The Keck Observatory Twilight Observing Program

The W.M. Keck Observatory has implemented a cadence observing program for observations conducted during twilight - periods that are not otherwise utilized by the classically-scheduled observers due to limitations on the sky brightness. We initiated observations with NIRC2 NGS-AO during semester 18A as a pilot program to monitor spatially resolved, temporal phenomena on Solar System objects. The observations are performed when the classically-scheduled observers on Keck II using optical spectrometers DEIMOS or KCWI surrender the twilight time, because the sky is too bright to obtain any useful data at visible wavelengths. The observations can be executed by the Keck Observing Assistants (OAs). To facilitate the work of the OAs, web-based planning tools, observation scripts, and instructions (https://www2.keck.hawaii.edu/inst/tda/TwilightZone.html) were developed by N. Molter, with contributions from K. de Kleer, in the framework of the Keck Visiting Scholars program. The twilight observing program has already proven very successful, with observations carried out on 26 different nights within the last year, leading to a science publication (Molter et al. 2018, submitted) and to a considerable gain on telescope time utilization. Due to the success of the pilot twilight observing program, cadence twilight observing is included in our call for proposals starting in semester 18B. The twilight observing program is open to the four major W. M. Keck Observatory constituents; UC, CIT, NASA and UH. Each institution is limited to one program, where programs designed for a duration of 1 to 2 years have priority. The main guiding principles of this program are (1) voluntary participation by classically-scheduled PI and OA, and (2) execution of observations completely at the discretion of the classically-scheduled PI and OA. Any PI proposing a cadence twilight observing program is required to develop the appropriate tools for the OAs to conduct the observations autonomously. Specifically, PIs are required to (1) develop target and observation managers, (2) develop, test and debug observation scripts, (3) employ simple instrument configurations, and (4) dissect the observations in short individual integrations (<5min).