ztf-avro-alert

ZTF Avro Schemas

These documents are for schema v3.2.

Schema Heirarchy

ZTF uses nested schemas to organize the data in the alert packet.

ztf.alert (defined in alert.avsc) is the top-level namespace. ztf.alert in turn relies on candidate.avsc, prv_candidate.avsc, and cutout.avsc.

ztf.alert

The top-level alert contains the following fields:

Field Type Contents
schemavsn string schema version used
publisher string origin of alert packet
objectId long unique identifier for this object
candid long unique identifier for the subtraction candidate
candidate ztf.alert.candidate candidate record
prv_candidates array of ztf.alert.prv_candidate or null candidate records for 30 days’ past history
cutoutScience ztf.alert.cutout or null cutout of the science image
cutoutTemplate ztf.alert.cutout or null cutout of the coadded reference image
cutoutDifference ztf.alert.cutout or null cutout of the resulting difference image

ztf.alert.candidate

Field Type Contents
jd double Observation Julian date at start of exposure [days]
fid int Filter ID (1=g; 2=r; 3=i)
pid long Processing ID for image
diffmaglim [float, null], default: null 5-sigma mag limit in difference image based on PSF-fit photometry [mag]
pdiffimfilename [string, null], default: null filename of positive (sci minus ref) difference image
programpi [string, null], default: null Principal investigator attached to program ID
programid int Program ID: encodes either public, collab, or caltech mode
candid long Candidate ID from operations DB
isdiffpos string t or 1 => candidate is from positive (sci minus ref) subtraction; f or 0 => candidate is from negative (ref minus sci) subtraction
tblid [long, null], default: null Internal pipeline table extraction ID
nid [int, null], default: null Night ID
rcid [int, null], default: null Readout channel ID [00 .. 63]
field [int, null], default: null ZTF field ID
xpos [float, null], default: null x-image position of candidate [pixels]
ypos [float, null], default: null y-image position of candidate [pixels]
ra double Right Ascension of candidate; J2000 [deg]
dec double Declination of candidate; J2000 [deg]
magpsf float magnitude from PSF-fit photometry [mag]
sigmapsf float 1-sigma uncertainty in magpsf [mag]
chipsf [float, null], default: null Reduced chi-square for PSF-fit
magap [float, null], default: null Aperture mag using 8 pixel diameter aperture [mag]
sigmagap [float, null], default: null 1-sigma uncertainty in magap [mag]
distnr [float, null], default: null distance to nearest source in reference image PSF-catalog within 30 arcsec [pixels]
magnr [float, null], default: null magnitude of nearest source in reference image PSF-catalog within 30 arcsec [mag]
sigmagnr [float, null], default: null 1-sigma uncertainty in magnr within 30 arcsec [mag]
chinr [float, null], default: null DAOPhot chi parameter of nearest source in reference image PSF-catalog within 30 arcsec
sharpnr [float, null], default: null DAOPhot sharp parameter of nearest source in reference image PSF-catalog within 30 arcsec
sky [float, null], default: null Local sky background estimate [DN]
magdiff [float, null], default: null Difference: magap - magpsf [mag]
fwhm [float, null], default: null Full Width Half Max assuming a Gaussian core, from SExtractor [pixels]
classtar [float, null], default: null Star/Galaxy classification score from SExtractor
mindtoedge [float, null], default: null Distance to nearest edge in image [pixels]
magfromlim [float, null], default: null Difference: diffmaglim - magap [mag]
seeratio [float, null], default: null Ratio: difffwhm / fwhm
aimage [float, null], default: null Windowed profile RMS afloat major axis from SExtractor [pixels]
bimage [float, null], default: null Windowed profile RMS afloat minor axis from SExtractor [pixels]
aimagerat [float, null], default: null Ratio: aimage / fwhm
bimagerat [float, null], default: null Ratio: bimage / fwhm
elong [float, null], default: null Ratio: aimage / bimage
nneg [int, null], default: null number of negative pixels in a 5 x 5 pixel stamp
nbad [int, null], default: null number of prior-tagged bad pixels in a 5 x 5 pixel stamp
rb [float, null], default: null RealBogus quality score; range is 0 to 1 where closer to 1 is more reliable
