Although this is a bit to the south for northern observers, this much-observed field deserves to be used for calibration and extinction determination. The field has data from Landolt, Stetson, Clem & Landolt, Mario Hamuy, and Arne Henden, all quite concordant. Arlo measured the very blue hot B-type subdwarf star called 'Mark A' and some fairly faint nearby field stars. I add other rather brighter field stars that should be more convenient for most observers. The field is best centered on the active galaxy Markarian 509 rather than the subdwarf. This will appear nearly stellar in small telescopes. It is only mildly variable; there is a VSX entry for it.
--- Mrk 509 field center: 20 44 09 -10 43 (J2000)
--- color ranges: -0.24 < B-V < 1.63; -1.16 < U-B < 1.77
--- Stetson 2021 Nov 26 data (incl re-reduction of Landolt & Clem data)
Name RA (J2000) Dec V B-V U-B V-R R-I V-I source
MarkA-CL163 20 43 38.98 -10 43 29.7 12.593 0.965 0.641 0.527 0.499 1.026 CL2013 n=44 errors 0.0012,0.017,0.0018,0.0017,0.0015,0.0015
MarkA-CL197 20 43 42.99 -10 48 17.9 13.859 0.669 0.116 0.384 0.369 0.753 CL2013 n=44 errors 0.0008,0.0014,0.0020,0.0013,0.0012,0.0011
MarkA-CL204 20 43 44.23 -10 46 10.2 12.905 0.939 0.583 0.513 0.490 1.003 CL2013 n=44 errors 0.0011,0.0014,0.0016,0.0015,0.0015,0.0016
Mark A2 20 43 55.00 -10 45 31.3 14.540 0.666 0.096 0.379 0.371 0.751 L92=L2009 n=21,10 errors 0.0028,0.0031,0.0046,0.0024,0.0050,0.0059
Mark A 20 43 59.23 -10 47 41.7 13.256 -0.246 -1.159 -0.114 -0.124 -0.238 L2009 n=88,71 errors 0.0008,0.0008,0.0021,0.0008,0.0027,0.0029
Mrk 509 Hamuy B 20 44 00.39 -10 43 06.0 13.126 0.547 -0.021 0.321 0.316 0.637 Stetson S26 n~100 errors 0.0010,0.0015,0.0020,0.0013,0.0014,0.0015
Mrk 509 Hamuy E 20 44 02.98 -10 40 22.7 11.377 0.907 0.606 0.516 0.478 0.994 CL2013 n=39 errors 0.0009,0.0011,0.0012,0.0017,0.0021,0.0017
Mrk 509 Hamuy H 20 44 03.29 -10 37 42.3 13.318 0.535 -0.033 0.329 0.319 0.648 CL2013 n=35 errors 0.0013,0.0024,0.0026,0.0021,0.0024,0.0021
Mark A3 20 44 03.82 -10 45 37.9 14.818 0.938 0.651 0.587 0.510 1.098 L92=L2009 n=21,10 errors 0.028,0.0031,0.0046,0.0024,0.0050,0.0059
Mrk 509 Hamuy I 20 44 09.18 -10 39 10.8 14.417 0.601 -0.053 0.357 0.364 0.721 Stetson S864 n~75 errors 0.0011,0.0016,0.0022,0.0014,0.0017,0.0018
Mrk 509 Hamuy G 20 44 10.69 -10 39 23.2 12.551 0.572 -0.045 0.372 0.369 0.741 CL2013 n=39 errors 0.0007,0.0013,0.0018,0.0011,0.0011,0.0010
Mrk 509 Hamuy D 20 44 11.21 -10 41 52.4 12.404 0.565 0.097 0.329 0.313 0.642 CL2013 n=43 errors 0.0008,0.0012,0.0017,0.0018,0.0018,0.0012
Mrk 509 Hamuy A 20 44 15.08 -10 44 41.7 14.547 0.661 0.136 0.373 0.346 0.719 Stetson S1 n~100 errors 0.0011,0.0017,0.0028,0.0021,0.0022,0.0017
MarkA-CL567 20 44 28.16 -10 41 05.7 11.064 0.969 0.816 0.529 0.457 0.986 CL2013 n=35 errors 0.0015,0.0021,0.0026,0.0040,0.0046,0.0031
GSC 5760-1565 20 44 39.32 -10 46 20.6 11.817 1.630 1.772 0.949 1.174 2.123 Stetson S1267 n~40 errors 0.0053,0.0076,0.0188,0.0082,0.0084,0.0078
It would seem these fields should be candidates to be added to our standard fields resource:
To do that I would need more information about the data sources. Inclusion in VSD requires a reference to a paper or some public source.
