Scene Modeling Pipeline

Systematic Uncertainties in the Absolute Calibration


The flux calibration of PS1 measurements relies on an iterative process that includes work from T12 and (Astier 2006) and is built on by this work and that of R14. T12 observes several HST Calspec standards.

The original observations of Calspec standards by T12 are supplemented by observations of those and other Calspec standards as part of the Pan-STARRs 3Pi survey. In this work, we determine the AB offsets between the observed magnitudes of the entire set of Calspec standards calibrated by ubercal and the synthetic photometry of these standards. This iterative process is thus a more accurate test of the absolute flux calibration of PS1. Once these offsets are found, R14 applies the offsets to the Medium Deep field catalogs, and analyzes further calibration uncertainties that may affect the SN measurements. Zeropoints for the nightly photometry of the supernovae are determined by comparing the photometry of a single image to the photometry from the Medium Deep field catalog at that location.

The main demarcation between the analysis of Rest et al. and Scolnic et al. when analyzing the calibration uncertainties is that Rest et al. analyzes the uncertainties in the photometric measurements of the stars and supernovae, while Scolnic et al. focus on how these uncertainties propagate to measurements of the absolute calibration of the PS1 system and the supernova distances.

Overview of Calibration Uncertainties

Uncertainties in the calibration of the various samples comprise the largest systematic uncertainty in our analysis. In Fig, we show a schematic describing the calibration of the various subsamples. The overall systematic uncertainty in the calibration of our combined PS1+lz sample may be expressed as the combination of three uncertainties. The first component encompasses systematic uncertainties in the nightly photometry and how well the filter bandpasses are measured. For the PS1 sample, R14 presents analysis of the systematic uncertainty due to spatial and temporal uncertainties in the nightly photometry. T12 presents the uncertainty in how well the bandpasses are measured (uncertainty of filter edges.

Spatial and temporal variation of the filter bandpasses propagate into our total calibration uncertainty in three ways: how the catalog photometry is determined, how the photometry of the Calspec standards is determined, and how the photometry of the supernovae is determined. We expect that the uncertainty in the nightly zeropoints to be small. We find by comparing Pan-STARRs and SDSS photometry that any variation of the PS1 photometry across the focal plane for colors \(0.4<g-i<1.5\) is less than 3mmag and is difficult to detect because of noise. The effect of variation of the filter bandpasses on photometry of the Calspec standards and the supernovae are significantly larger because of the very blue colors of a large fraction of the Calspec standards and the narrow spectral features of these supernovae. These effects are both considered.

The second major component of the total calibration uncertainty is in determining the flux zeropoints of each filter based on observations of astronomical standards. Since the accuracy of the internal PS1 measurements of the flux zeropoints is not better than \(1\%\), the zeropoints are adjusted so that the observed photometry of HST Calspec standards (e.g., AB - HST Calspec