Measuring and recreating the sky value
How Sky Values are Measured
It is quite clear what astronomers mean by `sky': the mean
value of all pixels in an image which are not explicitly identified as
part of any detected object. It is this quantity which, when
multiplied by the effective number of pixels in an object, tells us
how much of the measured flux is not in fact associated with the
object of interest. Unfortunately, means are not very robust, and the
identification of pixels not explicitly identified as part
of any detected object is fraught with difficulties.
There are two main strategies employed to avoid these difficulties:
the use of clipped means, and the use of rank statistics such as the
median.
Photo performs two levels of sky subtraction; when first processing
each frame it estimates a global sky level, and then, while searching
for and measuring faint objects, it re-estimates the sky level locally
(but not individually for every object).
The initial sky estimate is taken from the median value of every pixel
in the image (more precisely, every fourth pixel in the image),
clipped at 2.32634 sigma. This estimate of sky is corrected for the
bias introduced by using a median, and a clipped one at that. The
statistical
error in this value is then estimated from the values of sky determined
separately from the four quadrants of the image.
Using this initial sky estimation, Photo proceeds to find all the
bright objects (typically those with more than 60 sigma detections).
Among these are any saturated stars present on the frame, and Photo
is designed to remove the scattering wings from at least the brighter
of these --- this should include the scattering due to the
atmosphere, and also that due to scattering within the CCD membrane,
which is especially a problem in the i band. In fact, we have chosen
not to aggressively subtract the wings of stars, partly because
of the difficulty of handling the wings of stars that do not fall on
the frame, and partly due to our lack of a robust understanding of the
outer parts of the PSF . With the parameters
employed, only the very cores of the stars (out to 20 pixels) are ever
subtracted, and this has a negligible influence on the data. Information
about star-subtraction is recorded in the fpBIN files, in
HDU 4.
Once the BRIGHT detections have been processed, Photo
proceeds with a more local sky estimate. This is carried out by
finding the same clipped median, but now in 256x256 pixel
boxes, centered every 128 pixels. These values are again debiased.
This estimate of the sky is then subtracted from the data, using
linear interpolation between these values spaced 128 pixels apart; the
interpolation is done using a variant of the well-known Bresenham
algorithm usually employed to draw lines on
pixellated displays.
This sky image, sampled every 128x128 pixels is written out to
the fpBIN file in HDU 2; the estimated uncertainties in the sky
(as estimated from the interquartile range and converted to a standard
deviation taking due account of clipping) is stored in HDU 3. The
value of sky in each band and its error, as interpolated to the center
of the object, are written to the fpObjc files along with all
other measured quantities.
After all objects have been detected and removed,
Photo has the option of redetermining the sky using the same
256x256 pixel boxes; in practice this has not proved to
significantly affect the photometry.
Last modified: Tue Apr 15 12:00:12 CDT 2003
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