The tables are rather cryptic! Looking only at the Mrk 509 field for the moment, the source I call 'CL2013' is Clem & Landolt 2013, basically the equatorial standards paper that was done with CCDs:
The full tables (45,000 stars) are linked from the paper at the CDS. The sources called 'L92' and 'L2009' are the 1992 and 2009 Landolt photoelectric papers, which are:
For two of the stars the data from the 1992 paper are simply copied into the 2009 paper (thus L92=L2009). Arlo probably used the very blue sdB star 'Mark A' as a standard himself, so the 2009 paper has more observations than in the 1992 paper, and is thus preferred (the values are microscopically different).
For the Stetson data, I use his files at the Canadian Astronomy Data Center:
I guess one would cite the first 'homogeneous photometry' standards description:
...which has 547 citations in ADS, so is the paper folks are using to reference the CADC files.
Stetson lists separate U,B,V,R,I magnitudes rather than V and color-indices, so the colors (and their errors) were calculated by me. You might prefer the direct magnitudes anyway. Stetson revises his files on the timescale of a few months as he gets more data. The current version for the 'Mark A' field is from 2023 May 19, and my figures are somewhat older. Because he has hundreds of observations for each star, the changes are only at the few-millimag level since he does a complete re-reduction of all his data from all nights for every star whenever he posts a new file. It is perhaps worth noting that Stetson has re-reduced the Clem & Landolt raw data for the Mrk 509 field to his specs (something about scattered light corrections across the field, and because for many of the very faintest entries galaxies are not well separated from stars).
More to follow.
Looking briefly again at the other lists I posted, most can be traced to Landolt's papers, including the original 1973 UBV paper and the first UBVRI paper from 1983, plus the later ones already mentioned. Stetson's work around the globular clusters can be linked to the CADC site and the 2000 PASP 'homogeneous photometry' paper. There are a few stars also from Arne Henden's big file of unpublished stuff, which he provided to me a few years ago. All these resources are a boon.
However, those fields besides Mrk 509 are perhaps not quite ready for prime time. The reason is (my opinion here) that they exclude brighter stars that I think make them more generally useful for folks with smaller telescopes. Neither Stetson or Henden could not observe stars brighter than V mag 10 or 11, and the next couple of magnitudes brighter include useful stars that extend color ranges etc. I have lots of data to do this calibration, but have not arranged things to be able to do the reductions properly. The sequences are fine as they are, but can be made better, which I hope to do.
I am preparing the 3 fields that Brian Skiff submitted for review for adding to the Standard Fields set that the AAVSO maintains at https://app.aavso.org/vsd/stdfields
In the future if fields are submitted I recommend two formats that would help greatly on HQ's side:
If the data is sourced in color than pack the data in csv with columns:
name,RA h,RA m,RA s,Dec d,Dec m,Dec s,V,Verr,B-V,B-Verr,U-B,U-Berr,V-R,V-Rerr,R-I,R-Ierr,V-I,V-Ierr, # Comments
With this we can create VSDAdmin1 format file easily. Having the error data in the comment section is a lot of cut-and-paste!
Note this is a fixed format for Johnson/Cousins only.
If the data is magnitudes, please do not use the VSDAdmin1 format. Understand that the VSD holds magnitude data, so color data submissions will be converted to magnitudes and the errors will be increased as the conversion uses a quadrature conversion of the error. If magnitude data is converted to color in quadrature then its a double whammy as we convert it back in quadrature.
The alternate format is VSDAdmin2 that has csv columns of:
name,RA h,RA m,RA s,Dec d,Dec m,Dec s,[filter triplets with filter name, mag, err],#comment
eg 140, 0,0,13.33, -0,4,43.4, V,13.965,0.166, B,14.575,0.004, SG,14.199,0.03, SR,13.759,0.164, SI,13.583,0.011, R,13.553,0.244, I,13.167,0.294,#comment
This format is designed to accommodate all filters known to the AID. And we are interested in accumulating Sloan reference data.
In both cases the comment field is where you can be descriptive of the data source including number of observations behind the data (n=xx)