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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).
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The starting point for the PS1 data archive is at Pan-STARRS1 data archive home page.
Definition
Look up the formal definition (Kron 1980, ApJS 43, 305).
The Kron 1980 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)
Be aware that by definition Kron magnitudes do NOT encompass the total flux from an object, and in DR1 no attempt is made to correct the Kron fluxes for this missing light. Some information on the size of this effect for galaxies can be seen in this article, but very roughly expect 10% of the flux to be missing.
Values Available
Kron parameters are listed in the Detection table as followsseveral different tables, depending on their origin (warp images, stack images, mean values, etc.) The PS1 MeanObject table fields table includes:
Name | Unit | Data Type | Size | Default Value | Description |
|---|
gMeanKronMag |
AB magnitudes | REAL | 4 | -999 | Mean Kron (1980) |
magnitude from g filter detections. |
gMeanKronMagErr |
AB magnitudes | REAL | 4 | -999 | Error |
in mean Kron (1980) |
magnitude from g filter detections. |
gMeanKronMagStd |
AB magnitudes | REAL | 4 | -999 | Standard deviation of Kron (1980) |
magnitudes from g filter detections. | |||||
gMeanKronMagNpt | dimensionless | SMALLINT | 2 | -999 | Number of measurements included in mean Kron (1980) magnitude from g filter detections. |
and similar entries in the same row for rizy filters.
StackObjectThin contains:
Name | Unit | Data Type | Size | Default Value | Description |
|---|---|---|---|---|---|
gKronMag | AB magnitudes | REAL | 4 | -999 | Kron (1980) magnitude from g filter stack detection |
gKronMagErr | AB magnitudes | REAL | 4 | -999 | Error in Kron (1980) magnitude from g filter stack detection. |
and similar entries in the same row for rizy filters rizy filters.
StackObjectAttributes contains:
Name | Unit | Data Type | Size | Default Value | Description |
|---|---|---|---|---|---|
gKronFlux | Janskys | REAL | 4 | -999 | Kron (1980) flux from g filter stack detection. |
gKronFluxErr | Janskys | REAL | 4 | -999 | Error in Kron (1980) flux from g filter stack detection. |
gKronRad | arcsec | REAL | 4 | -999 | Kron (1980) radius from g filter stack detection. |
and similar entries in the same row for rizy filters rizy filters.
MeanObject ForcedMeanObject contains:
Name | Unit | Data Type | Size | Default Value | Description |
|---|
gFKronFlux |
Janskys | REAL | 4 | -999 | Mean Kron (1980) |
flux from forced single epoch g filter detections. |
gFKronFluxErr |
Janskys | REAL | 4 | -999 | Error in mean Kron (1980) |
flux from forced single epoch g filter detections. |
gFKronFluxStd |
Janskys | REAL | 4 | -999 | Standard deviation of Kron (198) fluxes from forced single epoch g filter detections. | |
gFKronMag | AB magnitudes | REAL | 4 | -999 | Magnitude from mean Kron (1980) |
flux from forced single epoch g filter detections. |
gFKronMagErr |
AB magnitudes |
REAL |
4 | -999 |
Error in magnitude from mean Kron (1980) |
flux from forced single epoch g filter detections. |
and similar entries in the same row for rizy filters.
rizy filters.
The tables below with multi-epoch measurements are not included in the DR1 database but are included here for completeness.
PS1 Detection table fields ForcedWarpMeasurement contains:
Name | Unit | Data Type | Size | Default Value | Description |
|---|
kronFlux | Janskys | REAL | 4 | -999 | Kron (1980) flux. |
kronFluxErr | Janskys | REAL | 4 | -999 | Error |
on Kron (1980) flux. |
kronRad | arcsec | REAL | 4 | -999 | Kron (1980) radius. |
ForcedMeanObject ForcedWarpMeasurement contains:
Name | Unit | Data Type | Size | Default Value | Description |
|---|
FkronFlux | Janskys | REAL | 4 | -999 |
Kron (1980) flux |
. |
FkronFluxErr | Janskys | REAL | 4 | -999 | Error in |
Kron (1980) flux |
. |
FkronRad |
arcsec | REAL | 4 | -999 |
Kron ( |
and similar entries in the same row for rizy filters.
1980) radius. |
DiffDetection contains:
Name | Unit | Data Type | Size | Default Value | Description |
|---|---|---|---|---|---|
DkronFlux | Janskys | REAL | 4 | -999 | Kron (1980) flux. |
DkronFluxErr | Janskys | REAL | 4 | -999 | Error in Kron (1980) flux. |
DkronRad | arcsec | REAL | 4 | -999 | Kron (1980) radius. |
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.
