Sun, 08/23/2015 - 22:34
Hi,
in my recent observation of V1070 Cyg, I have used the photometric table X15307AQ. In this table, the color index of the star 000-BCN-865 is reported as B-V=0.000. There must surely be a mistake, because the star is a K5 spectral type. Indeed, Simbad reports B-V=1.490.
Further, I had chosen that as a comparison star (in ensamble) and, with B-V=0.000, I get results completely wrong. Instead, with B-V=1.490, I get good results.
Please, can somebody confirm it?
Regards,
Luigi
Hi Luigi,
The Bright Star Catalog from which this star's photometry was selected has a blank entry for (B-V). The machine-readable version of the catalog that I used to obtain the photometry for this star did not indicate "blank" correctly, and so the stored (B-V) was set to zero. As you mention, this is incorrect for this star. A value of 1.49 is fine to use for now, but since it is different than VSP reports (even if VSP is wrong), you should note it in the "comments" field of your report. You should also submit a CHET report so that the sequence team knows that they need to correct this error.
Arne
Luigi,
It is difficult to assist you if the name you give for an object is incorrect. Is the star's name HD 203857? The HR number for HD 203857 is HR 8913. Notice that the star's correct name is HD 203857 not HD203857. Is the AUID for this star 000-BCN-865? VSX cannot find an entry for that star nor for HD 203857. Please be very careful when giving star names. This is especially true for AUIDs, which can be cross-identified with other star names only by VSX. In this case, it is impossible to do that because neither name is in VSX.
Cheers,
Thom
Thanks, Arne, for your confirm.
Thanks, Thom, for your comment. The AUID for this star is 000-BCN-865 and, as said Arne, the B-V is wrong.
Regards,
Luigi
Hi Luigi,
The bigger question is why you want to use this star as a comp. The chart you provided is a 15-degree (A scale) chart, with lots of comparison stars listed. In fact, there are many comp stars in the range of 5.0 to 7.6 much closer to the target. If you use one or more of these, you don't have to use the discrepant star.
The rule of thumb is to use a comparison star or stars that are close to the color of the target, and preferably bracketing it somewhat. However, for an M-star like V1070 Cyg, you cannot easily do this, since most red stars are variable. For new sequences, the sequence team typically tries instead to use stars in the ~0.5-1.0 range of (B-V), stars that are typically non-variable, and with the intent to keep the comp stars similar in color to each other. That makes it easier for the visual observer, and does not greatly impact the digital observer. The HD star is a bit red under those guidelines, but there are always exceptions, especially on the bright end where you don't have much choice when selecting comps.
Arne
Hi Arne,
I do DSLR ensemble photometry using the spreadsheet of Citizen Sky. In this spreadsheet is required a large color range of the ensemble for the determination of transformation coefficient of the DSLR.
Further, in rich star fields I have often blending issues and I can't always have much choice.
Cheers,
Luigi