Following on Tom Calderwood's nice presentation today about photometric transformations, I pass along some details about the Landolt Selected Area 110 stars that can be used for determining these, and atmospheric extinction as well. The field is in Aquila, so is up by late-evening just now, and will be available until October-ish. This is merely one of many such calibration fields that can be used. The specific star-field here is led by SA 110-353 = HD 172829, which is the reddest fairly bright Landolt star, with B-V = 2.00. It has the erroneous variable-star designation of HK Aql, but is simply not variable at any level measureable in the usual way. Getting it and the other stars properly exposed at shorter/longer wavelengths could require two different sets of exposures in different filters. The field center is for a 20'x20' field to capture all the stars.
To use the field simply to measure extinction, you could get away with exposing just to get the red/blue pair SA 110-353 and -340, but you'll get several others stars for free in the process to help beef-up the determination.
I have trimmed my more detailed working file to show just a name, coordinates, and the photometric magnitudes and color-indices. I include Sloan z values as available. For most of the stars the data are taken from Peter Stetson's extensive field calibrations that greatly extend the Landolt sequences around the sky. For stars in common, his values are typically within a few millimags of what Arlo published, but usually with much higher statistical weight (many more observations with big telescopes). The nominal errors are < 0.01 for each item. For one star I show provisional data from APASS and TASS MkIV, which will need to be calibrated more accurately against the other stars in the same field. That star is probably good to +/- 0.03 mag for V and B-V. The last entry has curiously inconsistent B-V values from multiple reliable sources, which also needs to be looked at with independent data. Finally, I note that three stars in the middle range of B-V color have negative U-B color-indices. These are unreddened very-metal-poor stars, two of which have significant proper motion. They are useful since they extend the range in U-B color relative to the other mostly reddened stars in the field.
\Brian
(the table will need to be looked at with an equal-space font, or reformatted as csv etc as desired)
--- SA 110-353 field center: 18 41 50 +00 12.0 (J2000)
--- Stetson 2021 Feb 22 data
--- color ranges: 0.31 < B-V < 2.00; -0.21 < U-B < 2.29
Name RA (J2000) Dec V B-V U-B V-R R-I V-I z spec source
SA 110-S305 18 41 20.17 +00 13 03.0 12.229 0.663 0.088 0.402 0.400 0.801 11.822 Stetson
SA 110- 339 18 41 26.52 +00 08 25.3 13.605 0.987 0.766 0.591 0.523 1.114 Stetson
SA 110- 340 18 41 28.45 +00 15 23.0 10.025 0.308 0.124 0.171 0.183 0.354 10.102 A5II Landolt 2009
GSC 0447-0188 18 41 32.16 +00 16 39.1 12.70 0.81 0.96 12.022 APASS/TASS MkIV provisional
SA 110-S323 18 41 33.94 +00 04 24.7 13.132 1.022 0.408 0.624 0.673 1.297 12.093 K? Stetson
SA 110-S329 18 41 35.84 +00 20 40.7 11.308 0.492 -0.212 0.323 0.340 0.663 F8 wl Stetson, large proper motion
SA 110- 246 18 41 50.76 +00 05 04.5 12.710 0.605 -0.156 0.380 0.415 0.795 12.285 G wl Stetson, large proper motion
SA 110-S353 18 41 51.84 +00 14 10.8 12.586 0.635 -0.062 0.383 0.375 0.758 G wl Stetson
SA 110- 248 18 42 00.56 +00 03 18.4 10.800 0.698 0.507 0.391 0.484 0.875 10.242 A0 Stetson
SA 110- 352 18 42 17.16 +00 11 09.6 11.334 0.571 0.073 0.337 0.340 0.677 F2: Stetson
SA 110- 353 18 42 17.70 +00 09 18.4 8.447 2.002 2.292 1.186 1.120 2.306 K5III Landolt 1983a
SA 110-S421 18 42 18.42 +00 05 04.7 13.205 1.153 0.745 0.678 0.768 1.446 11.945 Stetson
SA 110- 355 18 42 18.93 +00 08 24.2 11.944 1.023 0.504 0.652 0.727 1.378 A2V Landolt 1992 = Landolt 2009
11.942 1.056 0.473 0.647 0.748 1.395 Stetson
11.948 1.051 0.673 0.688 1.361 Henden n=3
11.957 0.979 0.492 0.663 0.733 1.396 Clem & Landolt 2013 n=5