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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. |
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This diagram shows the differences bewtween the two photometric
systems, where i should be understood to mean data taken through a Sloan
i filter but tied to the Laondolt standards.
The data can be represented by the following linear fit.
(V-i) = 0.971(V-I) + 0.008 The 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.If you want to try other descriptions of the data, you can download either the entire catalogue or just the PMS seclection in our usual cluster format. |