wiki:SDQAMetricsForDC3b
Last modified 8 years ago Last modified on 09/14/2010 03:00:12 PM

SDQA Metrics for LSST

This document defines the data quality metrics that will be implemented for DC3b.

Metric Attributes

The attributes for each metric are:

  • Name: name of the field in the database for each metric
  • Units: physical units of each metric; quantities with no physical units should be so indicated. Note that the units should follow the IAU recommendation for basic units, with additional allowed units and formatting guidelines as recommended in the FITS standard (see http://fits.gsfc.nasa.gov/fits_standard.html, Table 4.2).
  • Type: data type to be used in the database for each metric
  • Stage: the pipeline stage where the metric is computed (see: DC3bProcessingStages). Value will be one of:
    • "ISR" for Instrument Signature Removal
    • "CrSplit" for Cosmic Ray Removal
    • "ImgChar" for image characterization
    • "AstroCal" for astrometric calibration
    • "DiffIm" for Image Difference processing
    • "MOPS" for Moving Object processing
  • Entity: the entity or collection of entities over which the metric is computed. Possible values are:
    • "Amp" for a 512x2048 amplifier section of an image
    • "CCD" for a single 4Kx4K CCD sensor
    • "Raft" for a physical block of 3x3 co-mounted CCDs
    • "FPA" for the entire focal plane array of sensors
    • "Source" for all entries in the Source database
  • Description: Brief description of the meaning for each metric, and how it will be calculated

Exposure-level Metadata

Certain exposure-level metadata will be recorded for each image in order to make best use of the QA metrics. Most of the quantities listed below will be harvested from the image headers (for the CFHT-LS data), or from TBD for the simulation data.

Name Units Type Description
ccdId [None] Int32 Unique identifier for this CCD
offsetFpaX pixel Float Separation between a reference point on the FPA and the first X pixel of a CCD
offsetFpaY pixel Float Separation between a reference point on the FPA and the first Y pixel of a CCD
dateObs dateTime double Timestamp for the start of this exposure, formatted per ISO8601
expTime s Float Duration of the exposure
filterId [None] Int32 Unique identifier for the filter (i.e., bandpass) in use

Metrics for Basic Image Characterization

The following are basic, descriptive statistics for LSST raw images. All quantities that describe native brightness (BUNIT) are anticipated to be in units of ADU for Amp, and in electrons for CCDs after ISR pipeline processing. Note that several of the metrics below (in italic) are naturally derived from image histograms. Metrics in bold are required for DC3b.

Name Units Type Stage Entity Description
nGoodPix [None] Int32 ISR CCD Number of good pixels, i.e., 1.0 - (nBadCalibPix+nSaturatePix+nSpurPix)
nBadCalibPix [None] Int32 ISR CCD Number of pixels with compromised values (charge traps, bad columns, hot pixels).
nSpurPix [None] Int32 ImgChar CCD Number of pixels compromised by non-astronomical transients (Rad hits, mux bleeds, satellite trails, etc.).
nSaturatePix [None] Int32 ISR Amp, CCD Number of pixels compromised by detector saturation, including pixels with uncorrectable non-linearity.
fracGhostBg percent Float ImgChar CCD Fraction of image area affected by ghost images with surface brightness gradients exceeding 0.333 of sky noise. [SRD: "GhostAF"]
overscanMean ADU Float ISR Amp Mean of overscan pixels, with outlier rejection
overscanStdDev ADU Float ISR Amp Standard deviation about mean of overscan pixels, computed from Good pixels
overscanMedian ADU Float ISR Amp Median of overscan pixels, computed from Good pixels; used as robust estimate of CCD bias level
overscanMin ADU Float ISR Amp Minimum value of non-rejected overscan pixels
overscanMax ADU Float ISR Amp Maximum value of non-rejected overscan pixels
imageClipMean4Sig3Pass BUNIT Float ISR CCD Mean of image, computed from Good pixels, with 4-sigma clipping and 3 reject cycles (i.e., passes)
imageSigma [None] Float ISR CCD Standard deviation about imageClipMean4Sig3Cycle, computed from Good pixels
imageMedian BUNIT Float ISR CCD Median of image, computed from Good pixels
imageMode BUNIT Float ISR CCD Mode of image, computed from Good pixels. This is an excellent measure of the background level if the mode is well behaved (i.e., unless there are very extended targets in the image)
imageMin BUNIT Float ISR CCD Minimum of image, computed from Good pixels
imageMax BUNIT Float ISR CCD Maximum of image, computed from Good pixels
imageGradientX BUNIT/pixel Float ISR CCD Image gradient, X-direction, computed from Good pixels in background regions
imageGradientY BUNIT/pixel Float ISR CCD Image gradient, Y-direction, computed from Good pixels in background regions
diffimageMean BUNIT Float DiffIm CCD Mean of difference image, computed from Good pixels
diffimageSigma [None] Float DiffIm CCD Standard deviation about difference image mean, computed from Good pixels
diffimageKernelSum BUNIT/pixel Float DiffIm CCD Difference image kernel sum
wcsOffsetX arcsec Float AstroCal CCD Mean displacement between WCS solution and catalog stars along image Axis1
wcsSigmaX arcsec Float AstroCal CCD RMS of WCS solution along image Axis1
wcsOffsetY arcsec Float AstroCal CCD Mean displacement between WCS solution and catalog stars along image Axis2
wcsSigmaY arcsec Float AstroCal CCD RMS of WCS solution along image Axis2
nWcsAvailStars arcsec Float AstroCal CCD Number of catalog stars within the CCD footprint that could be used for WCS solution
nWcsGoodStars arcsec Float AstroCal CCD Number of catalog stars within the CCD footprint that were actually used for WCS solution
amplFringe [None] Float ISR CCD Amplitude of fringe pattern, relative to subtraction template
photDepth ABmag Float ImgChar CCD Median photometric depth
diffCatVsInst [mag] Float SFM CCD Median of differences of catalog mag and instrumental mag for all detected sources. Used to detect attenuation across the FOV due to partial cloud cover.
photZeropt mag Float ImgChar CCD Photometric zero-point--i.e., magnitude corresponding to one count in the calibrated image.
photZeroptSigma mmag Float ImgChar CCD Error distribution RMS of the internal photometric zero-point
psfWidth arcsec Float SFM CCD Median of delivered seeing distribution, measured from bright point-sources on CCD
psfWidthSigma arcsec Float SFM CCD Full-width at half-maximum of the delivered seeing distribution
psfEllipticity [None] Float ImgChar CCD Median of point source ellipticity distribution
trackletEfficiency percent Float DayMOPS DIASource Percent of sythetic DIASources with corresponding correct tracklet*
trackletAccuracy percent Float DayMOPS DIASource Percent of tracklets containing at least one synthetic DIASource which are correct
trackEfficiency percent Float DayMOPS DIASource Percent of sythetic DIASources with corresponding correct track*
trackAccuracy percent Float DayMOPS DIASource Percent of track containing at least one synthetic DIASource which are correct
orbitRecoveryEfficiency percent Float DayMOPS DIASource Percent of sythetic DIASources with corresponding correct orbit*
orbitRecoveryAccuracy percent Float DayMOPS DIASource Percent of orbits with track containing at least one synthetic DIASource which are correct

*: all MOPS "Efficiency" metrics disregard DIASources for which tracklet or track creation was impossible, e.g. moving too quickly, not in sufficient images, etc.