Transforms between Sloan and Cousins for V-I
In some of our work we have used the Sloan i filter in preference to Cousins I, to cut down the night sky fringing. We have, however, tied this back to the Landolt standards, to produce a V-I colour. There are already papers which compare the Sloan system to the Cousins system, where both have been calibrated using their "natural" standards ( Fukugita et al, Smith et al), but nothing which uses a mixed system such as ours. Our concerns were that there is a relatively large colour term in our transformations (~1.2, rather than 1.0), and there might be a systematic deviation with colour.
To examine this was have compared the V-I data we obtained for Cep OB3b in Pozzo et al (2003) with a Cousins I filter, to the newer data used in Burningham et al 2005, which was obtained using a Sloan i filter, and which for this work we calibrated to give V-I. The colour-magnitude diagram on the left shows the stars in common between the two datasets, with a photometrically defined pre-main-sequence in red.
(V-i) = 0.971(V-I) + 0.008The pre-main sequence stars (in red) do not show a significantly different trend from the others. But, the caveat is that the reddest standard in our V-i calibration is 2.87, and in V-I 2.5.
The reason for the difference is almost certainly the relatively high extinction to the field (approximately 3 mags in V). Our standard star calibration should have ensured that a Landolt standard at V-I=3 (which will be a cool star) comes out at the right colour. But a highly reddened star has a different spectrum, and though it might be V-I=3 with the Cousins filter, it is obviously slightly different when measured with the Sloan one.