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The Zwicky Transient Facility: System Overview, Performance, and First Results

Eric C. Bellm, Shrinivas R. Kulkarni, Matthew J. Graham, Richard Dekany, Roger M. Smith, Reed Riddle, Frank J. Masci, George Helou, Thomas A. Prince, Scott M. Adams, C. Barbarino, Tom Barlow, James Bauer, Ron Beck, Justin Belicki, Rahul Biswas, Nadejda Blagorodnova, Dennis Bodewits, Bryce Bolin, Valery Brinnel, Tim Brooke, Brian Bue, Mattia Bulla, Rick Burruss, S. Bradley Cenko, Chan-Kao Chang, Andrew Connolly, Michael Coughlin, John Cromer, Virginia Cunningham, Kishalay De, Alex Delacroix, Vandana Desai, Dmitry A. Duev, Gwendolyn Eadie, Tony L. Farnham, Michael Feeney, Ulrich Feindt, David Flynn, Anna Franckowiak, S. Frederick, C. Fremling, Avishay Gal-Yam, Suvi Gezari, Matteo Giomi, Daniel A. Goldstein, V. Zach Golkhou, Ariel Goobar, Steven Groom, Eugean Hacopians, David Hale, John Henning, Anna Y. Q. Ho, David Hover, Justin Howell, Tiara Hung, Daniela Huppenkothen, David Imel, Wing-Huen Ip, Željko Ivezić, Edward Jackson, Lynne Jones, Mario Juric, Mansi M. Kasliwal, S. Kaspi, Stephen Kaye, Michael S. P. Kelley, Marek Kowalski, Emily Kramer, Thomas Kupfer, Walter Landry, Russ R. Laher, Chien-De Lee, Hsing Wen Lin, Zhong-Yi Lin, Ragnhild Lunnan, Matteo Giomi, Ashish Mahabal, Peter Mao, Adam A. Miller, Serge Monkewitz, Patrick Murphy, Chow-Choong Ngeow, Jakob Nordin, Peter Nugent, Eran Ofek, Maria T. Patterson, Bryan Penprase, Michael Porter, Ludwig Rauch, Umaa Rebbapragada, Dan Reiley, Mickael Rigault, Hector Rodriguez, Jan van Roestel, Ben Rusholme, Jakob van Santen, S. Schulze, David L. Shupe, Leo P. Singer, Maayane T. Soumagnac, Robert Stein, Jason Surace, Jesper Sollerman, Paula Szkody, F. Taddia, Scott Terek, Angela Van Sistine, Sjoert van Velzen, W. Thomas Vestrand, Richard Walters, Charlotte Ward, Quan-Zhi Ye, Po-Chieh Yu, Lin Yan, Jeffry Zolkower

arXiv:1902.01932v1astro-ph.IM

TL;DR

ZTF addresses the need for rapid, wide-field optical time-domain observations by combining a new camera and observing system on the Palomar 48-inch Schmidt telescope. Its survey strategy and performance support frequent coverage of the Northern Hemisphere and discovery of bright, rapidly evolving transients.

  • Problem

    ZTF targets the need for broad, repeated optical monitoring to discover and characterize transients, variables, and other time-varying objects.

  • Method

    ZTF uses a custom wide-field camera and optimized optics on the Palomar 48-inch Schmidt telescope, conducting Northern Sky and Galactic Plane surveys with repeated g- and r-band visits.

  • Results

    ZTF surveys the Northern Hemisphere hundreds of times in three bands over minutes-to-years timescales, while obtaining about four times more observations per sky area than LSST.

  • Takeaways & Limitations

    ZTF will provide large samples of bright transients and variables, with discoveries accessible to spectroscopic follow-up using moderate-aperture telescopes.

  • Takeaways & Limitations

    Vignetting at extreme field angles limits ZTF's useful field of view, reaching 30% in the corners.

Abstract

from arXiv · show

The Zwicky Transient Facility (ZTF) is a new optical time-domain survey that uses the Palomar 48-inch Schmidt telescope. A custom-built wide-field camera provides a 47 deg$^2$ field of view and 8 second readout time, yielding more than an order of magnitude improvement in survey speed relative to its predecessor survey, the Palomar Transient Factory (PTF). We describe the design and implementation of the camera and observing system. The ZTF data system at the Infrared Processing and Analysis Center provides near-real-time reduction to identify moving and varying objects. We outline the analysis pipelines, data products, and associated archive. Finally, we present on-sky performance analysis and first scientific results from commissioning and the early survey. ZTF's public alert stream will serve as a useful precursor for that of the Large Synoptic Survey Telescope.

