Engineering topics

Bernstein, Gary

Publications and source records attributed to Bernstein, Gary.

8 records

DOE OSTI2018

Dark energy survey operations: years 4 and 5

The Dark Energy Survey is a “stage III” dark energy experiment, performing an optical imaging survey to measure cos-mological equation of state parameters using four independent methods. The scope and complexity of the survey, originallyscheduled for 525 nights of observing spread over five years and combining a 5000 degree wide survey with a 10 field weeklytime-domain survey, introduced complex strategic and tactical scheduling problems that needed to be addressed. We beginwith an overview of the process used to develop DES strategy and tactics, from the inception of the project, to task forces thatstudied and developed strategy changes over the course of the survey, to the nightly pre-observing meeting in which imme-diate tactical issues were addressed. We then summarize the strategic choices made for each sub-survey, including metrics,scheduling considerations, choice of time domain fields and their sequences of exposures, and wide survey footprint andpointing layout choices. We go on to describe the detailed process that determined which specific exposures were taken atwhich specific times. We give a chronology of the strategic and tactical peculiarities of each year of observing, including theproposal and execution of a sixth year. We give an overview ofobstac, the implementation of the DES scheduler used to sim-ulate and evaluate strategic and tactical options, and automate exposure scheduling; and describe developments inobstacfor use after DES. Appendices describe further details of data quality evaluation,τ, andteff; airmass calculation; and model-ing of the seeing and sky brightness. The significant corpus of DES data indicates that the simple scaling relations for seeingas a function of wavelength and airmass derived from the Kolmogorov turbulence model work adequately for exposureplanning purposes: deviations from these relations are modest in comparison with short time-scale seeing variations

Gruendl, Robert A.↗

DOE OSTI2018

The ellipticity of galaxy cluster haloes from satellite galaxies and weak lensing

Here, we study the ellipticity of galaxy cluster haloes as characterized by the distribution of cluster galaxies and as measured with weak lensing. We use Monte Carlo simulations of elliptical cluster density profiles to estimate and correct for Poisson noise bias, edge bias and projection effects. We apply our methodology to 10 428 Sloan Digital Sky Survey clusters identified by the redMaPPer algorithm with richness above 20. We find a mean ellipticity =0.271 ± 0.002 (stat) ±0.031 (sys) corresponding to an axis ratio = 0.573 ± 0.002 (stat) ±0.039 (sys). We compare this ellipticity of the satellites to the halo shape, through a stacked lensing measurement using optimal estimators of the lensing quadrupole based on Clampitt and Jain (2016). We find a best-fitting axis ratio of 0.56 ± 0.09 (stat) ±0.03 (sys), consistent with the ellipticity of the satellite distribution. Thus, cluster galaxies trace the shape of the dark matter halo to within our estimated uncertainties. Finally, we restack the satellite and lensing ellipticity measurements along the major axis of the cluster central galaxy's light distribution. From the lensing measurements, we infer a misalignment angle with an root-mean-square of 30° ± 10° when stacking on the central galaxy. We discuss applications of halo shape measurements to test the effects of the baryonic gas and active galactic nucleus feedback, as well as dark matter and gravity. The major improvements in signal-to-noise ratio expected with the ongoing Dark Energy Survey and future surveys from Large Synoptic Survey Telescope, Euclid, and Wide Field Infrared Survey Telescope will make halo shapes a useful probe of these effects.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

Limiting Magnitude, τ, t eff , and Image Quality in DES Year 1

The Dark Energy Survey (DES) is an astronomical imaging survey being completed with the DECam imager on the Blanco telescope at CTIO. After each night of observing, the DES data management (DM) group performs an initial processing of that night's data, and uses the results to determine which exposures are of acceptable quality, and which need to be repeated. The primary measure by which we declare an image of acceptable quality is $\tau$, a scaling of the exposure time. This is the scale factor that needs to be applied to the open shutter time to reach the same photometric signal to noise ratio for faint point sources under a set of canonical good conditions. These conditions are defined to be seeing resulting in a PSF full width at half maximum (FWHM) of 0.9" and a pre-defined sky brightness which approximates the zenith sky brightness under fully dark conditions. Point source limiting magnitude and signal to noise should therefore vary with t in the same way they vary with exposure time. Measurements of point sources and $\tau$ in the first year of DES data confirm that they do. In the context of DES, the symbol $t_{eff}$ and the expression "effective exposure time" usually refer to the scaling factor, $\tau$, rather than the actual effective exposure time; the "effective exposure time" in this case refers to the effective duration of one second, rather than the effective duration of an exposure.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

Atmospheric Dispersion Effects in Weak Lensing Measurements

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 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 (dr) and in the second moment (width) of the wavelength-averaged PSF (dv) for galaxies.We estimate the level of dv that will induce spurious weak lensing signals in PSF-corrected galaxy shapes that exceed the statistical errors of the DES and the LSST cosmic-shear experiments. We also estimate the dr signals that will produce unacceptable spurious distortions after stacking of exposures taken at different airmasses and hour angles. We also calculate the errors in the griz bands, and find that dispersion systematics, uncorrected, are up to 6 and 2 times larger in g and r bands,respectively, than the requirements for the DES error budget, but can be safely ignored in i and z bands. For the LSST requirements, the factors are about 30, 10, and 3 in g, r, and i bands,respectively. We find that a simple correction linear in galaxy color is accurate enough to reduce dispersion shear systematics to insignificant levels in the r band for DES and i band for LSST,but still as much as 5 times than the requirements for LSST r-band observations. More complex corrections will likely be able to reduce the systematic cosmic-shear errors below statistical errors for LSST r band. But g-band effects remain large enough that it seems likely that induced systematics will dominate the statistical errors of both surveys, and cosmic-shear measurements should rely on the redder bands.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

