Quality Error Bit Flags
This page details the quality error bit flags assigned during post processing. These
flags appear as the ppErrBits attributes in the detection and source tables. The flags can be
used to refine object samples extracted from the archive. Examples of this usage are given in the
Bit flag design
ppErrBits is a 32-bit (4-byte) integer in every detection table (e.g.
vhsDetection) that contains
the quality error information for each source detection encoded as a bit flag, according to the prescription given in the table below.
So the bit flag will give a 'yes' or 'no' answer to up to 32 different quality issues for every source detection. The
ppErrBits attribute from the detection table is propagated into the relevant passband ppErrBits in the corresponding source table
during source merging. For example the ppErrBits flag in vhsDetection for Y-band observations is propagated into yppErrBits in
The quality issues are listed in order of severity, such that a selection of sources can be easily filtered according to the level
of quality the user desires. These quality issues are divided amongst the four bytes such that the least significant byte represents
information about the source that is most probably harmless (such as it is a deblended source), the most significant byte contains
bits that highlight some kind of severe warning about the quality of the source, and the two remaining bytes in the middle
contain various warnings that could just possibly imply the source is spurious.
For example, to select only those sources with absolutely no quality issues
the user can filter on
ppErrBits = 0, and to include sources with only minor
quality issues the user can filter as
ppErrBits < 256 (i.e. where only the
first byte contains 1s, so that is a range up to 2^8). Alternatively, the user
could use hexadecimal bit masks if they prefer, which will also allow them to
ignore the list of priorities and pick out specific quality issues that they
are interested in.
Bit flag table
The following different quality issues are implemented VSA public
survey release databases:
Source image is deblended
This is determined from the
detection attributes, all of which should be -1 if the source image is
Bad pixel in default aperture
These sources have at least one bad pixel in the default aperture, so contain
missing information. The total number of bad pixels in the default aperture is
supplied in the
errBits attribute (for all surveys that use
VDFS-generated catalogues), which can be used to refine the criterion.
Low confidence in default aperture
These sources are from pixels with an overall low average confidence level
within the default aperture. The level was arbitrarily set at
averageConf < 80
based on visual inspection of the distribution. (Only applied to v1.1 and newer VISTA data).
These sources come from regions of the tile that includes data from detector 16,
which is known to give poor photometric results. (Only applied to v1.1 and newer VISTA data).
Saturated source image
Source image contains at least one pixel that is close to being saturated,
defined as having a count in ADU that is greater than 90% of the average
saturation level of the frame (
pHeight + sky > 0.9 * avSaturLevel).
Such detections have derived fluxes and magnitudes that are corrected for saturation.
This flag is applied to VVV detections that come from multiframes with a
poor photometric calibration due to heavy and highly variable extinction in
Galactic centre fields. A detector level criterion is applied to all frames in
Unknown at present
Source lies within a dither offset of the frame boundary
Source lies within a jitter + microstep offset of the stacked frame boundary with an additional safety margin of twice the default aperture radius to include all cases where the source image is incomplete. This is an important warning because all of these sources should certainly contain missing information - i.e. there may be partial sources and/or the source may not be present in all component images of the stack.
For deep stacks we also consider any positional offsets in creating the final deep stack image from intermediate stacks.
Source lies within the underexposed strip (or "ear") of a tile
Source lies outside the fully exposed area of a tile defined by the dimensions
provided at http://www.vista.ac.uk. Note
that the x & y axes given at this website are the opposite way around to those
in the real tiles.
Source lies within an underexposed region of a tile due to missing detector
When constructing a deep tile some regions may not be of a consistent exposure
due to the component deep pawprints losing detector frames through quality
control. This flag is for detections in regions made from these underexposed
Bright tile detection, but no detections in pawprints
Detections brighter than the 5-sigma detection limit of the frame are found in tiles, but not on the pawprints, and are
therefore likely to be spurious. This flag was introduced because a nebulisation error produced a strange bright streak
on a VMC tile in VMCv20171101. A small number of other weird sources were found that could be removed using this flag.
Possible future flags
Source close to bright star
Bright star halos generate many spurious sources.
Detection close to dither edge
Edge effects occur in images around the dither edge leading to spurious detections. This is a very low importance bit flag as there many sources close to the dither edge that are genuine, but it is still useful information to have. Therefore this could be the lowest significant bit.
Spurious sources occur in lines along diffraction spikes.
WFAU, Institute for Astronomy,
Royal Observatory, Blackford Hill
Edinburgh, EH9 3HJ, UK