1. INTRODUCTION

ZTF is a new optical time-domain survey built on the Palomar 48-inch Schmidt telescope. This paper presents its observing and data systems, on-sky performance, public surveys, and initial science results.

  • Large optical sky surveys provide catalogs and finder charts that guide detailed follow-up observations with larger telescopes.
  • Photographic Palomar Observatory Sky Surveys mapped the Northern Hemisphere and enabled decades of astronomical discovery.
  • Advances in CCD quantum efficiency, readout, and computing enabled deeper exposures, larger data volumes, and timely processing.
  • ZTF fills the Palomar 48-inch Schmidt focal plane with a new CCD camera, improving survey speed by three orders of magnitude over photographic surveys.
  • The paper surveys ZTF’s observing and data systems, performance, public surveys, and initial transient, variable, and solar-system science results.

2. OBSERVING SYSTEM

ZTF combines a wide-field CCD camera, optical and mechanical design choices, and low-overhead operations to maximize volumetric survey speed on the Palomar telescope. Its as-built system achieves a 14.9-fold improvement over PTF for a fiducial object.

  • The system was designed to maximize field of view and image quality while minimizing beam obstruction, readout time, and slew overheads.
  • Large-format wafer-scale CCDs fill the wide focal plane while limiting chip-gap losses and using silicon-efficient filters near peak quantum efficiency.
  • 14.9 times larger volumetric survey speed than PTF is achieved for a fiducial Mr = −19 mag object, with ZTF reaching V̇−19 = 3.5 × 10^4 Mpc^3 s−1.
  • Half of the science CCDs use single-layer coatings and half use dual-layer coatings that improve quantum efficiency in g and r bands.
  • The CCDs have few blocked columns, large well capacity, negligible dark current for contemplated exposures, and charge-transfer inefficiency below 5 ppm per pixel shift.
  • Perimeter CCDs provide guiding and tip, tilt, and focus measurements, while the compact cryostat and external components reduce prime-focus obscuration.

2.3. Readout Electronics

ZTF’s observing system uses specialized optics and telescope upgrades to preserve image quality across its larger focal plane while limiting obstruction and operational overheads. The design corrects field curvature through multiple optical elements, but vignetting limits the useful field at extreme angles.

  • 2.4. Optics: The larger ZTF field required optics beyond PTF’s powered dewar window to correct the Schmidt telescope’s curved focal surface.
  • 2.4. Optics: The final optical design combines a trim plate, meniscus vacuum window, faceted cold plate, and field-flattener lenses to compensate field curvature.
  • 2.4. Optics: 30% vignetting in the corners limits ZTF’s useful field of view at extreme field angles.
  • The shutter opens and closes within 430 msec while imparting less than 3 grams of unbalanced force, with negligible impact on image stability.
  • Telescope drive and dome upgrades increase slew speeds to (2.5, 3.0, 3.0)° s−1 for hour angle, declination, and dome axes.
  • Additional upgrades reduce beam obstruction and scattered light while improving electrical protection, condensation control, wind screening, and dome thermal management.

2.7. Filters and Filter Exchanger

ZTF’s filters and observing software are designed around efficient, repeatable survey operations and reliable image processing. The filter system prioritizes signal-to-noise and low obstruction, while calibration hardware addresses limitations in earlier PTF flat-fielding.

  • 2.7. Filters and Filter Exchanger: ZTF uses custom g, r, and i filters chosen to avoid major Palomar sky-background lines, maximize signal-to-noise, and control large-filter costs.
  • 2.7. Filters and Filter Exchanger: A robotic arm transfers filters between an access-hatch cabinet and the camera using solenoid-deactivated magnets and redundant latches.
  • The modular, fail-safe, multithreaded observing software supports continuous operation, monitoring, error tracking, and parameter reconfiguration.
  • Integer Linear Programming with slot-based lookahead schedules fields and observation order to maximize volumetric survey speed.
  • Fixed-grid fields with minimal dithering simplify image subtraction and cover the sky with designed overlaps and Galactic Plane alignment.
  • 2.10. Flat Field Illuminator: The Flat Field Illuminator provides stable pre-night calibration frames, replacing PTF’s science-image flats that delayed reduction and suffered from fringing and scattered light.
  • 2.10. Flat Field Illuminator: 6% edge increases in flat fields arise from scattered light near field-flattener frames and can be fitted with sub-percent accuracy.

