Colors/magnitudes: explanations and caveats

Version of September 19, 2012

Prologue

MAST is in the process of adding new ground-based survey colors and magnitudes for the Kepler Field of View; see below. Our executive summary is: for optical wavelengths that the UBV survey should be preferred over the KIS survey. However, both the KIS and and KIC magnitudes have suspected problems (e.g. at the bright end of the KIS magnitudes and for the (g-r) color; the extent of saturation issues for KIC has not yet been well determined); see the graphs page below. The few u magnitudes given in the KIC have been removed from our Target Search (but not from the still full KIC itself at MAST!) page because they are of poor quality. Users should plot KIC and KIS colors/magnitudes only with care: as they are based on different zeropoint magnitude systems (AB and Vega/Johnson). The reference for the AB system is Oke, J.B. and Gunn, J. 1983, ApJ, 266, 713O, and for the Johnson/Vega system Johnson & Morgan (ApJ, 117, 486, 1953J.

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 using a filter from one KIC/KIS/UBV survey and a second filter from another catalog. Therefore, because we represent no mixed colors, e.g., (g_KIS - r_KIC), the only gr colors represented will be understood to be either gr (KIC) or gr_KIS.

Finally, we make no 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, or to the corresponding nonproprietary arXiv site.

Additional Magnitude Surveys

SURVEY NAME	AREAL COVERAGE
GALEX (NUV, some FUV)	~1/2
2MASS (J,H,K)	(selected objs.)
UK_IRT (J-magnitudes)	full
INT/KIS (U,g,r,i, Hα)	~1/2
UBV (Everett et al.)	full

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 (arXiv) and Everett, Howell, & Kinemuchi 2012, PASP, 124, 316E (arXiv).
At some points in the future MAST plans to bring in new colors from SDSS General Release 8 (Sloan ugriz; perhaps late fall, 2012), Part 2 of the KIS, and PanStarrs.

Filter transmissions

Filter transmission curves may be accessed via the links below for the KIS, Sloan and UBV filters to a table (first three tabular entries):
INT/ING/KIS filters (UgriH_α) link here. Select the Halpha, U (RGO) or g, r, i (Sloan-Gunn) filters in various ascii and image formats.
Similarly:

SDSS project (ugri) link here.

UBV link here. (Select the first three rows in the table.)

Graphs (magnitude scattershots)

1) u (u_KIC) vs. U_KIS Plot:
The u_KIC vs. U_KIS-magnitude scatter plot shows 3-4 parallel sequences each offset from the 1:1 relation with KIS magnitudes but distributed over a range of 1½ magnitudes. For this reason the authors of the KIC catalog have asked MAST to withdraw their u-magnitudes (only 2092 objects) from the Target Search pages. They may still be accessed via the MAST/KIC web page, but we doubt that they are useful.

2) g vs. g_KIS Plot:
We find an offset of -0.03 magnitudes. However, this offset is fully expected because of the zeropoint differences between the KIS (Johnson/Vega) and KIC(AB) systems, as given by Gonzales-Solares et al. (2011) as:

g_KIS(Vega) = g(AB) + 0.060 - 0.136 × (g(AB) - r(AB)).

Here AB referred explicitly to observations by the SDSS/DR8 in other regions of the sky. However, in principle it could apply to other AB-based systems, such as the KIC. If we adopt an average color from our sample = 0.64, the 0.03 mag. offset vanishes; the two systems are in substantial agreement.

This plot and the r and i magnitude scatter plots disclose two other points that are more bothersome. At faint magnitudes (> 16-17, depending on the filter) a faint, nonparallel secondary sequence develops. We will show evidence that the secondary magnitude sequence is a problem with the KIC data. At bright magnitudes (m_KIS < 12) the KIS magnitudes are too bright. Greiss et al. were aware of this problem for <12 KIS objects and ascribed it to partial saturation of images. Note, however, the KIS "class" value for saturation is only used for a minority of objects in the range 10-12th magnitude.

3) r vs. r_KIS Plot,
Likewise, the offset in the r magnitude scatter plot of -0.15 altogether vanishes when an average color = 0.64 for our population is applied to the Gonzalez-Solares et al. mean relation.

r_KIS(Vega) = r(AB) - 0.144 - 0.076 × (r(AB) - i(AB)).

4) i vs. i_KIS Plot:
Again the seemingly large offset of -0.48 magnitudes is largely due to the zeropoint difference between the KIS (Johnson/Vega) and KIC (AB) magnitude systems, namely:

i_KIS(Vega) = i(AB) - 0.411 - 0.073 × (r(AB) - i(AB)).
Given a mean value <(r - i)> = 0.35 for our population, the predicted offset is -0.44, which differs by -0.03 mags. from the value given in our plot.

