Sun, 09/06/2015 - 19:19
Hi all,
I have noted that the values of B-V reported in the photometric tables from WBVR catalog are wrong. I think they refer to W-B and not B-V.
For example, the star 000-BCN-956 (HD 204536) is reported with B-V = - 0.721. I checked this value in the WBVR catalog on the web and I found B-V = - 0.076 and W-B = - 0.734.
I checked other stars and I found the same discrepancies.
Can you confirm it?
Cheers,
Luigi
Luigi,
You are quite correct in that an original error was made with the B-V value.
I am going to go ahead and correct this...
HOWEVER, there is a mechanism in place, FYI, going into the future whereby chart errors, discrepancies and inadequacies can be dealt with and has been in place for a number of years now:
https://www.aavso.org/chet
This is the best place to bring such concerns to the attention of the Sequence Team.
Ad Astra,
Tim Crawford, CTX
Sequence Team
Hi Tim,
thanks for your attention.
Have I need to open a check report now?
Luigi
Keep in mind that the WBVR system is close to Johnson's UBVR system but not the same.
Over the years I have checked the values of all stars brighter than 5.0 and came up with the following convertion between B-V from Johnson's and B-V from WBVR.
B-V= -0.25 --> WBVR - 0.025
B-V= -0.2 --> WBVR - 0.017
B-V= -0.15 --> WBVR - 0.012
B-V= -0.1 --> WBVR - 0.009
B-V= -0.05 --> WBVR - 0.005
B-V= -0.02 --> WBVR - 0.000
B-V= 0.0 --> WBVR - 0.000
B-V= 0.1 --> WBVR - 0.000
B-V= 0.2 --> WBVR - 0.000
B-V= 0.3 --> WBVR - 0.000
B-V= 0.4 --> WBVR - 0.000
B-V= 0.5 --> WBVR - 0.001
B-V= 0.6 --> WBVR - 0.004
B-V= 0.8 --> WBVR - 0.010
B-V= 1.0 --> WBVR - 0.020
B-V= 1.2 --> WBVR - 0.033
B-V= 1.4 --> WBVR - 0.042
B-V= 1.6 --> WBVR - 0.055
B-V= 1.7 --> WBVR - 0.064
B-V= 1.8 --> WBVR - 0.077
B-V= 1.9 --> WBVR - 0.090
The main conclusion from this table is that the B-V color of the average AAVSO comparison star is mostly the same in both systems but if you are in need of using a redder or bluer comparison star, it would be appropriate to apply a small correction.
Cheers,
Sebastian
This catalog was very difficult to work with when I was populating the comparison star database. It contains lots of blank columns and missing characters. Probably cleaner versions of the catalog exist now (10 years later). My guess is that, in some cases, a blank column was missed and the WB value used instead of the BV value.
When BV was present, the transformation came from Warren, where
(B-V) = 0.9759 * (b-v) - 0.005
which might come close to what Sebastian found. Approximately 3120 bright VSD stars had potential WBVR data, but only a few of those were actually used as GCPD took precedence. You are finding these with the wide-field DSLR photometry that you are doing; the incorrect (B-V) values were not seen before because they were not used by the visual/binocular observers.
As mentioned in this thread, report the incorrect values via CHET, along with their corrections, and the sequence team will update VSD. I think that process will hold us over until the next massive VSD update in a few months. Good going, Luigi - you are an excellent quality control person!
Arne
Sebastian, Arne,
thanks for your clarification.
Luigi