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Questions about Swift? Try our guide to Swift or the guide to data processing and analysis. If these don't solve your problem, please feel free to e-mail us at swifthelp@le.ac.uk.

List of acronyms and abbreviations

We are located in the Department of Physics & Astronomy , at the University of Leicester (directions).

Coincidence Loss and Artifacts

The UV/Optical Telescope is a photon counting instrument. If the count rate from a source approaches the instrument frame time, multiple photons may arrive within the same frame, and be counted as a single one. This is known as coincidence loss, and is analogous to pile-up in X-ray detectors. Coincidence loss becomes a problem around magnitude 10-12 (dependent on filter and whether the full-frame or windowed modes are used).

When running uvotsource or uvotmaghist, if the source is too bright, a lower limit will be written to the screen output, along the lines of: 

UVOT u magnitude (Vega system)
              Source: < 11.91
          Background: 22.60 arcsec^-2
    Background-limit: 19.15
   Coincidence-limit: 11.91

UVOT u magnitude (AB system)
              Source: < 12.93
          Background: 23.62 arcsec^-2
    Background-limit: 20.17
   Coincidence-limit: 12.93

However, an (inaccurate) magnitude and (large) error will still be given in the output FITS file; these values should not be used.

When looking at an image, if the source of interest is surrounded by square-shaped brightness, this is a definitive sign that the source is suffering from significant coincidence loss. It appears like this because the Mod-8 (fixed pattern) noise correction fails when coincidence loss is an issue.

Further information about coincidence loss and other artifacts which may appear in some images (halo or smoke rings, diffraction spikes, read-out streaks...), see Page et al. (2015).

Index