Bernstein, Gary

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Now showing 1 - 4 of 4
  • Publication
    Noise and Bias in Square-Root Compression Schemes
    (2010-03-01) Bernstein, Gary M; Bebek, Chris; Rhodes, Jason; Stoughton, Chris; Vanderveld, R. Ali; Yeh, Penshu
    We investigate data compression schemes for proposed all-sky diffraction-limited visible/NIR sky surveys aimed at the dark-energy problem. We show that lossy square-root compression to 1 bit pixel-1 of noise, followed by standard lossless compression algorithms, reduces the images to 2.5–4 bits pixel-1, depending primarily upon the level of cosmic-ray contamination of the images. Compression to this level adds noise equivalent to ≤ 10% penalty in observing time. We derive an analytic correction to flux biases inherent to the square-root compression scheme. Numerical tests on simple galaxy models confirm that galaxy fluxes and shapes are measured with systematic biases ≲10-4 induced by the compression scheme, well below the requirements of supernova and weak gravitational lensing dark-energy experiments. In a related investigation, Vanderveld and coworkers bound the shape biases using realistic simulated images of the high-Galactic–latitude sky. The square-root preprocessing step has advantages over simple (linear) decimation when there are many bright objects or cosmic rays in the field, or when the background level will vary.
  • Publication
    Evaluating Dark Energy Probes Using Multidimensional Dark Energy Parameters
    (2007-05-17) Albrecht, Andreas; Bernstein, Gary
    We investigate the value of future dark-energy experiments by modeling their ability to constrain the dark-energy equation of state. Similar work was recently reported by the Dark Energy Task Force (DETF) using a two dimensional parameterization of the equation-of-state evolution. We examine constraints in a nine-dimensional dark-energy parameterization, and find that the best experiments constrain significantly more than two dimensions in our 9D space. Consequently the impact of these experiments is substantially beyond that revealed in the DETF analysis, and the estimated cost per ‘‘impact’’ drops by about a factor of 10 as one moves to the very best experiments. The DETF conclusions about the relative value of different techniques and of the importance of combining techniques are unchanged by our analysis.
  • Publication
    Photometric Redshift Biases from Galaxy Evolution
    (2010-04-01) MacDonald, C. Jonathan; Bernstein, Gary
    Proposed cosmological surveys will make use of photometric redshifts of galaxies that are significantly fainter than any complete spectroscopic redshift surveys that exist to train the photo-z methods. We investigate the photo-z biases that result from known differences between the faint and bright populations: a rise in active galactic nucleus (AGN) activity toward higher redshift, and a metallicity difference between intrinsically luminous and faint early-type galaxies. We find that even very small mismatches between the mean photometric target and the training set can induce photo-z biases large enough to corrupt derived cosmological parameters significantly. Our results suggest that a metallicity shift of ∼0.003 dex in an old population, or contamination of any galaxy spectrum with ∼0.2% AGN flux, is sufficient to induce a 10-3 bias in photo-z. These results highlight the danger in extrapolating the behavior of bright galaxies to a fainter population, and the desirability of a spectroscopic training set that spans all of the characteristics of the photo-z targets, i.e., extending to the 25th mag or fainter galaxies that will be used in future surveys.
  • Publication
    Dark Energy Constratins from Lensing-Detected Galaxy Clusters
    (2006-06-21) Marian, Laura; Bernstein, Gary
    We study the ability of weak-lensing surveys to detect galaxy clusters and constrain cosmological parameters, in particular, the equation of state of dark energy. There are two major sources of noise for weak-lensing cluster measurements: the ‘‘shape noise’’ from the intrinsic ellipticities of galaxies; and the large scale projection noise. We produce a filter for the shear field which optimizes the signal-to-noise of shape-noise-dominated shear measurements. Our Fisher-matrix analysis of this projected-mass observable makes use of the shape of this mass function, and takes into account the Poisson variance, sample variance, shape noise, and projected-mass noise, and also the fact that the conversion of the shear signal into mass is cosmology-dependent. The Fisher analysis is applied to both a nominal 15 000 deg2 ground-based survey and a 1000 deg2 space-based survey. Assuming a detection threshold of S/N = 5, we find both experiments detect ≈ 20 000 clusters, and yield 1-σ constraints of Δ w0 ≈ 0.07, Δ wa ≈ 0.20 when combined with cosmic microwave background data (for flat universe). The projection noise exceeds the shape noise only for clusters at z ≲ 0.1 and has little effect on the derived dark-energy constraints. Sample variance does not significantly affect either survey. Finally, we note that all these results are extremely sensitive to the noise levels and detection thresholds that we impose. They can be significantly improved if we combine ground and space surveys as independent experiments and add their corresponding Fisher matrices.