Optimal Optical Search Strategy for Finding Transient in Large Sky Error
  Region Under Realistic Constraints
  In order to identify the rapidly-fading, optical transient counterparts of
gravitational wave (GW) sources, an efficient follow-up strategy is required.
Since most ground-based optical observatories aimed at following-up GW sources
have a telescope with a small field-of-view (FOV) as compared to the GW sky
error region, we focus on a search strategy that involves dividing the GW patch
into tiles of the same area as the telescope FOV to strategically image the
entire patch. We present an improvement over the optimal telescope-scheduling
algorithm outlined in Rana et al. (2016), by combining the tiling and
galaxy-targeted search strategies, and factoring the effects of the source
airmass and telescope slew, along with setting constraints, into the scheduling
algorithm in order to increase the chances of identifying the GW counterpart.
We propose two separate algorithms: the airmass-weighted algorithm, a specific
solution to the Hungarian algorithm that maximizes probability acquired, while
minimizing the image airmass, and the slew-optimization algorithm that
minimizes the overall slew angle covered between images for the given
probability acquired by the optimal telescope-scheduling algorithm in Rana et
al. (2016). Using the observatory site of the GROWTH-India telescope as an
example, we generate 100s of skymaps to test the performance of our algorithms.
Our results indicate that slew-optimization can reduce the cumulative slew
angle in the observing schedule by 100s of degrees, saving several of minutes
of observing time without the loss of tiles and probability. Further, we
demonstrate that as compared to the greedy algorithm, the airmass-weighted
algorithm can acquire up to 20 % more probability and 30 sq. deg. more in areal
coverage for skymaps of all sizes and configurations.