Combining weak-lensing tomography and spectroscopic redshift surveys

Redshift space distortion (RSD) is a powerful way of measuring the growth of structure and testing General Relativity, but it is limited by cosmic variance and the degeneracy between galaxy bias b and the growth rate factor f. The cross-correlation of lensing shear with the galaxy density field can in principle measure b in a manner free from cosmic variance limits, breaking the f-b degeneracy and allowing inference of the matter power spectrum from the galaxy survey. We analyze the growth constraints from a realistic tomographic weak lensing photo-z survey combined with a spectroscopic galaxy redshift survey over the same sky area. For sky coverage f sky = 0.5, analysis of the transverse modes measures b to 2-3% accuracy per Δz = 0.1 bin at z < 1 when ~10 galaxies arcmin –2 are measured in the lensing survey and all halos with M > M min = 10 13 h –1 M ⊙ have spectra. For the gravitational growth parameter parameter γ (f = Ω γ m ), combining the lensing information with RSD analysis of non-transverse modes yields accuracy σ(γ) ≈ 0.01. Adding lensing information to the RSD survey improves \sigma(\gamma) by an amount equivalent to a 3x (10x) increase in RSD survey area when the spectroscopic survey extends down to halo mass 10 13.5 (10 14 ) h –1 M ⊙ . We also find that the σ(γ) of overlapping surveys is equivalent to that of surveys 1.5-2 times larger if they are separated on the sky. This gain is greatest when the spectroscopic mass threshold is 10 13 -10 14 h –1 M ⊙ , similar to LRG surveys. The gain of overlapping surveys is reduced for very deep or very shallow spectroscopic surveys, but any practical surveys are more powerful when overlapped than when separated. As a result, the gain of overlapped surveys is larger in the case when the primordial power spectrum normalization is uncertain by > 0.5%.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

Reducing Zero-point Systematics in Dark Energy Supernova Experiments

We study the effect of filter zero-point uncertainties on future supernova dark energy missions. Fitting for calibration parameters using simultaneous analysis of all Type Ia supernova standard candles achieves a significant improvement over more traditional fit methods. This conclusion is robust under diverse experimental configurations (number of observed supernovae, maximum survey redshift, inclusion of additional systematics). This approach to supernova fitting considerably eases otherwise stringent mission cali- bration requirements. As an example we simulate a space-based mission based on the proposed JDEM satellite; however the method and conclusions are general and valid for any future supernova dark energy mission, ground or space-based.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

Optimal linear reconstruction of dark matter from halo catalogues

The dark matter lumps (or "halos") that contain galaxies have locations in the Universe that are to some extent random with respect to the overall matter distributions. We investigate how best to estimate the total matter distribution from the locations of the halos. We derive the weight function w(M) to apply to dark-matter haloes that minimizes the stochasticity between the weighted halo distribution and its underlying mass density field. The optimal w(M) depends on the range of masses of halos being used. While the standard biased-Poisson model of the halo distribution predicts that bias weighting is optimal, the simple fact that the mass is comprised of haloes implies that the optimal w(M) will be a mixture of mass-weighting and bias-weighting. In N-body simulations, the Poisson estimator is up to 15× noisier than the optimal. Optimal weighting could make cosmological tests based on the matter power spectrum or cross-correlations much more powerful and/or cost effective.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1993

The near-infrared Tully-Fisher relation - A preliminary study of the Coma and Abell 400 clusters

We have started a large project to study the NIR Tully-Fisher (TF) relation using H- and I-band surface photometry of spiral galaxies. A preliminary study of 20 spirals in the Coma and Abell 400 clusters is presented. The NIR images have been used to derive accurate inclinations and total magnitudes, and rotational linewidths are measured from high-quality 21-cm Arecibo data. The scatter in the Coma TF plot is found to be 0.19 mag in the H band and 0.20 mag in the I band for a set of 13 galaxies, if we assume that they are all at the same distance. The deviation of the Coma galaxies from the best-fit Tully-Fisher relation is correlated with their redshift, indicating that some of the galaxies are not bound to the cluster. Indeed, if we treat all the galaxies in the Coma sample as undergoing free Hubble expansion, the TF scatter drops to 0.12 and 0.13 mag for the H- and I-band datasets, respectively. The Abell 400 sample is best fit by a common distance model, yielding a scatter of 0.12 mag for seven galaxies in H using a fixed TF slope. We are in the process of studying cluster and field spirals out to about 10,000 km/s in order to calibrate the NIR TF relation and will apply it to more nearby galaxies to measure the peculiar velocity field in the local universe.

Guhathakurta, Puragra↗