3. DATA SYSTEM

ZTF’s data system processes images into calibrated photometry, difference-image detections, and rich near-real-time alerts. Automated reference construction, differencing, classification, archival, and moving-object pipelines support transient, variable-star, and solar-system studies.

  • Data flow: The IPAC data system transfers images from Palomar and processes each CCD readout quadrant independently, preserving observing-program access permissions.Typical transfers take <25 seconds, while the fastest observing cadence is 38.3 seconds.
  • Image processing: The image pipeline applies bias subtraction, flat-field correction, Gaia DR1 astrometry, Pan-STARRS1 photometric calibration, and quality masks.It produces PSF-fit and aperture photometry catalogs from processed direct images, with raw and processed products archived.
  • Reference images: Reference images are automatically built for each field, filter, and quadrant from typically at least 15 registered images using outlier-rejected averaging.These references support image differencing and lightcurve source association.
  • Difference imaging: The differencing pipeline matches photometric throughput, warps references, matches backgrounds, and uses ZOGY PSF matching and filtering to identify changing and moving sources.Candidate features are supplied to the Real-Bogus machine-learning algorithm before database storage and alert packaging.
  • Alert stream: About four minutes separate raw images from transient alerts in the realtime pipeline.Rich alert packets include Real-Bogus scores, recent and historical lightcurves, catalog cross-matches, star-galaxy scores, and image cutouts.
  • Public access and solar-system processing: Public alerts stream near real time to community brokers, with bulk nightly releases available while brokers come online.Solar-system processing separately handles streaked objects and point-like moving objects, with ZMODE linking tracklets across three nights and fitting orbits.

4. ON-SKY PERFORMANCE

Commissioning verified ZTF’s observing and data systems and established delivered image quality and limiting magnitudes across its three filters. The fixed-grid strategy simplifies processing but can duplicate photometry for sources observed in secondary fields or near quadrant boundaries.

  • Commissioning: ZTF commissioning ran through March 2018 and combined technical verification of the observing and data systems with science validation experiments.Formal survey operations began on March 20, 2018.
  • Data-system boundary: Fixed-grid observing simplifies image subtraction but can place the same source’s photometry in multiple files.This occurs when observations use the secondary pointing grid or when a source lies near a readout-quadrant boundary.
  • Image quality: 2.1′′, 2.0′′, and 2.1′′ were the median FWHM image qualities in g, r, and i bands, respectively, for observations above airmass 1.2.Figure 5 summarizes the normalized FWHM distributions for all three filters during June 2018.
  • Limiting magnitude: 20.8 mag, 20.6 mag, and 19.9 mag were the median five-sigma model limiting magnitudes in g, r, and i over one lunation.In dark time, the corresponding medians were 21.1 mag, 20.9 mag, and 20.2 mag.

5. SURVEY STRATEGY

ZTF divides observing time among public, collaboration, and Caltech programs, while its public surveys provide broad Northern Sky and Galactic Plane coverage. The strategy combines repeated cadence with spectroscopic classification of bright extragalactic transients.

  • Program allocation: Public, collaboration, and Caltech surveys receive 40%, 40%, and 20% of ZTF observing time, respectively.A scheduler interleaves the programs and optimizes each night’s schedule for volumetric survey speed.
  • Public surveys: The public program conducts Northern Sky and Galactic Plane surveys with three-day and nightly cadences, respectively.Northern fields lie north of δ = −31° outside the Galactic Plane Survey, while Galactic Plane fields satisfy |b| < 7° and δ > −31°; visits use g and r separated by at least 30 minutes.
  • Transient classification: ZTF will attempt low-resolution spectra for likely extragalactic transients brighter than 18.5 mag using the SED Machine.The resulting classifications will be reported publicly.

