  The dynamic range of photon counting micro-channel-plate (MCP) intensified
charged-coupled device (CCD) instruments such as the Swift Ultraviolet/Optical
Telescope (UVOT) and the XMM-Newton Optical Monitor (XMM-OM) is limited at the
bright end by coincidence loss, the superposition of multiple photons in the
individual frames recorded by the CCD. Photons which arrive during the brief
period in which the image frame is transferred for read out of the CCD are
displaced in the transfer direction in the recorded images. For sufficiently
bright sources, these displaced counts form read-out streaks. Using UVOT
observations of Tycho-2 stars, we investigate the use of these read-out streaks
to obtain photometry for sources which are too bright (and hence have too much
coincidence loss) for normal aperture photometry to be reliable. For
read-out-streak photometry, the bright-source limiting factor is coincidence
loss within the MCPs rather than the CCD. We find that photometric measurements
can be obtained for stars up to 2.4 magnitudes brighter than the usual
full-frame coincidence-loss limit by using the read-out streaks. The resulting
bright-limit Vega magnitudes in the UVOT passbands are UVW2=8.80, UVM2=8.27,
UVW1=8.86, u=9.76, b=10.53, v=9.31 and White=11.71; these limits are
independent of the windowing mode of the camera. We find that a photometric
precision of 0.1 mag can be achieved through read-out streak measurements. A
suitable method for the measurement of read-out streaks is described and all
necessary calibration factors are given.
