, it also has error values of 0.141m and 0.100m in it's Band V measuements as well. Iguess it is possible for the true magnitude of 68 UMa to be close to the Tycho2 magnitude, assuming it isn't a variable. When Ilook at the list of stars from the VSDin the area around Z UMa, all the stars from the GSCand WVBR have photometric errors of 0.141 and 0.100 in the Band V filtered magnitudes. Does this mean that we should not consider those to be appropriate comparison stars for photometric use? If so, is there a catalog of bright stars (< mag 10), that includes Band V measurements that would be considered accurate enough for photometric use with an accuracy better than 0.05m? -Mike Durkin

Hi Mike, I updated most of the comparison star magnitudes about two years ago with the best photometry that I could find, from about a dozen sources of information. This was done using a few selection rules, such as choosing WBVR data over Tycho data where it existed, as WBVR yielded more passbands of information and with a cleaner transformation to the standard Johnson/Cousins system. Unfortunately, the WBVR catalog does not include errors, so I just picked 0.10mag as the error for each magnitude to cover photometric and transformation uncertainties. What is really needed is a single source of photometry, homogeneous, covering the entire sky. That is why the AAVSO is currently running the AAVSO Photometric All-Sky Survey (APASS). Once that is complete, you will have consistent, high quality photometry everywhere. Until then, you will run across many cases like this, where a specific star may or may not have quality measurements. That is why we always recommend that you only use fields containing Landolt standards (or perhaps a few well-studied clusters like M67 and NGC7790) for determining your transformation coefficients whenever possible. In general, the highest quality VSD magnitudes will be those from USNO or SRO. Don't get too concerned over these anomalies. The VSD database is adequate for visual work, and in most cases, also for use as secondary standards for CCD/DSLR photometry, but just be aware that potential differences will exist. Arne