6. FIRST RESULTS

Early ZTF observations demonstrate broad utility for transient, variable-star, and solar-system science, while commissioning results also reveal practical limits from incomplete references, classifier training, weather, and short baselines.

  • Transient Science: 38 supernovae were classified during two commissioning months; 15 were discovered only by ZTF and 13 were first discovered by ZTF before other surveys.The modest yield reflects limited reference images, high Real/Bogus thresholds during training, and poor winter weather.
  • Transient Science: ZTF18aaayemw was discovered as a rising transient and later classified as a SN II after broad Hα appeared in spectra obtained a month later.Its early spectrum was blue and featureless, with an estimated blackbody temperature of ∼12,000 K and redshift z = 0.0512.
  • Target of Opportunity and Multi-Messenger Science: ZTF covered a GRB localization over 2900 square degrees, identifying no viable optical counterpart and setting median five-sigma limits of r > 20.3 and g > 20.6 mag.The observations began 9.1 hours after the trigger and covered approximately 70% of the localization probability.
  • Variable-Star Science: Commissioning data explored day-to-month variability in 83 Be-star candidates and provided homogeneous gri-band light curves for studying RR Lyrae pulsational properties.A longer baseline and refinements to the light-curve pipeline are expected to constrain Be-star variable fractions, amplitudes, and outburst activity.
  • Solar-System Science: ∼600,000 measurements submitted to the MPC yielded designations for about 320 new objects, including seven Near-Earth Asteroids.Five of the seven new NEOs were detected by the dedicated streak-detection pipeline.
  • Solar-System Science: More than 2600 asteroid light curves were extracted, but rotation periods were reliably detected only below 3 hours and not conclusively for faint asteroids ≳19.5 mag.The pilot campaign found no super-fast rotating asteroids, reflecting the short observation span and larger uncertainties for faint objects.

7. SUMMARY

ZTF combines repeated, multi-band Northern Hemisphere coverage with a high-volume alert stream and serves as a practical precursor to LSST. Its cadence and accessibility emphasize bright-transient discovery and follow-up.

  • Summary: ZTF will survey the Northern Hemisphere hundreds of times in three bands, sampling timescales from minutes to years.The resulting datasets are intended to support young-supernova, relativistic-transient, tidal-disruption, active-galactic-nucleus, variable-star, and solar-system studies.
  • Summary: ZTF will stream one million time-domain detections nightly through a prototype LSST alert-distribution system, ahead of LSST’s ten million nightly alerts.This is intended to provide several years of community experience with high-volume time-domain alerts.
  • Summary: ZTF obtains about four times more observations of any sky area than LSST on average because of its larger field of view.Its finer time sampling is described as enabling earlier transient discovery and better event classification from light curves.
  • Summary: ZTF’s transient discovery rate for events brighter than 21st magnitude is greater than LSST’s, while its smaller aperture keeps discoveries accessible for spectroscopy.The survey is therefore positioned to provide bright-transient and variable-star samples for interpreting LSST’s deeper survey.

A.1. Light curves

The appendix describes host-subtracted PSF photometry for ZTF18aaayemw and supplements Palomar observations with photometry from additional telescopes.

  • A.1. Light curves: Host-subtracted PSF photometry used P48 observations with a Pan-STARRS1 r-band stack as the reference image.The P48 reference images contained supernova light, so the Pan-STARRS1 stack was used instead.
  • A.1. Light curves: Additional photometry came from the Centurion 28-inch telescope, the WISE 1-m telescope, and Swift UVOT.The data were reduced with IRAF routines and their world-coordinate system was calibrated with astrometry.net.

A.2. Spectroscopy

Spectroscopic follow-up of ZTF18aaayemw combined observations from several telescopes with standard reduction, wavelength calibration, and flux-calibration procedures.

  • A.2. Spectroscopy: Spectra were obtained with ALFOSC, DBSP, DOLORES, DeVeny, and the Dual Imager Spectrograph across five observatories.The instruments provided the multi-epoch spectroscopy used for the object’s spectral evolution.
  • A.2. Spectroscopy: Spectral reductions included bias and flat-field corrections, one-dimensional extraction, wavelength calibration with arc lamps, and flux calibration using standard stars.The APO+DIS, DCT DeVeny, NOT, and TNG data used the stated instrument-specific reduction routines and standards.
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