The primary instrument aboard Kepler is the focal plane array consisting of 21 science and 4
Fine Guidance Sensor CCD modules. Field flattener lenses on each module map the spherical
telescope image surface onto the flat CCD chips, and define the overall wavelength bandpass.
Each science module is an array of 2200 by 2048 pixels. These 21 modules each have 4 output
channels, for a total of 84 channels and 94.6 million active pixels that view the sky, with
additional masked real pixels and virtual pixels for collection of collateral data.
The shape of the bandpass, described below, was chosen to contain most of the optical spectrum.
This choice maximizes the sensitivity of the telescope + detector combination for detecting
planets transiting solar-type stars. Kepler contains no "true" filter, in the sense
that HST imaging instruments include filter wheel assemblies with multiple, specifically-defined
bandpasses. Here, the intent is to utilize the entire optical range, except for the short
wavelengths, which were truncated to avoid chromspheric emission lines in solar-type stars.
The photometer provides no color information, but does provide excellent depth. Kepler's
wide-band images are similar to the clear filter frames taken with HST/STIS.
To achieve maximum sensitivity, the Kepler bandpass is wider than the
typical broad-band filters commonly used in optical astronomy (e.g. Johnson
UBVRI, Sloan ugriz). The absolute Kepler sensitivity curve is
displayed in the clickable figure (Fig 2) to the right. The displayed response
curve was derived during pre-flight testing, and represents the laboratory
calibration of the Kepler photometer. Links to the tabulated values for the
wavelength response are provided at the top of the page in both high- and
low- spectral resolution forms.
Figure 3: Optical element components of the Kepler Instrument Response compared
to approximate M5 and G2 stellar spectra.
The total photometer spectral response is a combination
of the transmission functions of all optical elements, including the Schmidt
corrector, the primary mirror assembly, the field flatterner lenses
on each CCD module, and the wavelength dependent quantum efficiency
of the detectors (Fig 3). The front surfaces of the field flatteners are
anti-reflection coated; a bandpass filter coating was applied to the back
surfaces. This bandpass was chosen to minimize the effects of stellar
variability in the near-ultraviolet (λ ≤ 420 nm), especially the
Ca II H & K emission lines, which would impact exooplanet transit
detection. At long wavelengths the coating was designed to minimize fringing.
Fig 4: Comparison of the Kepler, MOST, CoRoT and Johnson response curves. Kindly
provided by Jason Rowe (NASA Ames) and extracted from
Fig 4 compares the Kepler response function with those of two similar
missions: the MOST spacecraft, and the
CoRoT mission. The response functions for all
three experiments are displayed from 4000 to 9000 Å. The MOST bandpass bandpass is
marked by the dashed line, the Kepler bandpass is shown in black and the CoRoT bandpass
is shown by the dot-dashed line. The transmission functions for the Johnson B,V,R,I
filters are shown from left to right in blue, green, red and magenta respectively
and have been scaled to peak at 100% transmission. The spectrum for an A2V star is shown
in cyan, which peaks in the UV and the spectrum for a M2V star is shown in orange which
peaks in the infrared. The two spectra have been scaled to have equal flux in the Johnson
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