Exploring & Exploiting the Dynamic Optical Sky

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For millennia, human beings have noticed "new stars" appearing and disappearing in the night sky. Thanks to advances in sensors, computation and algorithms astronomers are now able to discover millions of changes in the night sky. The objects range from near-earth asteroids to explosive stellar deaths across the Universe. This vibrant field of “time-domain astronomy” is now addressing fundamental scientific questions such as the origin of the elements in the Universe, as well as playing a critical role in identifying exotic astronomical events which are bright in gravitational waves or neutrinos and providing unique insights into stars. Time-domain astronomy is one of the four scientific pillars of the Vera Rubin Observatory, the NSF’s high priority ground-based observatory for the next decade. The Zwicky Transient Facility (ZTF) is preparing the astronomy community including students to become adept users of the Rubin Observatory. ZTF-II builds on the success of ZTF-I which began survey operations in March 2018 and has been responsible for 50% of global supernova discoveries over the past year, along with a slew of other discoveries from both the real-time alert stream and the multi-billion object catalog releases. This award advances the goals of the Windows on the Universe Big Idea via ZTF’s strong relevance to Multi-Messenger Astrophysics.ZTF-II is a state-of-the art time-domain survey employing a 47 square degrees field-of-view camera on the Palomar 48-inch Samuel Oschin Schmidt telescope. It operates a public survey covering the visible northern sky (declination above -30 degrees) every 2 nights in g- and r-bands to about 20.5 magnitude. Each ZTF observation is processed by an image differencing pipeline which generates real-time alerts for all 5-sigma detections of point-source transient events, amounting to several hundred thousand alerts per night. These alerts contain thumbnail images of the discovery image, a reference image, and the difference between the two, as well as a 30-day light curve history, and various metadata, such as a real-bogus score, details of the nearest PS1 source, and pipeline parameters. Daily spectral classifications of all supernovae brighter than 18.5 mag are obtained using a robotic spectrometer and announced via the Transient Name Server. Each ZTF observation is also processed by a Point Spread Function photometry pipeline which produces a single epoch catalog covering all identified sources. These are archived at IPAC and used to create light-curves for all detected objects in ZTF data releases - in ZTF-II, these will be released on a 2-month cycle starting in 2021.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.