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Throughout this section, we have listed various known problems with
the processing, many of which will be fixed in future releases of the
data. We now list problems that were not otherwise mentioned
earlier.
- There are several quantities that have not yet been finalized,
and are currently place-holders in the pipeline outputs. These
include error estimates for PetroR50 and PetroR90, all
errors associated with isophotal quantities, a
texture parameter to measure the small-scale roughness of an
object, and fracPSF, the fraction of light included in the PSF.
Quantities which are not calculated are designated -9999 in the
database, while errors which are not calculated are designated
-1000. Note that some quantities are not calculated, e.g., for
objects very close to the edge of a frame.
- We have not done a thorough testing of every one of the outputs
of the pipeline. Quantities such as psfMag and petroMag,
which are used in target selection (§ 4.8),
are very extensively tested, while quantities such as the parameters
of the elliptical isophote fit, or the errors in the model fit
parameters, have not been as carefully vetted.
- Due to difficulties in cosmic-ray rejection, one should be
suspicious of objects detected in only one band. This is particularly
problematic in z, where the occasional cosmic ray can splatter in
the thick chip to be indistinguishable from the PSF (see the
discussion in fan01b). Future releases
of frames will set a flag MAYBE_CR to flag ambiguous
cases.
- Electronics cross-talk can give faint ghosts of saturated
stars in z. Draw a line down the central column of the CCD; the
ghost will appear at the mirror position relative to this line. These
will be flagged in future releases.
- The code does not subtract diffraction spikes from stars
(although bleed trails are interpolated over).
These will be subtracted in future releases.
- There are a variety of faint ghosts due to reflection off the edges of
the filters. Thus one should be suspicious of the reality of very low
surface brightness (fainter than 24 mag per square arcsec in r)
extended features close to the edges of frames, especially in the u
band.
- Similarly, the deblender occasionally pulls off low surface
brightness features from large galaxies. This problem is much
reduced with improvements to the deblender in subsequent versions of
the pipeline.
- The proper motion errors are occasionally underestimated (yielding a
few quasars with nominally significant proper motions!). This is due
to uncertainties in the astrometric offsets between bands, and will be
fixed in future releases. However, overlapping scans have shown that
the proper motion errors for the majority of asteroids are overestimated
by roughly a factor of two.
- Ghosting in the u filter causes systematic errors in the
flat-fields in some of the runs, affecting the photometry by up to
10%. This is discussed in detail in § 4.5.3 below.
Finally, a disclaimer: we have tested the outputs of the pipeline to
the best of our ability, but there are no doubt subtle problems not
listed here that we are currently unaware of. For example, many of the
quoted error estimates have not yet been tested adequately. We are
eager for feedback from the community if any problems are found.
Next: Photometric Calibration
Up: The Frames Pipeline
Previous: Measurements of Shape and
Michael Strauss
Thu Jan 30 11:15:34 EST 2003