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UVOT Operational Constraints and Performance Issues
Sun, Moon, Planet and Bright Star Constraints for UVOT
Here we list the known bright objects that present a hazard for UVOT, and the angular distance limits at which constraints are imposed. We also list the angle which the Spacecraft avoids, and the angle used by the planning tool TAKO.
Sun, Moon, Earth, Planets and the Brightest Stars
44 deg for the Sun (Spacecraft avoids by 45, TAKO by 46) 19 deg for the Moon (Spacecraft avoids by 21, TAKO by 23); may change to 14 deg (Spacecraft avoids by 16, TAKO by 18) 92 deg for the Earth (Spacecraft avoids by 94, TAKO by 99) (subtract 66 to get avoidance from centre of disc) 25' for Mercury, Venus, Mars, Jupiter, Saturn (no restrictions for Uranus, Neptune, and Pluto) 25' for stars between -1.4 and 0.0 mag
Other bright stars and field sources
Stars in the field are checked in the onboard star catalogue for colour and brightness, from which a predicted theoretical count rate is calculated. If a star would produce 200,000 count/s or more, then the field is not observed. If the total predicted counts received for any star during this pointing (0-45 min) reaches 1,000,000,000, the exposure is terminated and the next filter tested.
If the total count rate on the whole detector is greater than about 200,000 count/s, counts are lost in the electronics and a dark band appears in the image. At 200,000 count/s the following magnitude stars would be just observable (although coincidence loss would make measuring a reliable magnitude impossible):
White filter B1V 9.1 mag V filter B1V 4.5 mag White filter A1V 7.6 mag V filter A1V 5.1 mag UVW2 filter A1V 3.2 mag White filter G2V 7.0 mag V filter G2V 5.1 mag V filter M2V 5.0 mag white filter M2V 7.0 mag
A star that is too bright to be observed on one filter may be observable in a different filter. The bright limit for observations is strongly dependent on the spectrum of the source, and the choice of filter. Users who are proposing UVOT observations are strongly encouraged to use the UVOT Bright Star Checker Tool to see if there are any bright objects in or near the UVOT field of view that may interfere with observing the field.
To estimate if a particular source can be observed with UVOT please refer to the UVOT Point Source Simulation Tool. Users should use caution when attempting to observe a field that has a bright source within 20 arcmin since UVOT's onboard bright source algorithm is not identical to what is used in the simulation tool. This means that attempting to push UVOT observations to the edge of the bright source limit may not be successful.
Safety Circuit Trip-Offs
The UVOT detector has an autonomous onboard safety circuit that is designed to activate and turn off the detector when it senses an observation is dangerous. It is not unusual for the UVOT safety circuit to trip during GRB chasing operations. The safety circuit trips during verification were all due to slewing over bright (mag 2-4) blue (O and B type) stars except for:
- 1 trip in a grism exposure with a slowly increasing background caused by the Earth approaching
- 2 trips observing and preparing to observe the SMC which has many bright stars which appears too bright to the safety circuit because of overlap
- 2 trips in the B filter for reasons yet to be determined.
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