5) U_KIS vs. U_Johnson Plot:
This figure shows an excellent relation between the U magnitudes in the KIS and UBV/Kitt Peak system. The latter was adopted for the Everett et al. study. There is a shift of about +0.08 (i.e., in the sense, KIS is too faint). This is a sizeable discreprancy. There is reason to believe that the KIS zeropoint is suspect, first, because the authors state that the KIS survey's U filter suffered physical problems that compromised its results. Second, they adopted a zeropoint correction from their g-magnitude system - a guess which cannot be expected to be accurate. MAST will eventually provide SDSS u-magnitude data for the Kepler field which should help decide which is the better zeropoint.

6) g_KIS vs. B Plot:
This scatter plot shows an excellent general correlation of the blue magnitudes in the blue-green filters in the UBV and KIS systems. For example, there is no secondary sequence at faint magnitudes. This shows that the spurious feature is inherent in the KIC g, r, and i magnitudes. In addition, the slope is 1.0, which was also found in a study by Jester et al. (2005, AJ, 130, 873) for AB-based g magnitudes observed in the SDSS survey. In fact Jester et al.'s mean relation is:
B(Vega) = g(AB) - 0.33 - 0.073 × (g(AB) - r(AB)),
where the g and r magnitudes are meant for SDSS but could as well correspond to the KIS survey. Taking a mean value of <(g-r)_KIS> = 0.67, Jester's relation predicts an intercept of 0.42 magnitude, which agrees very well with our value of 0.41. The turn-up of the sequence at bright filters appears is the reflex of the turn-down noted above for the g_KIC vs. g_KIS plot.

7) r_KIS vs. V Plot:
This plot shows another satisfying agreement, on the whole, in both random and systematic errors. Transposing Jester et al.'s relation, one finds:
V(Vega) = r_KIS + 0.42× (B - V) + 0.11,
where all quantities are in the Vega system. Assuming <(B-V)> = 0.56 for our population, we find a close correspondence with our intercept of 0.33 to the predicted offset of 0.35 mags. A turn-up is again visible at the bright magnitude end of the relation, casting doubt on the reliability of KIS magnitudes at the bright end, an issue that Greiss et al. noted.

Simple Color Plots

a) (g-r)_KIC vs. (g-r)_KIS Plot:
Because of the different magnitude systems adopted by the KIC and KIS, this plot shows that the intercept in the (g-r)_KIC vs. (g-r)_KIS plot is nonzero. Somewhat surprisingly, the slope is 0.83, very different from 1.0, in the sense that the KIS color shows a smaller range than the KIC color does. In an earlier preprint of their paper, Greiss et al. showed a dependence of the magnitude difference, (g_SDSS-g_KIC) with color (g_KIS - r_SDSS. This is another way of showing the difference in slope depicted above, and it occurs in the same sense. In this case, the "odd man out" is the (g - r)_KIS color, which suggests in this case that the nonunitary slope in color comes from the KIS and not the KIC. MAST will investigate this further when the SDSS/DR8 data for the Kepler field are ingested.

b) (g-i)_KIC vs. (g-i)_KIS Plot:
This plot shows a slope that is close to but not quite equal to the value of 1.0 we would expect based on the individual magnitude plots, #1 and #3, given above. The nonzero intercept follows from those plots too.

c) (r-i)_KIC vs. (r-i)_KIS Plot:
This plot shows a slope that is closer to unitary than the previous one. The nonzero intercept was discussed in the individual magnitude plots, #1 and #3 above.

d) (U-g)_KIS vs. (U-B) Plot:
Here we plot the UV-blue color in the KIS and Johnson systems. Although they exhibit a decidedly nonunitary slope, 0.80, this is to be expected as this is close to the predicted slope of 0.78 by Jester et al. The offset of -0.24 is quite different from the one found by Jester et al. because they considered the (u-g) color based on SDSS's AB system; (U-g)_KIS and (U-B) are both based on the Vega system.

e) (g-r)_KIS vs. (B-V) Plot:
This plot shows almost a perfect regression relation a slope of 0.97 and +0.02 mags. intercept. We notice that the exact solution depends somewhat on the amount of scatter admitted in the initial solution (here ±0.2 mags.). For example, mall departures from coefficients of 1.0 and 0.0 , respectively, can be induced by including or excluding various amounts of the asymmetric scatter around the mean relation. Although it is yet to be confirmed from SDSS DR8 data, we suspect the asymmetric scatter at the red end of the blue is associated with the unexpected (g-r)_KIS dependence in relation to the (g-r)_KIC noted for Plot a).