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There are several pointing constraints on the Swift spacecraft that proposers should be aware of. The spacecraft will not point any instrument within:
Targets near the declination of Swift's orbital pole (+- 69 deg) could be unobservable for long periods of time as they violate its Earth constraint. An additional constraint is that the targets roll angle must keep the solar panels pointed within 10 deg of the Sun. Proposers are encouraged to use the Roll Angle Calculator to estimate the allowed range of roll angles values for their target. Bright objects presents a hazard for the UVOT, and the instrument automatically sets its blocked filter when pointing close to them. The angular distance limits is 25 arcminutes for Venus, Mars, Jupiter, Saturn, and for stars between ?1.4 mag and 0.0 mag. No restrictions apply to Uranus, Neptune, and Pluto, while Mercury is too close to the Sun to be observed. Stars in the UVOT FoV are checked in the onboard star catalog for color and brightness, from which a predicted theoretical count rate is calculated. If a star would produce 200000 counts per second or more the field is not observed. If the total counts received for any star during this pointing (up to approximately 45 minutes) reaches 1000000 the exposure is terminated and the next filter is tested.
If the total count rate on the whole UVOT detector is greater than about 200000 counts then counts are lost in the electronics and a dark band will appear in the image. At 200000 counts per second the magnitudes listed in Table 4 would be just observable, although coincidence loss would make measuring a reliable magnitude impossible.
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The UVOT detector has an autonomous onboard safety circuit that is designed to activate and turn off the detector when it senses that an observation is dangerous. The UVOT's safety circuit can trip during GRB chasing observations.
