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Describe the aperture photometry information for detections.
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Definition
Aperture photometry here refers to the measurement of the total count rate - or equivalently instrumental magnitude - for a point source based on integration over an aperture. This photometry should not be used for extended sources. Note that these values are quite distinct from the fixed radial aperture fluxes which are also measured in PS1.
It is usually computed using one of a number of predefined routines, typically designed to account for partial pixels via some form of interpolation. Characteristically, there appears to be little information on the specifics of the aperture photometry definitions adopted in PanSTARRS1 PSPS processing. The total count rate is typically corrected for the "sky", or local background. The local background can be estimated, e.g., from a robust mean or median carried out on a formally empty region of the image, and then multiplied by the area of the aperture and subtracted from the count rate included in that aperture. The total count rate can then be corrected, both for variations in sensitivity (e.g., air mass) and for the specifics of the aperture used (e.g., by applying an aperture correction based on the PSF at that position). Aperture magnitudes are typically more robust than PSF magnitudes to variations in seing, but are more subject to variations in crowded fields. Determining what is a source and the correct background level is also challenging in highly crowded areas. It would be very desirable to have specific information on how crowding and aperture effects are treated in PSPS processing.
In PS1, an 'optimal' aperture radius is determined based on the local PSF. The wings of the same analytic PSF are then used to extrapolate the flux measured inside this aperture to a 'total' flux (note that the value of this extrapolation is not recorded in PSPS) [Source: N. Metcalfe - CHECK this with Hawaii].
Parameters
in Detection Tablein MeanObject
| Name | Unit | Data Type | Size | Default Value | ||
|---|---|---|---|---|---|---|
| apFluxgMeanApMag | JanskysAB magnitudes | REAL | 4 | -999Flux in | seeing-dependent apertureMean aperture magnitude from g filter detections. | |
| apFluxErrgMeanApMagErr | JanskysAB magnitudes | REAL | 4 | -999 | Error on flux in seeing-dependent aperturemean aperture magnitude from g filter detections. | |
| apFillFgMeanApMagStd | dimensionlessAB magnitudes | REAL | 4 | -999 | Aperture fill factorStandard deviation of aperture magnitudes from g filter detections. | |
| apRadiusgMeanApMagNpt | arcsecdimensionless | REALSMALLINT | 42 | -999 | Aperture radius. | Number of measurements included in mean aperture magnitude from g filter detections. |
and similar entries for the rizy filtersThe equivalent stack measurements are found in StackObjectAttributes.
Parameters
inin StackObjectThin
| Name | Unit | Data Type | Size | Default Value | |
|---|---|---|---|---|---|
| gApMag | AB magnitudes | REAL | 4 | -999 | Aperture magnitude from g filter stack detection. |
| gApMagErr | AB magnitudes | REAL | 4 | -999 | Error in aperture magnitude from g filter stack detection. |
and similar entries for the rizy filters.
Parameters in
MeanObjectDetection Table
| Name | Unit | Data Type | Size | Default Value | ||
|---|---|---|---|---|---|---|
| gMeanApMagapFlux | AB magnitudesJanskys | REAL | 4 | -999 | Mean aperture magnitude from g filter detections Flux in seeing-dependent aperture. | |
| gMeanApMagErrapFluxErr | AB magnitudesJanskys | REAL | 4 | -999 | Error on flux in mean aperture magnitude from g filter detectionsseeing-dependent aperture. | |
| gMeanApMagStdapFillF | AB magnitudesdimensionless | REAL | 4 | -999 | Standard deviation of aperture magnitudes from g filter detectionsAperture fill factor. | |
| gMeanApMagNptapRadius | dimensionlessarcsec | SMALLINTREAL | 24 | -999 | Number of measurements included in mean aperture magnitude from g filter detections. | Aperture radius. |
The equivalent stack measurements are found in StackObjectAttributesand similar entries for the rizy filters.
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