Swift/UVOT has brightness limits to protect the detector. The limit is approximately V~ 5 but depends on the spectral type and which filter(s) you want to use. A 20' radius is checked to allow for different position angles and uncertainty in the spacecraft pointing. Based on the color and optical brightness, the count rate in each filter is predicted. If the predicted count rate is higher than the limit for a requested filter, that filter is skipped in the observing sequence and only the requested filters that are deemed safe are observed. There is a bright star checker at the following link, but it only tells you which bright stars are in the field--it doesn't actually tell you which filters can and can't be observed. Rough limits for bright stars, as well as the allowed angles from the sun, earth, moon, and planets, are posted at http://swift.gsfc.nasa.gov/analysis/uvot_digest/numbers.html
Because the white filter and the grisms let in photons from the widest wavelength range, they have the strictest limits. The UV filters, on the other hand, are the least likely to be prohibited. Note, these are the brightness limits of stars which can be safely observed. The bright limits at which you can do photometry are much fainter (though different analysis using the halos or the readout streaks can push brighter than normal aperture photometry, see http://adsabs.harvard.edu/abs/2013MNRAS.436.1684P ).
In addition to the strict safety limits, bright stars can also interfere with the data analysis. Stars of moderate brightness (I should quantify this better, but the star below is about 9th mag in V) will have a filled in halo with a 20 arcsec diameter. A fainter halo is visible around it, with a radius of 140 arcsec, which gets brighter for brighter stars and will mess up the photometry of faint sources. Bright sources in the halo may still be recoverable by choosing a background region of similar brightness.
[ details of the above image which I first sent out in reference to SN2007C ]
So you know the issues involved, I've attached an
image from the field of GRB060729. (it is a UVW1
image so that we can see the effect in the UV of
bright stars--the effect is worse in the optical for
us, but the usefullness of the UV data is our main
concern.) It shows the 20" radius bright core around
a 9th mag star and the 2' radius halo around an 8th mag star
(the halo is faintly visible around the fainter star
but not a dominant problem). So being within 20" of a
star brighter than 10th or so would be nearly
impossible. The larger halo would be hard to subtract
correctly but not a show stopper if the SN is
interesting and bright enough. And if there are stars
brighter than 7th or so within 20' of the target
pointing we can't observe at all. (That's something
the Swift people will check since it depends on the
spectral type, number of sources, and such, but just
so people are aware.)
image from the field of GRB060729. (it is a UVW1
image so that we can see the effect in the UV of
bright stars--the effect is worse in the optical for
us, but the usefullness of the UV data is our main
concern.) It shows the 20" radius bright core around
a 9th mag star and the 2' radius halo around an 8th mag star
(the halo is faintly visible around the fainter star
but not a dominant problem). So being within 20" of a
star brighter than 10th or so would be nearly
impossible. The larger halo would be hard to subtract
correctly but not a show stopper if the SN is
interesting and bright enough. And if there are stars
brighter than 7th or so within 20' of the target
pointing we can't observe at all. (That's something
the Swift people will check since it depends on the
spectral type, number of sources, and such, but just
so people are aware.)
