Pointing Constraints

There are several pointing constraints on the Swift spacecraft that proposers should be aware of. The BAT, XRT, and UVOT will avoid the following:

• 28 degrees of the Earth's limb (95 degrees from the center of the Earth),

• 46 degrees of the Sun, and

• 23 degrees of the Moon.

The UVOT will also avoid planets as listed in Table 3. The zone of avoidance for stars between $-1.4$ mag and 0.0 mag is 25 arcminutes.

Table 3: The UVOT's zones of avoidance for objects in the Solar System.

Object	Angular Offset
Mercury	25 arcminutes
Venus	25 arcminutes
Mars	25 arcminutes
Jupiter	25 arcminutes
Saturn	25 arcminutes

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.

Table 4: Approximate bright magnitude limits for UVOT observations of stars.

Filter	Spectral Class	Magnitude
$v$	B1V	4.5
	A1V	5.1
	G2V	5.1
	M2V	5.0
uvw2	A1V	3.2
white	B1V	9.1
	A1V	7.6
	G2V	7.0
	M2V	7.0

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.

Approximately 80% of all GRBs will occur in a region of the sky that is observable by Swift.