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Kron photometry is determined from the observed (smoothed) profile of each Detection.  A Kron radius is determined for all objects, irrespective of whether they are point sources or not, and the total magnitude is inferred from the growth curve (enclosed flux vs. radius).

Contents

Definition

Look up the formal definition (Kron 1980, ApJS 43, 305).  The paper shows how to define a nominal magnitude for an extended object on the basis of the shape of the light profile. 

In the PanSTARRS implementation, the definition of the Kron radius and Kron magnitude is carried out as follows (from the PS1 guide (Kron Parameters)  [PS1 wiki login required]:

  • Kron Radius determined from smoothed image
  • start with isophotal aperture at sky level
  • measure moments
  • redefine aperture to be 6x 1st radial moment
  • re-measure moments
  • do we iterate like this again - not sure?
  • Kron flux is measured inside 2.5 * 1st radial moment
  • 1.0 * 1st radial moment and 4.0 * 1st radial moment fluxes are also measured as KRON_INNER and KRON_OUTER (don't make it into PSPS though, only in cmfs)

Values Available

Kron parameters are listed in the Detection table as follows:

NameUnitData TypeSizeDefault ValueDescription
kronFluxJanskysREAL4-999Kron (1980) flux.
kronFluxErrJanskysREAL4-999Error on Kron (1980) flux.
kronRadarcsecREAL4-999Kron (1980) radius.

 

StackObjectThin contains:

NameUnitData TypeSizeDefault ValueDescription
gKronMagAB magnitudesREAL4-999Kron (1980) magnitude from g filter stack detection
gKronMagErrAB magnitudesREAL4-999Error in Kron (1980) magnitude from g filter stack detection.

and similar entries in the same row for rizy filters.

 

StackObjectAttributes contains:

NameUnitData TypeSizeDefault ValueDescription
gKronFluxJanskysREAL4-999Kron (1980) flux from g filter stack detection.
gKronFluxErrJanskysREAL4-999Error in Kron (1980) flux from g filter stack detection.
gKronRadarcsecREAL4-999Kron (1980) radius from g filter stack detection.

and similar entries in the same row for rizy filters.

 

MeanObject contains:

NameUnitData TypeSizeDefault ValueDescription
gMeanKronMagAB magnitudesREAL4-999Mean Kron (1980) magnitude from g filter detections.
gMeanKronMagErrAB magnitudesREAL4-999Error in mean Kron (1980) magnitude from g filter detections.
gMeanKronMagStdAB magnitudesREAL4-999Standard deviation of Kron (1980) magnitudes from g filter detections.
gMeanKronMagNptdimensionlessSMALLINT2-999Number of measurements included in mean Kron (1980) magnitude from g filter detections.

and similar entries in the same row for rizy filters.

 

ForcedWarpMeasurement contains:

NameUnitData TypeSizeDefault ValueDescription
FkronFluxJanskysREAL4-999Kron (1980) flux.
FkronFluxErrJanskysREAL4-999Error in Kron (1980) flux.
FkronRadarcsecREAL4-999Kron (1980) radius.

 

ForcedMeanObject contains:

NameUnitData TypeSizeDefault ValueDescription
gFKronFluxJanskysREAL4-999Mean Kron (1980) flux from forced single epoch g filter detections.
gFKronFluxErrJanskysREAL4-999Error in mean Kron (1980) flux from forced single epoch g filter detections.
gFKronFluxStdJanskysREAL4-999Standard deviation of Kron (198) fluxes from forced single epoch g filter detections.
gFKronMagAB magnitudesREAL4-999Magnitude from mean Kron (1980) flux from forced single epoch g filter detections.
gFKronMagErrAB magnitudesREAL4-999Error in magnitude from mean Kron (1980) flux from forced single epoch g filter detections.

and similar entries in the same row for rizy filters.

 

DiffDetection contains:

NameUnitData TypeSizeDefault ValueDescription
DkronFluxJanskysREAL4-999Kron (1980) flux.
DkronFluxErrJanskysREAL4-999Error in Kron (1980) flux.
DkronRadarcsecREAL4-999Kron (1980) radius.

References

Most of the information contained here is based on the PS1 Guide http://ps1sc.ifa.hawaii.edu/PS1wiki/index.php/PS1Guide#Kron_parameters (requires PS1 wiki login).  Information on the contents of the Table is obtained from the PSPS Schema Browser at http://web01.psps.ifa.hawaii.edu/PSI/schema_browser.php?describeTable=Detection (requires PSPS login).  The STScI verison of the schema browser is at http://plps3v.stsci.edu/PSI/schema_browser.php.

The original definitions presumably are from Kron (1980): http://adsabs.harvard.edu/abs/1980PhyS...21..652K

A modern implementation is described, e.g., for 2MASS at: http://www.ipac.caltech.edu/2mass/releases/allsky/doc/sec4_5e.html. If you have more specific information on the exact definitions used in PanSTARRS processing, please add a comment or email stefano@stsci.edu.

 

 
  

 

 

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