The study of explosive extragalactic transients—supernovae (SNe), novae, and related events—has seen great progress thanks to wide-area optical sky surveys. The scientific frontier is now discovery and rapid spectroscopic followup of the fast-declining, young, and rare. Discovering SNe shortly after explosion is of particular value. Photometric observations constrain the radius and binarity of the progenitor \cite{Bloom_2011,Cao_2015}. Rapid spectroscopic observations of core-collapse SNe can reveal signatures of the progenitor in the photoionized circumstellar wind \cite{Gal_Yam_2014}. This “flash” spectroscopy reveals properties of the progenitor after the explosion, but the signatures disappear after about one day from the explosion.
Gamma-ray burst (GRB) optical afterglows—both those aligned with the gamma-ray-producing jet axis and off-axis “orphan” events–are rare, faint, and fast-fading. Accordingly, most GRB afterglows are discovered with rapid followup of external gamma-ray triggers. With high-cadence optical monitoring, however, it is now possible to discover GRBs in the optical band independent of external triggers \cite{Cenko_2015}, potentially revealing event populations undiscoverable by all-sky gamma-ray monitors \cite{Cenko_2013}.
While most extragalactic transients discovered by optical surveys are SNe Ia, large scale surveys can discover (or place limits on) more rare types of explosions that reveal the diversity of the late stages of stellar and binary evolution. Similarly, surveys covering nearby galaxies (such as M31) and our own Galactic Plane can observe lower-luminosity transients such as novae and stellar transients.
The transient science cases above demand an optical survey that can simultaneously survey at high cadence (in order to capture fast-evolving and young events) while maintaining wide areal coverage (in order to detect rare events). While any given survey can trade areal coverage against cadence, to make progress on these questions we require a new camera with greater capability, defined by its instantaneous volumetric survey speed \cite{2014htu..conf...27B}. This is simply the spatial volume in which transients of a fiducial absolute magnitude are detectable in a single image divided by the total time per exposure. This capability metric governs the trade between snapshot survey volume and cadence.
SRD Requirement 1.1 Subject to the constraints in Section \ref{sec:constraints} and an overall cost envelope, the design of the ZTF Observing System shall maximize the instantaneous volumetric survey speed of the instrument for transient detection.
The flowdown from this requirement through the ZTF Observing System is described more fully in the ZTF Instrument Requirements Document. We briefly highlight several consequences. First, maximizing the survey speed implies maximizing the field of view of the camera, so long as delivered image quality is not compromised. The ZTF optical design enables a field of view of 47 deg\({}^{2}\) on the P48 with area-weighted mean image quality of 2.0\({}^{\prime\prime}\) (\(r\) band), matching that of PTF. By using wafer-scale 6k\(\times\)6k CCDs with 15 micron pixels, we minimize losses due to chip gaps, critically sample the effective seeing, and lower data rates by a factor of 3 versus 10 micron devices.
A second implication of maximizing the survey speed is that we should minimize overhead between exposures due to readout time and telescope slews. The readout time requires trading speed versus noise; the ZTF readout electronics will allow a 10 second readout (10 MHz pixel rate) while keeping the read noise below sky even in dark time.
We are upgrading the telescope and dome motors to ensure fast slews.
SRD Requirement 1.1 The telescope shall slew between adjacent ZTF fields and settle in less than the ZTF camera readout time. Telescope and dome drives should minimize the time required for long slews where possible.
The ZTF scheduling system is responsible for designing a pointing plan that minimizes overall slew times.
SRD Requirement 1.1 The median time between exposures for the ZTF survey shall be less than 15 seconds (goal: 10 seconds).
Finally, we note that, given a specified overhead time between exposures, there is an integration time that maximizes the volumetric survey speed. (For median 15 second overhead, this implies a 30 second exposure time.)
SRD Requirement 1.1 The exposure time shall be chosen to maximize the volumetric survey speed.
Finally, maximizing throughput in the optical band implies observing near the peak of CCD Quantum Efficiency:
SRD Requirement 1.1 ZTF shall survey primarily in \(r\) and \(g\) bands.
See CIN 670 for detailed specifications of the ZTF filters.
Prompt detection, discovery, and follow-up of high-priority transients places several requirements on the ZTF Data System:
SRD Requirement 1.1 Images taken by the ZTF Observing System shall be transferred to the ZTF Data System as rapidly as possible, with \(>\)95% of the transfers arriving within 10 minutes (goal: 5 minutes) of the image being taken.
SRD Requirement 1.1 The ZTF Data System shall produce co-added “reference” frames for all observed regions of the sky.
SRD Requirement 1.1 The ZTF Data System shall perform image differencing between the new images and reference images, filter the resulting transient candidates for artifacts, and present prioritized candidates to human scanners through a web-based scanning interface.
SRD Requirement 1.1 The image differencing pipelines and alert streams shall process data and identify transients in the Galactic Plane and M31 in addition to fields at high Galactic latitudes.
SRD Requirement 1.1 The ZTF Data System shall identify transients due to known asteroids.
SRD Requirement 1.1 The latency between images arriving at the ZTF Data System and candidates being presented for scanning shall be less than 10 minutes (goal: 5 minutes) for \(>\)95% of the images.
The role of the real-time image differencing pipeline is for transient discovery, so its requirements on photometric precision are more modest than for “archival” imaging products:
SRD Requirement 1.1 The required RMS relative photometric precision of bright, unsaturated sources from the realtime image differencing pipeline is TBD.