Describe the aperture photometry information for detections.
Aperture photometry refers to the measurement of the total count rate - or equivalently instrumental magnitude - based on integration over an aperture. 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).
Parameters in Detection Table
|Name||Unit||Data Type||Size||Default Value|
Flux in seeing-dependent aperture.
|apFluxErr||Janskys||REAL||4||-999||Error on flux in seeing-dependent aperture.|
|apFillF||dimensionless||REAL||4||-999||Aperture fill factor.|