Science observations are taken at one of two timing settings: long (30-min) or short (1-min) cadence.
There are 94.6 million pixels within the CCD detector array across the focal plane. Data collected by 94% of those pixels cannot be saved because of hardware storage limits onboard the spacecraft and the bandwidth of communications between the spacecraft and Earth. Instead, data for specific targets are saved onboard as subimages every minute or every 30 minutes. The number of pixels in each subimage depends approximately on the brightness of the target therein. For example, 12th magnitude point sources in the Kepler bandpass (Kp = 12) have an average mask area of 32 pixels. The pixel size can be increased if necessary to accommodate extended objects. The 6% of stored pixels deliver time-tagged sub-images for 160,000-170,000 targets every quarter of operation.
The instrument has neither changeable filters
nor dispersing elements. The one Kepler photometric bandpass has a half-maximum transmission
range of 430 to 840 nm. The detector has an image scale of 3.98
arcseconds per pixel. The image quality varies with position in the focal plane,
with 95% encircled energy diameters ranging from 3.1 to 7.5 pixels,
with a median of 4.2 pixels. The percentage of point-source flux
concentrated in the brightest pixel is between 20% and 62%, with a median
value of 45%. Because of a very stable focal plane, the relative centroid positions of stars yields a precision better than 1 milliarcsecond over periods
up to three months. Target photometry (i.e. time-tagged light curves) are produced by software pipeline. The Kepler exoplanet survey has been designed for performance across targets 9 < Kp < 15 stars, but the potential magnitude range for astrophysics is broader. Neglecting sources of astrophysical noise, Kepler will achieve a benchmark photometric
precision of 80 parts-per-million (ppm) on a Kp = 12 G2V star in 30 minutes of integration.
A Kp = 20 target yields a photometric precision of 6% over the same integration time. Targets brighter than Kp = 11.5 are saturated on the detector but photometric precision is recoverable, albeit expensive in terms of pixels, for targets as bright as Kp = 4. Sub-images and target light curves are archived at the MAST
Kepler operations are divided into four quarters each year, separated by the quarterly rolls of the spacecraft. A new target list is uploaded to the spacecraft before every quarter. Long cadence targets can be removed or added to the target list every quarter. Short cadence targets can be removed or added to the target list every month. Kepler data are separated by the same scheme. Long cadence images and light curves are stored in files that span quarter. Short cadence images and light curves are stored in files that span a month.