Colors/magnitudes: explanations and caveats
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IntroductionMAST is now adding new ground-based survey colors and magnitudes for the Kepler Field of View: so far this includes UK-IRT, KIS (DR1 and DR2), and Sloan (SDSS/DR9). The KIC u-magnitudes have been removed from MAST's Target Search form (but not the KIC form) because of their poor quality.
An executive summary of results for optical wavelengths is that the UBV survey should be preferred over the KIS survey. However, a broad secondary scatter distribution when some photometric target groups are include makes our recommendations for using SDSS g, r, and i magnitudes somehwhat nuanced (see UPDATES below). Nonetheless, the SDSS magnitudes do bring in a number of new Kepler targets (thanks to the SDSS U mangnitudes, particularly high temperature targets) and furthermore clarify some problems with the KIS and KIC magnitudes, particularly for bright magnitudes; see the graphs below. Users should combine KIC and KIS colors and magnitudes only with care, as they are based on different zeropoint magnitude systems (AB and Vega/Johnson; see table below). Graphical comparisons of KIC vs. UBV and KIC vs. SDSS magnitudes and colors are given separately in this tarball.
We give a link for the rules connecting survey acronyms with the filter magnitude names. You should also know that because of the zeropoint differences for the magnitude systems MAST will minimize the combinations of optical magnitude systems in the following way: colors formed from one filter from one KIC/KIS/UBV survey and a second filter from another. Therefore, because there will be no mixed colors, e.g., no (g_KIS - r_KIC). The only gr colors represented will be understood to be either gr (KIC) or gr_KIS.
Also, we won't comment on the Halpha equivalent widths in the KIS survey, which are parameterized by the rHα KIS index. A useful reference on the calibration of this index is Drew et al. 2005, MNRAS, 362, 753D.
In all cases filter wavelength centroids and transmissions should be shifted by -5Å from the tabular values because optical rays converge as they pass through the filter.
The Kepler Isaac Newton Telescope Survey (KIS) and
Everett (UBV) surveys are described in Greiss et al. 2012, AJ, 144, 24G and
Everett, Howell, & Kinemuchi 2012, PASP, 124, 316E.
UBV link here. (Select the first three rows in the table.)
(Latest) KIS DR1+2-KIC Comparisons (Summer 2014) The older plots (from 2012) using the KIS catalog did not take into account the "gclass" flags indicating whether an object was a likely star, a likely galaxy, or saturated. A revised version of the plots are shown below. Much of the scatter at the bright end below the 1:1 line is caused by saturated sources (plotted as squares in the figure below). Note that the regression solutions from these figures are based on the magnitude range of 12-16, even though greater ranges are plotted. MAST is currently investigating any possible differences between the KIS DR1 and DR2 fluxes.
Panel a.) We find an offset of of -0.09 magnitudes for the combined g KIS Parts 1 and 2 datasets compared to the KIC, taking into account the zeropoint differences between the KIS (Vega) and KIC (AB) systems. We use the relationship given by González-Solares et al. (2011, MNRAS, 416, 927):
g_KIS(Vega) = g(AB) + 0.060 - 0.136 × (g(AB) - r(AB))
If we adopt an average color from our sample = 0.64, the offset is only reduced to -0.06 mags, which is still larger than our expectation.
Panel b.) With the addition of KIS Part 2 data, the offset in the r KIS compared to KIC magnitudes is -0.27. This assumes an average color = 0.6 for our population, and uses the González-Solares et al. mean relation:
r_KIS(Vega) = r(AB) - 0.144 + 0.006 × (g(AB) - r(AB))
Panel c.) We also compare g SDSS and KIC, which shows no significant offset (the intercept at g=14 is 14.0). For more details on the comparison between SDSS and KIC, see the 2012 plots below.
Panel d.) Because of the different magnitude systems adopted by the KIC and KIS, this plot shows that the intercept in the (g-r)_kis vs. (g-r)_kic plot is nonzero (0.17). The slope is 0.90, rather different from 1.0, in the sense that the KIS color shows a smaller range than the KIC color does, and this tends to compensate for the large zero offset. For a mean (g-r)_kic color of 0.6, the offset is brought down to ~0.12. Early indications comparing different combinations of KIS, KIC, and SDSS suggest that the non-unitary slope in color comes from the KIS color calibration and not the KIC. MAST will investigate this further when the SDSS DR10 data for the Kepler field are ingested (as of August, 2014, still ongoing.)
Panel e.) This plot shows that there is a strong dependence on g magnitude (KIS or KIC) in the color difference (g-r)_kis - g(g-r)_kic. The dependence has almost a unitary slope (0.99). Following the discussions of Plots 2 and 3, the intercepts at the x, y axes are not what is expected, even when allowance is made for the difference in AB and Vega magnitude systems.
Panel f.) This plot shows there is almost no dependence on r magnitude (KIS or KIC) in the color difference (g-r)_kis - (g-r)_kic.Summer 2014 Plots KIS-KIC comparisions with "gclass = 1" extended sources removed:
Show/Hide Previous Work and Plots (2012)