To derive calibrated photometry, robust
background subtraction is a necessary and familiar step.
Source photometry is affected by a number of backgrounds contributors,
both celestial and instrumental
backgrounds. During nominal science data collection, pixels designated
as background are measured in addition to target pixels. These data
are used to correct the photometry within the Photometric Analysis
In addtion to correction of source photometry, measures of
the spatial and temporal background flux are by themselves an important
science topic, with a long and rich pedigree (e.g., the CMBR).
Backgrounds at many wavelengths are the sum of a number of processes;
peeling back each layer has provided valuable insight onto cosmic
evolution [ultraviolet: Bowyer, AnnRev 29, 59 (1991); Henry, AnnRev 29,
89 (1991); infrared: Hauser & Dwek, AnnRev 39, 249 (2001); soft x-rays:
McCammon & Sanders, AnnRev 657, (1990)]. Kepler provides a continuous
measure of the celestial background in a wide optical/near-IR band over
a 100 square degree FOV for 3.5 years nominal mission length.
Due to Kepler's unusual operational strategy,
applied for background removal differs from the most common approach
used in astronomical applications. Backgrounds are usually estimated
from a annulus of pixels immediately adjacent to the source, scaled
to a common area. Kepler does not use this approach, rather a separate
set of background apertures are collected across the focal plane, and
a background measure derived from these pixels. GOs should note that
the pipeline does not use pixels within the source aperture to
measure the local background.
Celestial backgrounds arise from a
number of sources, both from spatially smooth, diffuse light which
affects all pixels in the aperture, and from transient events,
which affect a few pixels.
Zodical light, produced by sunlight
scatered from dust in the ecliptic plane.
Diffuse scattered starlight, produced
by dust in the Galaxy. Due to Kepler's large FOV, the Galactic component
shows a spatial gradient, increasing at lower galactic latitudes.
Unresolved starlight. Given the 4x4
arcsecond dimensions of the
pixels, some light in the aperture arises from faint field stars.
As with the diffuse Galactic emission, the contribution from
unresolved starlight increases with decreasing Galactic latitude.
Cosmic ray impacts which corrupt
individual pixels. The pipeline
software flags and removes cosmic ray events from the pixel counts,
within the Photometric Analysis module.
Each cosmic ray event is replaced with a temporally local average of
the pixel's time series without the cosmic ray pixel events.
Surrounding sources. A type of
background which will affect
source photometry are the residual wings of the PSFs produced by
nearby stars, which may overlap the PSF of the target. Observers
may visualize this effect by considering an image taken with poor
seeing (equivalently poor focus). Faint PSF wings of surrounding
sources will fall across the target, requring either PSF-fitting
photometry or some form of aperture correction. A particular
challenge for Kepler observers is that the user does not receive
an entire image of the surrounding field, just the target pixel
image. Stars outside of the this aperture may affect the target
photometry, even though those stars were not actually measured.
Also remember that nearby sources may be variable,
producing a time-dependent background.
A correction for contaminating flux
in the source aperture produced by surrouding sourcs is applied
within the PDC pipeline module
- a single valued subtraction, termed the crowding metric.
In ideal situations the PSFs of neighboring stellar sources would
not change over time, expect for possible intrinsic source
variabilty. Observers should be aware that spacecraft operations
may induce changes in the source PSFs, through focus changes, and
spacecraft motions (jitter and drift). Motion of the source center
during an observing season, even at the millipixel level, will
induce variations in the contaminating flux, introducing small
levels of noise into thelight curve.
In preparing proposals, GO should choose sources as isolated as
possible, and be aware of the constraints for achieving
excellent time series photometry under strong crowding situations.
include the detector bias level (also termed black level),
which is removed in the CAL
pipeline module, some anticipated issues, such as scattered
light, unexpected electronic issues discovered during pre-flight
characterization of the detectors, and some features seen during
early flight operations, e.g.,v"argabrightening",
an anomalous full-field illumination, whose origin is under
investigation by the instrument team
(Jenkins etal 2010). Spatially varying backgrounds
produced by the detector electronis are full described in the
Kepler Instrument Handbook
(KIH). The Kepler project is investing considerable effort to
identify, assess, and develop corrections for instrumental signatures
discovered during the early phase of data collection. Additional
details on the spatially-varying image artifacts, and
their corrections can be found on the GO
Data Artifacts page.
MEASUREMENT & REMOVAL
Kepler constructs a background flux map using a set of
target pixels specifically assigned for this purpose. Background
"targets" are small (nominally 2 x 2 pixels) postage stamp images which measure
the background signal in the long cadence observations. Since backgrounds
must be estimated and removed from all observed sources in the FOV, a method
was adopted to interpolate background values from all targets pixels in a channel
using the background apertures in that channel. A maximum of 1125 background
apertures and 4,500 pixels (~4 pixels per target) are allocated for each of the
84 output channels. These background targets are selected to optimize a 2D polynomial
representation of the background flux distribution, derived separately for each
For robust fitting, background apertures should be
uniformly distributed on each CCD array. To mitigate edge effects, more background
apertures are positioned at the frame edges. During target management prior to each
quarter, background pixels are selected, avoiding stars and locations affected by