OmegaCAM Science Archive

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 SQL cookbook.

Bit flag design
Quality issues
Possible future flags

Bit flag design

ppErrBits is a 32-bit (4-byte) integer in every detection table (e.g. atlasDetection) 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 atlasDetection for G-band observations is propagated into gppErrBits in atlasSource.

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

Bytes	Quality Category	Bits	Detection Quality Issue	Bit Mask	Decimal Threshold
Byte 0 (LSB)	Information	Bit 0	Close to a dither edge (not yet implemented)	0x00 00 00 01	1
		Bit 1		0x00 00 00 02	2
		Bit 2	Close to a bright source (not yet implemented)	0x00 00 00 04	4
		Bit 3		0x00 00 00 08	8
		Bit 4	Deblended	0x00 00 00 10	16
		Bit 5		0x00 00 00 20	32
		Bit 6	Bad pixel(s) in default aperture	0x00 00 00 40	64
		Bit 7	Low confidence in default aperture	0x00 00 00 80	128
Byte 1	Warning	Bit 8		0x00 00 01 00	256
		Bit 9		0x00 00 02 00	512
		Bit 10		0x00 00 04 00	1 024
		Bit 11		0x00 00 08 00	2 048
		Bit 12	Lies within detector 16 region of a tile	0x00 00 10 00	4 096
		Bit 13		0x00 00 20 00	8 192
		Bit 14		0x00 00 40 00	16 384
		Bit 15		0x00 00 80 00	32 768
Byte 2	Important Warning	Bit 16	Close to saturated	0x00 01 00 00	65 536
		Bit 17	Photometric calibration probably subject to systematic error	0x00 02 00 00	131 072
		Bit 18		0x00 04 00 00	262 144
		Bit 19		0x00 08 00 00	524 288
		Bit 20	Possible diffraction spike artefact/contamination (not yet implemented)	0x00 10 00 00	1 048 576
		Bit 21		0x00 20 00 00	2 097 152
		Bit 22	Lies within a dither offset of the stacked frame boundary	0x00 40 00 00	4 194 304
		Bit 23	Lies within the underexposed strip (or "ear") of a tile	0x00 80 00 00	8 388 608
Byte 3 (MSB)	Severe Warning	Bit 24	Lies within an underexposed region of a tile due to missing detector	0x01 00 00 00	16 777 216
		Bit 25		0x02 00 00 00	33 554 432
		Bit 26		0x04 00 00 00	67 108 864
		Bit 27		0x08 00 00 00	134 217 728
		Bit 28		0x10 00 00 00	268 435 456
		Bit 29		0x20 00 00 00	536 870 912
		Bit 30		0x40 00 00 00	1 073 741 824
		Bit 31		0x80 00 00 00	2 147 483 648

Quality issues

The following different quality issues are implemented OSA public survey release databases:

Source image is deblended

This is determined from the aprof2 to aprof8 detection attributes, all of which should be -1 if the source image is deblended.

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.

Lies within detector 16 region of a tile

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.

Photometric calibration probably subject to systematic error

This flag is applied to VSA 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 that multiframe: 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

VSA only- 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 deep stacks.

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.

Diffraction spike

Spurious sources occur in lines along diffraction spikes.

WFAU, Institute for Astronomy,
Royal Observatory, Blackford Hill
Edinburgh, EH9 3HJ, UK

osa-support@roe.ac.uk
23/4/2015