Introduction

The reference for the AB magnitude system is Oke, J.B. and Gunn, J. 1983, ApJ, 266, 713O, and for the Vega system Johnson & Morgan (ApJ, 117, 486, 1953J). The table below lists the magnitude system for each survey in the Kepler Colors Table.

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.

Magnitude Surveys

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.

We have revised the September release of this page with "UPDATES" marked "Fall, 2012" to reflect our analysis of SDSS General Release (SDSS/DR9) data for the Sloan u, g, r, i, and z magnitudes. In late 2012 or early 2102 we intend to include part 2 of KIS, and PanStarrs as these catalogs become available.

It is important to add that although our matching radius for SDSS targets is only 1", it is clear that many matches, e.g. to the KIC, should be made out to 2". Additions to proposers' target list of SDSS objects that are non-KIC objects should be checked carefully against nearby KIC targets for this reason. As with the GALEX matchings we decided maintain a smaller "gold standard" radius to avoid false matches.

Filter transmissions

SDSS Project (ugri) link here.    

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

Graphs (magnitude scattershots)

Plot 1) u (u_kic) vs. U_kis :

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.

UPDATE (Fall, 2012): A scattershot plot of KIC objects with the latest Sloan survey release, SDSS/DR9, shows once again that serious problems exist with the U magnitudes between the two surveys. The linear regression between u_kic and u_sdss is described by a large intercept of 6.09 magnitudes (at m_{u_kic} = 0) and a slope of 0.62. The ranges for KIC and SDSS magnitudes are, respectively, 12--19, and 14--18. The SDSS Project has documented a red leak for its U filter, whereas no information exists on this point from the KIC Project.

Plot 2) g_(kic) vs. g_kis :
We find a offset of of -0.03 magnitudes. However, when one takes account of the zeropoint differences between the KIS (Vega) and KIC(AB) systems, as given by Gonzáles-Solares et al. (2011, MNRAS, 416, 927) as:

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

Here AB referred explicitly to observations by the SDSS/DR9 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 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 below 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.

UPDATE (Fall, 2012): A comparison of g_kic with g_sdss magnitudes in "Plot 2b" (below) shows an intercept at g_kic = 14 of 14.0 mags, i.e., no offset. Because of the asymmetrically distributed scatter we estimate the error on the intercept to be ±0.05 mags. In addition, notice that a "flange feature" in the upper right of the diagram mimics the feature in Plot 2. This tells us that many of the KIC magnitudes in the g_kic 18-21 arise from systematic errors in the KIC survey exist (the KIC does not provide estimated magnitude errors). Similar features are found in the comparisons for the KIC r and i (but not z) magnitude figures, Plots 3 and 4. On the other hand, a second flange, the scatter above the regression line in the magnitude range 12-14, is not present in Plot 2b as it is in Plots 2-4, indicating just as clearly that the fault seems to lie with the SDSS magnitudes; the errors for these particular objects are much larger than the errors the SDSS pipeline indicates.

Apart from these features, other investigators and we have found (g-r) color dependences in the differences between KIS and SDSS magnitudes (this seems to be true for at least r and i filter magnitudes as well as g). Our investigation of the differences between g magnitude distributions in any pair of the three Sloan filter surveys - KIC, KIS, and SDSS itself - confirms a color dependence among all three. Thus, it appears that any of the surveys suffers a color dependence relative to any of the others. The worst case again seems to be for the SDSS system, which shows both a larger slope with color and an asymmetric distribution about the mean. The Kepler team's Working Group on Stellar Properties is looking further into these issues.

Plot 2b) g_(kic) vs. g_sdss :

Plot 3) r vs. r_kis :
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 González-Solares et al. mean relation: r_KIS(Vega)   =   r(AB) - 0.144 - 0.076 × (r(AB) - i(AB)).

UPDATE (Fall, 2012): The mean regression line for r_kic with r_sdss in the magnitude range 11-18 has a near unitary (0.988) slope. Because of the asymmetrically distributed scatter an intercept at r_KIC = 14 of -0.04 mags. in the regression relation is probably statistically insignificant. Our remarks for Plots 2 and 2b concerning two "flange" features also apply here.

Plot 4) i vs. i_kis :
Again the seemingly large offset of -0.48 magnitudes is largely due to the zeropoint difference between the KIS (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.

UPDATE (Fall, 2012): The mean regression line for i_kic with i_sdss magnitudes has a slope of 0.963 between magnitudes 11 and 16. Because of the asymmetrically distributed scatter an intercept at i_kic = 14 of 14.02 mags. in the regression relation is probably statistically insignificant. The remarks for Plots 2 and 2b concerning two "flange" features apply here.

Plot 5)  z_kic vs. z_sdss :
Plot 5 shows that the faint magnitude "flange" feature in the earlier plots, and attributed to KIS magnitude problems is not present for the z magnitudes. On the other hand, the upper left flange, attributed to KIC problems above, is still present in this filter and The departure of the slope in this diagram from 1.0 is probably negligible. The offset at z_KIC = 14.0 is zero.