rbversion string version of RealBogus model/classifier used to assign rb quality score
ssdistnr [float, null], default: null distance to nearest known solar system object; set to -999.0 if none [arcsec]
ssmagnr [float, null], default: null magnitude of nearest known solar system object (usually V-band from MPC archive); set to -999.0 if none [mag]
ssnamenr [string, null], default: null name of nearest known solar system object (from MPC archive); ‘null’ if none
sumrat [float, null], default: null Ratio: sum(pixels) / sum(abs(pixels)) in a 5 x 5 pixel stamp where stamp is first median-filtered to mitigate outliers
magapbig [float, null], default: null Aperture mag using 18 pixel diameter aperture [mag]
sigmagapbig [float, null], default: null 1-sigma uncertainty in magapbig [mag]
ranr double Right Ascension of nearest source in reference image PSF-catalog; J2000 [deg]
decnr double Declination of nearest source in reference image PSF-catalog; J2000 [deg]
ndethist int Number of spatially-coincident detections falling within 1.5 arcsec going back to beginning of survey; only detections that fell on the same field and readout-channel ID where the input candidate was observed are counted
ncovhist int Number of times input candidate position fell on any field and readout-channel going back to beginning of survey
jdstarthist [double, null], default: null Earliest Julian date of epoch corresponding to ndethist [days]
jdendhist [double, null], default: null Latest Julian date of epoch corresponding to ndethist [days]
scorr [double, null], default: null Peak-pixel signal-to-noise ratio in point source matched-filtered detection image
tooflag [int, null], default: 0 1 => candidate is from a Target-of-Opportunity (ToO) exposure; 0 => candidate is from a non-ToO exposure
objectidps1 [long, null], default: null Object ID of closest source from PS1 catalog; if exists within 30 arcsec
sgmag1 [float, null], default: null g-band PSF magnitude of closest source from PS1 catalog; if exists within 30 arcsec [mag]
srmag1 [float, null], default: null r-band PSF magnitude of closest source from PS1 catalog; if exists within 30 arcsec [mag]
simag1 [float, null], default: null i-band PSF magnitude of closest source from PS1 catalog; if exists within 30 arcsec [mag]
szmag1 [float, null], default: null z-band PSF magnitude of closest source from PS1 catalog; if exists within 30 arcsec [mag]
sgscore1 [float, null], default: null Star/Galaxy score of closest source from PS1 catalog 0 <= sgscore <= 1 where closer to 1 implies higher likelihood of being a star
distpsnr1 [float, null], default: null Distance of closest source from PS1 catalog; if exists within 30 arcsec [arcsec]
objectidps2 [long, null], default: null Object ID of second closest source from PS1 catalog; if exists within 30 arcsec
sgmag2 [float, null], default: null g-band PSF magnitude of second closest source from PS1 catalog; if exists within 30 arcsec [mag]
srmag2 [float, null], default: null r-band PSF magnitude of second closest source from PS1 catalog; if exists within 30 arcsec [mag]
simag2 [float, null], default: null i-band PSF magnitude of second closest source from PS1 catalog; if exists within 30 arcsec [mag]
szmag2 [float, null], default: null z-band PSF magnitude of second closest source from PS1 catalog; if exists within 30 arcsec [mag]
sgscore2 [float, null], default: null Star/Galaxy score of second closest source from PS1 catalog; if exists within 30 arcsec: 0 <= sgscore <= 1 where closer to 1 implies higher likelihood of being a star
distpsnr2 [float, null], default: null Distance to second closest source from PS1 catalog; if exists within 30 arcsec [arcsec]
objectidps3 [long, null], default: null Object ID of third closest source from PS1 catalog; if exists within 30 arcsec
sgmag3 [float, null], default: null g-band PSF magnitude of third closest source from PS1 catalog; if exists within 30 arcsec [mag]
srmag3 [float, null], default: null r-band PSF magnitude of third closest source from PS1 catalog; if exists within 30 arcsec [mag]
simag3 [float, null], default: null i-band PSF magnitude of third closest source from PS1 catalog; if exists within 30 arcsec [mag]
