Photometric Calibration Methodology
Level-1 Data (Single Exposures)
PTF photometry is tied to the SDSS. The public data release specifically ties the Sextractor adaptive aperture “MAGAUTO” (a Kron-like aperture) to the SDSS. Other apertures may have small systematic offsets. This was motivated to account for the time-variable seeing due to the atmosphere and the spatially variable seeing resulting from the telescope/camera optical train. The photometric calibration is determined by using observations taken throughout the night that overlap with the SDSS. The PTF instrumental magnitudes are fit to the SDSS magnitudes. The fit parameters include the color term, the extinction, the zeropoint, a spatial component in x and y, and a time variable component. The photometric calibration system is discussed in further detail by Ofek et al. (2012).
Level-2 Data (Coadds)
During coaddition, all input frames are scaled to a common calibration. This is based on the existing single-frame calibration; no additional recalibration occurs after coaddition. The coadded reference frames all have a common zeropoint of 27.
