Day 1252

Thursday.  Friday.

1705.01109
An unbiased estimator for the ellipticity from image moments
Tessore

An unbiased estimator for the ellipticity of an object in a noisy image is given in terms of the image moments.  Three assumptions are made: i) the pixel noise is normally distributed, although with arbitrary covariance matrix, ii) the image moments are taken about a fixed centre, and iii) the point-spread function is known.  The relevant combinations of image moments are then jointly normal and their covariance matrix can be computed.  A particular estimator for the ratio of the means of jointly normal variates is constructed and used to provide the unbiased estimator of the ellipticity.  Furthermore, an unbiased estimate of the covariance of the new estimator is also given.


1705.01599
Characterization and photometric performance of the Hyper Suprime-Cam Software Pipeline
Huang, et al

The Subaru Strategic Program (SSP) is an ambitious multi-band survey using HSC on Subaru telescope.  The Wide layer of the SSP is both wide and deep, reaching a detection limit of i~26.0 mag.  At these depths, it is challenging to achieve accurate, unbiased, and consistent photometry across all 5 bands.  The HSC data are reduced using a pipeline that builds on the prototype pipeline for the LSST.  Developed a Python-based, flexible framework to inject synthetic galaxies into real HSC images called SynPipe.  Explain the design and implementation of SynPipe and generate a sample of synthetic galaxies to examine the photometric performance of the HSC pipeline.  For stars, achieve 1% photometric precision at i~19.0 mag and 6% precision at i~25.0 in the i-band.  For synthetic galaxies with single-Sersic profiles, forced CModel photometry achieves 13% photometric precision at i~20.0 mag and 18% precision at i~25.0 in the i-band.  Show that both forced PSF and Model photometry yield unbiased color estimates that are robust to seeing conditions.  Identify several caveats that apply to the version of HSC pipeline used for the first public HSC data release (DR1) that needs to be taking into consideration.  First, the degree to which an object is blended with other objects impacts the overall photometric performance.  This is especially true for point sources.  Highly blended objects tend to have larger photometric uncertainties, systematically underestimated fluxes and slightly biased colors.  Second, >20% of stars at 22.5<i<25.0 mag can be misclassified as extended objects.  Third, the current CModel algorithm tends to strongly underestimate the half-light radius and ellipticity of galaxy with i>21.5 mag.