PHOTOMETRIC ZEROPOINT

Kepler's primary mission of precision transit photometry requires differential photometric ability only. Absolute photometric calibration is not essential to the success of the exoplanet mission. A community-led program is underway to flux calibrate the Kepler instrument in-flight for the GO program. Although caveats are created by the broad 4,300 − 9,000 Å spectral response of the Kepler instrument, zeropoint calibrations for standard photometric systems will appear as updates on this page over time.

KEPLER MAGNITUDES

Each Kepler target has a pre-set observing aperture uploaded to the spacecraft. These apertures are defined in terms of the number of pixels and shape of the array. The brigter a source, the larger the aperture needed to collect the photons for an optimal detection of that source. Aperture size is primarily defined by the source's Kepler magnitude (Kp), a measure of the source intensity as observed through the wide Kepler bandpass.

• The Kepler Science Team conducted an extensive observing program prior to launch in order to classify stars in the FOV. The fundamental goal was to develope a list of FGKM dwarf stars as the primary source list for exoplanet detection. Objects were observed in the SDSS griz bands. This photometry, along with 2MASS data form the basis of the Kepler Input Catalog.

• No observations were obtained from the ground using a pseudo-Kepler band filter. The Project constructed a set of stellar spectral synthesis models covering a range of effective temperature, gravity and mean abundance, and derived g,r,i,Kp magnitudes by convolving the filter response functions with the models. Using correlations between these values, Kepler magnitudes are estimated from the observed SDSS magnitudes using empirical formulae.

• Most proposers will adopt the Kp values directly from the KIC. Exceptions are strongly variable stars, in which a magnitude range should be provided as described on the Target Checks page, and sources lacking a Kepler magnitude. For the latter, an approximate esimate of Kp can be derived using the following exxpression, which is based on the empirical relations used by the Kepler Stellar Classifiction Program.

If only B,V magnitudes are available, the user can convert B,V into SDSS g,r using the transformation derived by Smith etal (ApJ 123, 2121, 2002, Table 7). The expression below uses this transformation equation. If SDSS g,r values are available, just use the conditional statements in line 3 and 4 below.

This expression is accurate to about ±0.2 mag for stars hotter than 3500 K. For M stars, users are cautioned that systematic errors may exceed 0.6 mag, in the sense that Kp returns magnitudes that are too faint.

Derivation of Kepler magnitudes will be detailed further in a paper describing the Stellar Classification Program, and the generation of the Kepler Input Catalog. In the interim, interested users may examine the KIC algorithms document.

Given a calibration of (B−V) color with spectral type (effective temperature), a Kepler magnitude can be estimated. Values for (Kp−V) are presented below for main-sequence stars, based on the above relation. This color can be applied to stars whose apparent Johnson V magnitude is known to obtain Kp. Again, note that for stars cooler than M0, the estimated Kp may be too faint.

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