Targets of Opportunity in Two Gyro Mode

The requirement for use of FHSTs to remove pointing error after slews reduces the fraction of the sky that can be viewed in any week in Two Gyro mode. This effect is made visible in the Narrated Movie on the website. The plot below summarizes the effect by showing the fraction of the sky available each week in both Two Gyro and Three Gyro modes. In Three Gyro mode ~82% of the sky is available at any time (18% is lost due to Solar avoidance). In Two Gyro mode ~47% of the sky is available at any time.

Targets of Opportunity (ToOs) are generally in one of three classes: those that could occur at any time in any part of the sky, those that occur for specific objects but at unknown times, and those that are planned follow ups in survey regions. ToO programs designed to observe events that could occur anywhere, such as GRBs, will find that the inaccessible fraction of the sky has grown from ~18% to ~53%. ToO programs designed to observe events, such as flares, in specific targets will also have reduced availability. This impact should be determined for each specific target using the planning tools on the Two Gyro website. These tools can be used to determine exactly when during the year each target is accessible. Investigators designing ToO programs to carry out follow up observations in survey regions should consider target availability for the ToO follow-ups, as well as the initial survey observations, using the planning tools. The reduced scheduling flexibility in Two Gyro mode may also limit the duration of activated ToO sequences.

We do not expect Two Gyro mode to restrict the number of ToO activations that are implemented in a year.