CALIBRATION   (CAL)

OVERVIEW

The initial step in the Kepler science data pipeline is performed by the software module termed CAL . CAL converts raw data numbers into calibrated pixels, which are then passed to the Photometric Analysis module for compilation into a light curve. The Science Operations Center at Ames receives pixel data (Haas etal 2010) from the Data Management Center at STScI. Data from each Kepler CCD is formatted as FITS files, including collateral pixel data, collected for calibration. CAL operates on both the 30 min (long cadence) and 1 min (short cadence) observations, as well as on the full frame images. Users of CCD data will be familiar with most functions of the calibration module, however we note a few aspects peculiar to the operational modes of Kepler. In general, most users of Kepler photometry will not work directly with raw counts, so the information provided here is to inform users on the the process by which calibrated pixels are generated.
 

DOCUMENTATION

The purpose of this webpage is to provide a brief guide to CAL functions. GOs and archival users are urged to examine the primary documentation, and look on this page for discussion of specific issues. Primary documentation includes:

• Kepler Instrument Handbook, Version 1, 15-July-2009.

• Release Notes, which provide detailed information for each data release, either initial or reprocessed data per quarter.

• Jenkins etal, 2010, "Overview of the Kepler Science Processing Pipeline".

• Caldwell etal, 2010, "Instrument Performance in Kepler's First Months".

• "Pixel-Level Calibration in the Kepler Science Operations Center Pipeline", by E. Quintana etal, SPIE Conference on Astronomical Instrumentation, June 2010 (paper available July 2010).

CAL FUNCTIONS

CAL performs a number of functions familiar to CCD photometrists to transform raw data counts into calibrated pixels, operating on a single CCD channel at a time. Detailed models of each CCD were developed during pre-flight testing, and are combined with full-frame images obtained during the commissioning perior both before and after ejection of the aperture cover. Each CCD channel consists of 1070 rows by 1132 columns, of which only a subset (1024 x 1100) is used for photometry.

The focal plane CCD models are applied within CAL to execute the following tasks:

1. Bias level
   The CCD bias level is determined using collateral pixels obained with each CCD read. Users should note that the term "black level" is used in much of the Kepler documentation, as a synonym for the more commonly used "bias level". The bias level contains both a 2D map, and an additional 1D "dynamic" bias correction. Once the 2D black level is removed, a fit to the residual bias is used to estimate a 1D black correction. Note the difference between Kepler's bias subtraction method, and the approach commonly used for shuttered, non-continuous CCD operation, in which the user takes separate bias frames, before, during and after data collection.
    

2. Dark current
   Dark current is estimated from the masked and virual smear pixels. Since the focal plane is maintained at −85 C, the effective dark current is essentially zero. (Caldwell etal 2010).
    

3. Smear
   Kepler observes continuously, with no shutter; therefore stars illuminate the CCDs during readout. This "staring" mode produces trails along columns that contain stars, as charge is smeared out during the read. Each pixel in a given column of the image receives the same smear signal. These values are typically small, since each pixel only “sees” a star for the readout time, 520 milli-seconds, divided by the number of rows 1070. The smear level corrction in each image is determined using the masked and virtual smear pixels set aside for this purpose, as seen in the figure above.
    

4. Gain
   The gain function associates observed photoelectrons (e-) to the analog/digital units generated by the A/D converter. Gain is the average slope of the transfer function, and has a median value of 112 e-/ADU across the focal plane (Caldwell 2010). A measure of the deviation from the linear transfer function is estimated at each ADU signal level; this nonlinearity model provides appropriate corrections.
    

5. Undershoot
   An issue noted during pre-flight testing can be described as a large, signal-dependent trailing undershoot in the image, traced to an amplifier in the local detector electronics. An undershoot model is applied to correct affected pixels.
    

6. Flat field
   A flat field correction is the last step in CAL, applied to photometric pixels to correct for spatial variations in pixel sensitivity to a uniform light source. CAL uses a local flat field, also termed the pixel response non-uniformity (PRNU) map. The PRNU image maps each pixel’s relative brightness variation from the local mean, expressed in percent (Van Cleve & Caldwell 2009). The median standard deviation of the pixel values in the PRNU image across the focal plane is 0.96 %. The flat field was developed during ground testing prior to launch; there are no separate flatfield exposures obtained on orbit.
    

   CAL produces the basic photometric product of Kepler: a series of calibrated target pixel images within a pre-set aperture. The photometric time series is constructed from these images, in the next step, Photometric Analysis, or PA.