szmag3 [float, null], default: null z-band PSF magnitude of third closest source from PS1 catalog; if exists within 30 arcsec [mag]
sgscore3 [float, null], default: null Star/Galaxy score of third closest source from PS1 catalog; if exists within 30 arcsec: 0 <= sgscore <= 1 where closer to 1 implies higher likelihood of being a star
distpsnr3 [float, null], default: null Distance to third closest source from PS1 catalog; if exists within 30 arcsec [arcsec]
nmtchps int Number of source matches from PS1 catalog falling within 30 arcsec
rfid long Processing ID for reference image to facilitate archive retrieval
jdstartref double Observation Julian date of earliest exposure used to generate reference image [days]
jdendref double Observation Julian date of latest exposure used to generate reference image [days]
nframesref int Number of frames (epochal images) used to generate reference image
dsnrms [float, null], default: null Ratio: D/stddev(D) on event position where D = difference image
ssnrms [float, null], default: null Ratio: S/stddev(S) on event position where S = image of convolution: D (x) PSF(D)
dsdiff [float, null], default: null Difference of statistics: dsnrms - ssnrms
magzpsci [float, null], default: null Magnitude zero point for photometry estimates [mag]
magzpsciunc [float, null], default: null Magnitude zero point uncertainty (in magzpsci) [mag]
magzpscirms [float, null], default: null RMS (deviation from average) in all differences between instrumental photometry and matched photometric calibrators from science image processing [mag]
nmatches int Number of PS1 photometric calibrators used to calibrate science image from science image processing
clrcoeff [float, null], default: null Color coefficient from linear fit from photometric calibration of science image
clrcounc [float, null], default: null Color coefficient uncertainty from linear fit (corresponding to clrcoeff)
zpclrcov [float, null], default: null Covariance in magzpsci and clrcoeff from science image processing [mag^2]
zpmed [float, null], default: null Magnitude zero point from median of all differences between instrumental photometry and matched photometric calibrators from science image processing [mag]
clrmed [float, null], default: null Median color of all PS1 photometric calibrators used from science image processing [mag]: for filter (fid) = 1, 2, 3, PS1 color used = g-r, g-r, r-i respectively
clrrms [float, null], default: null RMS color (deviation from average) of all PS1 photometric calibrators used from science image processing [mag]
neargaia [float, null], default: null Distance to closest source from Gaia DR1 catalog irrespective of magnitude; if exists within 90 arcsec [arcsec]
neargaiabright [float, null], default: null Distance to closest source from Gaia DR1 catalog brighter than magnitude 14; if exists within 90 arcsec [arcsec]
maggaia [float, null], default: null Gaia (G-band) magnitude of closest source from Gaia DR1 catalog irrespective of magnitude; if exists within 90 arcsec [mag]
maggaiabright [float, null], default: null Gaia (G-band) magnitude of closest source from Gaia DR1 catalog brighter than magnitude 14; if exists within 90 arcsec [mag]
exptime [float, null], default: null Integration time of camera exposure [sec]

ztf.alert.prv_candidate

The prv_candidates field contains an array of one or more previous subtraction candidates at the position of the alert. These are obtained by a simple cone search at the position of the alert candidate on the last 30 days of history. If there are no previous candidates or upper limits, this field is null.

The fields for an individual prv_candidate are identical to candidate except for the omission of the PS1 and Gaia matches, previous detection history, tooflag, and reference image information.

Additionally, if the previous image has a nondetection at position of the new candidate, candid, isdiffpos, ra, dec, magpsf, sigmapsf, ranr, and decr will be null. In this case diffmaglim provides an estimate of the limiting magnitude over the entire image.

ztf.alert.cutout

Each cutout contains two fields:

Field Type Contents
fileName string Original cutout location
stampData bytes gzip-compressed FITS cutout image