Plot 6) U_kis vs. U_UBV   (where U represents U_UBV):
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. The authors report (Steeghs, priv. comm.) that they believe such differences are caused by incomplete evaluations of different throughput and transmission curves (including red leaks) in the two systems. These are particularly difficult to evaluate for the ultraviolet filters because of the lack of extensive standard star data and of course the dependence of the U-filter results on observing conditions. It is still not clear which survey, UBV, SDSS, or KIS provides the best U (or u) magnitudes.

UPDATE (Fall, 2012): A comparison of U_sdss with U_UBV magnitude regression exhibits a slope of 1.00. However the intercept is -0.84 magnitudes. We have no previous published U vs. U calibration to refer to, but we infer this large offset is mainly a consequence of the different zeropoints for the AB and Vega magnitude systems, respectively (differences in red red leaks between the filters could also be a factor). Likewise, we have no other valid U vs. U plots to cite concerning the the dropoff in U_UBV at the faint and especially bright magnitude ends of our U_sdss vs. U_UBV plot (see tarball file).

Plot 7) g_kis vs. B   (where B represents B_UBV):
This scatter plot shows an excellent general correlation of the magnitudes in the blue-green filters of 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.

Plot 8) r_kis vs. V (where V represents V_UBV):
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

UPDATE (Fall, 2012):   Although (g-r) color plots have been particularly worrisome for the KIC and KIS surveys, the correlated scatter properties are much worse for the SDSS (g-r) color versus the other surveys, as indicated by the discussion on the "flange" features in the SDSS magnitude plots noted above. However, when these features are filtered, the mean regression relations are not bad. For example, the intercept and slopes in the (g-r)_kic vs. (g-r)_sdss plots are -0.03 mags and 1.02, respectively. For a mean KIC (g-r) color of 0.6, this corresponds to an offset of -0.014 mags., which is negligible. If the flange features in the upper regions of the g, r magnitude plots are not filtered, broad and heavy banded features appear in the (g-r) plots. These distributions are broad and asymmetric around the mean relation and are a signal that faint stars or other groups of stars with poor SDSS errors have not been properly eliminated from the sample. (This is certainly still true in our SDSS color plots with i filter data.)

Plot b) (g-i)_kic vs. (g-i)_kis :
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.

UPDATE (Fall, 2012): The corresponding (g-i)_kic vs. (g-i)_sdss plot exhibits a derived regression slope of 1.027 and intercept of -0.016 mags. for a (g-i)_kic color of 0 (an intercept of 0 at a mean color of 0.6 mags.). However, there is a large secondary scatter around this relation, indicating that we have not yet isolated the population of moderately bright (< 16th magnitude) in our comparison.

Plot c) (r-i)_kic vs. (r-i)_kis :
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.

UPDATE (Fall, 2012): The corresponding (r-i)_kic vs. (r-i)_sdss plot exhibits grave enough problems to state that a mean regression line is worthless. The scatter plots shows two dense concentrations of points, with the secondary one being only much slightly redder in (r-i)_kic and much redder in (r-i)_sdss. Users should probably refrain altogether from using (r-i)_sdss for color selections of targets. Users are referred to the plot named ri_risdss.png in the tarball.

Plot d) (U-g)_kis vs. (U-B)   (where U and B refer to UBV):
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.

UPDATE (Fall, 2012): The (U-g)_SDSS vs. (U-B) plot, limited to stars brighter than g_SDSS < 18.0 mag, and with error cuts in u_SDSS and g_SDSS < 0.1 mags., exhibits a slope of 0.87 and an intercept at color (U-g)_SDSS = 0 of -1.08 magnitudes gives a good mean regression relation but values of the slope and intercept which differ substantially from the empirical relation from Jester et al. from the d-b) (U-g)_sdss vs. (U-B)   (where U and B refer to UBV):

UPDATE (Fall, 2012): The corresponding (U-g)_sdss vs. (U-B) plot exhibits an intercept of -1.08 mags. and a slope of 0.87. The Jester et al. 2005 predicts values of -0.88 mags. and 0.78, respectively. Perhaps these differences can be attributed to the apparently poor U_sdss magnitudes discussed in the UPDATE for Plot 1. As for Plot d, the causes for these different coefficients are not yet known, but there are a few reasonable suspects. First, the Everett et al. (2012) note that their calibrations for the U (and also V filters) are tied to the KIC T_eff scale, which according to Pinsonneault is in error. These errors are particularly severe for hot stars, for which the contribution in the U-bands are substantial. A second suspect is that the filter tranmission passbands are especially different for the U filter in the SDSS and KIS (RGO U filter) surveys.

Plot e) (g-r)_kis vs. (B-V) :
This plot shows almost a perfect regression relation an intercept of 0.02 mags. and a slope of 0.97. 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 DR9 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).

UPDATE (Fall, 2012): The mean regression for the (g-r)_sdss vs (B_V) color diagram gives intercept and slope values of 0.20 mags and 0.86, respectively. The departures from a 1:1 are due to the different filter bandpasses and also the different magnitude systems used (Vega va. AB). The Jester et al (2005) relation suggests values of 0.22 mags. and 0.98. The scatter in the plot in the tarfile is suppressed because by necessary filtering of the objects comprising the "flange features."