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Kron photometry is determined from the observed (smoothed) profile of each Detection.  A Kron radius is determined for each object, 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.

 

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 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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