UCL Department of Space and Climate Physics


Quality Flag Statistics

Quality flags pertaining to potentially unreliable photometry or astrometry are stored in the catalogue. Flags are constructed autonomously during pipeline processing and the details of the flagging algorithms are provided in the section Data Processing: Quality Flagging.

Filter dependence

The figure below displays a typical XMM-OM field through three filters, UVW2, UVW1 and B. As a consequence of both the spectral properties of typical field sources and the effective areas of the filters, the B image is the most crowded and source count rates through B, with a few exceptions, are larger. Five of the catalogue quality flags, readout streaks, smoke rings, diffraction spikes, mod-8 noise and bright neighbours, are the result of bright sources in the field. Many of these qualities can be seen by eye in the B image, as well as source saturation and an enhanced background at the center of the image caused by diffuse light reflected from outside the field of view. 

As we step through the filters towards the UV wavelengths, both the effective area, intrinsic brightness of typical celestial sources and background photon rates decrease. As a consequence so does the number of quality issues. Diffuse scattered light, a readout streak and smoke rings are still visible in the UVW1 image but at a reduced frequency and intensity relative to the B image. The UVW2 image contains a faint central enhancement but no other quality issues related to bright sources.

Image Quality Comparison

A typical OM field observed through the UVW2, UVW1, and B filters. The effective area of the OM, source brightnesses (in general), and source density increase towards the optical. The number of quality issues therefore increase towards optical wavelengths.

Quality Statistics

Inspection of the catalogued sources reveal the same trends across the filters. Those flags that are typically set when sources are bright or have bright neighbours dominate the optical images but become less frequent in the UV. In contrast, bad pixels, image edge and extended source flags reveal a relatively flat distribution across the filters. Central enhancement statistics also remain flat despite quantity of scattered light falling as the wavelength decreases. This is because the flagging algorithm makes no attempt to quantify scattered light which depends on pointing direction and exposure time. Sources will simply be flagged if they are located within the fixed-size, central cone whose characteristics have been iterated upon by screening multiple observations. So while, in real terms, the fraction of UV sources found over a scattering-enhanced background is smaller than the optical fraction, our conservative approach to source quality means that this property is not apparent in the quality flags.

At face value, the fraction of edge-of-image sources appears large. But each field is often comprised of multiple sub-images increasing the probability of an image edge impinging upon a source. Edges have an impact on both astrometry and photometry. With reference to both the many potential image windows and spacecraft pointing drift, particular notice of the edge flag should be taken when searching for source variability.

The table below gives the number of sources with each type of flag for each filter (see Data Processing: Quality Flagging for a description of the meanings of the flags). 

No flags5739033235197733936862532005814673983427421713973
Flag 0 (bad pixels)313742193252343462600432206674113424
Flag 1 (readout streaks)9617545457189454550060315725271957635113055710
Flag 2 (smoke ring)9178955844664278224000713434121491
Flag 3 (diffraction spikes)10542450043203550220720
Flag 4 (mod8 pattern)3619222823332403411830212487014850
Flag 5 (central region)390192276843436752732002198924113694
Flag 6 (near bright source)403387232303642219247811199861713679393693
Flag 7 (near edge)145078888807813300382412708436218262349
Flag 8 (in extended)353322357803620944822053260265152535
Flag 9 (bright pixel)820310255011900208009460
Flag 10 (multiple exposures)5111022550576001491503697121741
Flag 11 (too bright)97801670012520675088090

Table giving the statistics on the quality flagging of the sources. The statistics were compiled using XMM-SUSS2.1