An Empirical Pixel-Based Correction for Imperfect CTE. I. HST's Advanced Camera for Surveys

We use an empirical approach to characterize the effect of charge-transfer efficiency (CTE) losses in images taken with the Wide-Field Channel of the Advanced Camera for Surveys. The study is based on profiles of warm pixels in 168 dark exposures taken between September and October 2009. The dark exposures allow us to explore charge traps that affect electrons when the background is extremely low. We develop a model for the readout process that reproduces the observed trails out to 70 pixels. We then invert the model to convert the observed pixel values in an image into an estimate of the original pixel values. We find that when we apply the image-restoration process to science images with a variety of stars on a variety of background levels, it restores flux, position, and shape. This means that the observed trails contain essentially all of the flux lost to inefficient CTE. The Space Telescope Science Institute is currently evaluating this algorithm with the aim of optimizing it and eventually providing enhanced data products. The empirical procedure presented here should also work for other epochs (eg., pre-SM4), though the parameters may have to be recomputed for the time when ACS was operated at a higher temperature than the current -81 C. Finally, this empirical approach may also hold promise for other instruments, such as WFPC2, STIS, the ACS's HRC, and even WFC3/UVIS.Comment: 86 pages, 25 figures (6 in low resolution). PASP accepted on July 21, 201

Toward High-Precision Astrometry with WFPC2. I. Deriving an Accurate PSF

The first step toward doing high-precision astrometry is the measurement of individual stars in individual images, a step that is fraught with dangers when the images are undersampled. The key to avoiding systematic positional error in undersampled images is to determine an extremely accurate point-spread function (PSF). We apply the concept of the {\it effective} PSF, and show that in images that consist of pixels it is the ePSF, rather than the often-used instrumental PSF, that embodies the information from which accurate star positions and magnitudes can be derived. We show how, in a rich star field, one can use the information from dithered exposures to derive an extremely accurate effective PSF by iterating between the PSF itself and the star positions that we measure with it. We also give a simple but effective procedure for representing spatial variations of the HST PSF. With such attention to the PSF, we find that we are able to measure the position of a single reasonably bright star in a single image with a precision of 0.02 pixel (2 mas in WF frames, 1 mas in PC), but with a systematic accuracy better than 0.002 pixel (0.2 mas in WF, 0.1 mas in PC), so that multiple observations can reliably be combined to improve the accuracy by $\surd N$.Comment: 33 pp. text + 15 figs.; accepted by PAS

Accurate Internal Proper Motions of Globular Clusters

We have undertaken a long term program to measure high precision proper motions of nearby Galactic globular cluster (GC) stars using multi-epoch observations with the WFPC2 and the ACS cameras on-board the Hubble Space Telescope. The proper motions are used to study the internal cluster kinematics, and to obtain accurate cluster distances. In this paper, we also show how the proper motions of the field stars projected in the direction of the studied clusters can be used to set constraints on the Galaxy kinematics.Comment: 4 pages, 2 figures, A.S.P. Conf. Ser., in press in Vol. 296, 200

Guidelines for Economic Evaluation of Public Sector Water Resource Projects

Water development plays an important role in the economy of states and regions. However, procedures for estimating the expected net worth of proposed projects have never been simple, and results have rarely been without controversy. This report presents some guidelines for the application of economic evaluation procedures in project analysis of public sector water development in North Dakota. A brief history of North Dakota water development and two case studies of North Dakota water projects are included in this report.Resource /Energy Economics and Policy,

HST Astrometry of M4 and the Galactic Constant V_0/R_0

From multi-epoch WFPC2/HST observations we present astrometric measurements of stars in the Galactic globular cluster M4 (NGC 6121) and in the foreground/background. The presence of an extragalactic point source allows us to determine the absolute proper motion of the cluster, and, through use of the field stars in this region only 18 degree from the Galactic center, to measure the difference between the Oort constants, A-B. We find: (mu_alpha cos dec, mu_dec)_J2000 = (-13.21 +/- 0.35, -19.28 +/- 0.35) mas/yr, and A-B = V_0/R_0 = 27.6 +/- 1.7 km / s / kpc.Comment: 20 pages, 6 figures, A.J.__ACCEPTED__, 1 April, 2003, (...!

The Arches Cluster: Extended Structure and Tidal Radius

At a projected distance of ~26 pc from Sgr A*, the Arches cluster provides insight to star formation in the extreme Galactic Center (GC) environment. Despite its importance, many key properties such as the cluster's internal structure and orbital history are not well known. We present an astrometric and photometric study of the outer region of the Arches cluster (R > 6.25") using HST WFC3IR. Using proper motions we calculate membership probabilities for stars down to F153M = 20 mag (~2.5 M_sun) over a 120" x 120" field of view, an area 144 times larger than previous astrometric studies of the cluster. We construct the radial profile of the Arches to a radius of 75" (~3 pc at 8 kpc), which can be well described by a single power law. From this profile we place a 3-sigma lower limit of 2.8 pc on the observed tidal radius, which is larger than the predicted tidal radius (1 - 2.5 pc). Evidence of mass segregation is observed throughout the cluster and no tidal tail structures are apparent along the orbital path. The absence of breaks in the profile suggests that the Arches has not likely experienced its closest approach to the GC between ~0.2 - 1 Myr ago. If accurate, this constraint indicates that the cluster is on a prograde orbit and is located front of the sky plane that intersects Sgr A*. However, further simulations of clusters in the GC potential are required to interpret the observed profile with more confidence.Comment: 24 pages (17-page main text, 7-page appendix), 24 figures, accepted to Ap

A Re-examination of the "Planetary" Lensing Events in M22

We have carried out further analysis of the tentative, short-term brightenings reported by Sahu et al. (2001), which were suggested to be possible lensings of Galactic-bulge stars by free-floating planets in the globular cluster M22. Closer examination shows that--unlikely as it may seem--small, point-like cosmic rays had hit very close to the same star in both of a pair of cosmic-ray-split images, which cause the apparent brightenings of stars at the times and locations reported. We show that the observed number of double hits is consistent with the frequency of cosmic rays in WFPC2 images, given the number of stars and epochs observed. Finally, we point to ways in which cosmic rays can be more directly distinguished.Comment: Accepted Dec 11, 2001 by ApJ Letter