Andrés A. Plazas, Gary M. Bernstein
The wavelength dependence of atmospheric refraction causes elongation of finite-bandwidth images along the elevation vector, which produces spurious signals in weak gravitational lensing shear measurements unless this atmospheric dispersion is calibrated and removed to high precision. Because astrometric solutions and point spread function (PSF) characteristics are typically calibrated from stellar images, differences between the reference stars' spectra and the galaxies' spectra will leave residual errors in both the astrometric positions ($\Delta{\bar{R}}$) and in the second moment (width) of the wavelength-averaged PSF ($\Delta{v}$) for galaxies. We estimate the level of $\Delta{V}$ that will induce spurious weak lensing signals in PSF-corrected galaxy shapes that exceed the statistical errors of the {\em Dark Energy Survey (DES)} and the {\em Large Synoptic Survey Telescope (LSST)} cosmic-shear experiments. We also estimate the $\Delta{\bar{R}}$ signals that will produce unacceptable spurious distortions after stacking of exposures taken at different airmasses and hour angles. Using standard galaxy and stellar spectral templates we calculate the resultant errors in the $griz$ bands, and find that atmospheric dispersion differentials, left uncorrected, exceed the {\em DES} cosmic-shear requirements in the $g$ and $r$ bands, and exceed the stricter LSST requirements in $i$ band. We find that a simple correction linear in galaxy color is accurate enough to recover the use of $r$ band for DES and $i$ band for LSST. More complex approaches to correction of the atmospheric dispersion signal will be needed to use the $g$ band for DES cosmic-shear measurements or to use the $g$ or $r$ bands for LSST cosmic-shear measurements.
View original:
http://arxiv.org/abs/1204.1346
No comments:
Post a Comment