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Lasair 2.0

In addition to the information on this page, please also see the Lasair Cookbook.

The papers about ZTF

The Zwicky Transient Factory (ZTF) is the source of the events to which Lasair provides access. It is well summarised with the following set of papers:
  • The Zwicky Transient Facility: Data Processing, Products, and Archive, F. Masci et al arXiv
  • The Zwicky Transient Facility: System Overview, Performance, and First Results, E. Bellm et al arXiv
  • A Morphological Classification Model to Identify Unresolved PanSTARRS1 Sources: Application in the ZTF Real-Time Pipeline, Y. Tachibana et al, arXiv
  • Machine Learning for the Zwicky Transient Facility, A. Mahabal et al, arXiv
  • The Zwicky Transient Facility: Science Objectives, M. Graham et al, arXiv
  • The Zwicky Transient Facility Alert Distribution System, M. Patterson et al arXiv

You can query the alerts with a freeform SQL interface (select Query Objects). The Cone Search finds an object by position. You can see the coverage of the ZTF survey each night (click Coverage). A Watchlist is a collection of sky positions that you find interesting, and Lasair can do cone searches on all of these together. Power users can use a Jupyter notebook interface, click Jupyter for more information and to see the capabilities.

A Note about Photometry

ZTF photometry is measured after subtraction of a template image. Thus the reported magnitudes correspond to the difference in flux with respect to the template. Positive differences are shown as solid symbols, negative differences are shown as empty symbols. For variable (rather than transient) sources, reported magnitudes here can therefore be significantly fainter than the average magnitude of the source. See image at right.
In addition to the difference magnitude that ZTF provides, Lasair has computed apparent magnitude, or DC magnitude. It uses the reference magnitude of the nearest object at reference epoch -- for a variable star this will be the same astrophysical object as for the science image. It also uses zero point magnitudes. Note that objects that get processed with different reference images -- and hence the difference fluxes -- are offsets from different magnitudes. This re-engineered magnitude is NOT a forced PSF flux measurement on the unsubtracted science image. The code that converts the attributes supplied by ZTF to the DC magnitude and its error can be found here. To reconstruct the apparent magnitude, the zero point of the science image is required, which ZTF did not make available before Nov 2018. Therefore the apparant magnitudes are not reliable before this time.

Exponential Moving Averages

Lasair also computes three exponential moving averages of the apparent magnitudes, with a 2-day timescale, 8-day timescale, and 28-day timescale, for each of the filters: g and r. These are called dc_mag_g02, dc_mag_g08, dc_mag_g28, dc_mag_r02, dc_mag_r08, dc_mag_r28. Each is defined by a recursion relation:
  • ema(t) = f*ema(tp) + (1-f)*mag(t)
  • f = exp(-(t-tp)/tau)
More details are available from this paper.


  • Each ZTF alert is an information package that includes a rich set of parameters for candidate transient or variable objects. These are assimilated into objects and users can query on any column from the candidate or object table -- see here for schema.
  • Some functions of this website require a login. If you want to use these functions, use the "login/signup" links at the top left of this page.
  • Questions and enquiries, please click Contact. Make suggestions for improvements and fixes at the Lasair github issues page.
  • Lasair started ingesting ZTF on 28 May 2018, and now has 173774474 candidate alerts.
  • You may wish to use other brokers that have the ZTF stream: MARS from Las Cumbres Observatory, or Antares from NOAO Tucson.
  • The word "lasair" means flame or flash in Scots and Irish gaelic. It is pronounced lassa. More accurately, listen here or here.
  • If you use this service for science, please use the Acknowledgment at the bottom of this page.

Lasair Features

Click each image for full size.
Conesearch: You can copy and paste a position from Astronomer's Telegram or elsewhere to see if Lasair has a light curve for that source.
Rich Object Information: The light curve of each object is shown in two filters (g and r), with both detections and non-detections, in this case for supernova 2018jny / ZTF18acsovsw. Comments can be added by logged-in users, such as this one with the link to the IAU name of the supernova. The Sherlock classificaiton system has provided information about the host galaxy. Aladin Lite provides both images and catalogue sources: here we see Pan-STARRS image of the host galaxy with sources from Pan-STARRS and Gaia DR2
Detection Images: The object page has links to detailed information about each observation that makes up the light curve, with images, in this case for ZTF18acsovsw. We see the brightening of 2018jny.
Rich Query Capability: Lasair provides free-form SQL query access to the database. Available tables include the ZTF objects and candidates (detections), as well as the classification of the object from Sherlock and the comment list. Logged-in users can contribute useful queries which can then be re-used by others.
Watchlists: A watchlist is a collection of sources in the sky that can be created by a logged-in user, that can be private or public. The "Run Crossmatch" button searches the Lasair database for transients coincident with any of those sources. Here we see a catalogue of Cataclysmic Variables (left side of the table), and the associated Lasair sources (right side). The inset shows the light curve foe one of the CVs.
Coverage: The coverage of the survey can be seen against a variety of all-sky images from Aladin Lite. Green squares are for g filter (401.5–551.9 nm) and red squares for r filter (552.0–691.0 nm).
Sherlock Classification: The Sherlock system crossmatches every Lasair transient against many catalogues, classifying it, and producing a natural-language assessment of the object, that is available on the object page.
Skymaps: Gravitational-wave skymaps from LIGO-Virgo are represented as a contour map of percentiles 10%, 20%, ... 90%. ZTF coverage and detections can be overlaid, as well as galaxies where the counterpart might reside, with the area of the yellow square proportional to the probability. A doubleclick inside the square centres the galaxy for zooming, and shows its distance and a NED link above.