5,048 research outputs found
Combining Undersampled Dithered Images
Undersampled images, such as those produced by the HST WFPC-2, misrepresent
fine-scale structure intrinsic to the astronomical sources being imaged.
Analyzing such images is difficult on scales close to their resolution limits
and may produce erroneous results. A set of ``dithered'' images of an
astronomical source generally contains more information about its structure
than any single undersampled image, however, and may permit reconstruction of a
``superimage'' with Nyquist sampling. I present a tutorial on a method of image
reconstruction that builds a superimage from a complex linear combination of
the Fourier transforms of a set of undersampled dithered images. This method
works by algebraically eliminating the high order satellites in the periodic
transforms of the aliased images. The reconstructed image is an exact
representation of the data-set with no loss of resolution at the Nyquist scale.
The algorithm is directly derived from the theoretical properties of aliased
images and involves no arbitrary parameters, requiring only that the dithers
are purely translational and constant in pixel-space over the domain of the
object of interest. I show examples of its application to WFC and PC images. I
argue for its use when the best recovery of point sources or morphological
information at the HST diffraction limit is of interest.Comment: 22 pages, 9 EPS figures, submitted to PAS
Word of Mouth and Physician Referrals Still Drive Health Care Provider Choice
Examines how consumers choose physicians, specialists, or medical facilities, including the use of physician referrals, word-of-mouth recommendations, health plan information, and the Internet. Explores implications for consumer-directed health care
Designing Effective Health Care Quality Transparency Initiatives
Profiles two well-designed healthcare quality transparency initiatives from California and Massachusetts. Examines key design and implementation elements, including provider engagement, reliable data, consumer-friendliness, and feedback to providers
The Photometry of Undersampled Point Spread Functions
An undersampled point spread function may interact with the microstructure of
a solid-state detector such that the total flux detected can depend sensitively
on where the PSF center falls within a pixel. Such intra-pixel sensitivity
variations will not be corrected by flat field calibration and may limit the
accuracy of stellar photometry conducted with undersampled images, as are
typical for Hubble Space Telescope observations. The total flux in a stellar
image can vary by up to 0.03 mag in F555W WFC images depending on how it is
sampled, for example. For NIC3, these variations are especially strong, up to
0.39 mag, strongly limiting its use for stellar photometry. Intra-pixel
sensitivity variations can be corrected for, however, by constructing a
well-sampled PSF from a dithered data set. The reconstructed PSF is the
convolution of the optical PSF with the pixel response. It can be evaluated at
any desired fractional pixel location to generate a table of photometric
corrections as a function of relative PSF centroid. A caveat is that the
centroid of an undersampled PSF can also be affected by the pixel response
function, thus sophisticated centroiding methods, such as cross-correlating the
observed PSF with its fully-sampled counterpart, are required to derive the
proper photometric correction.Comment: 20 pages, 14 postscript figures, submitted to the PAS
Destiny: A Candidate Architecture for the Joint Dark Energy Mission
Destiny is a simple, direct, low cost mission to determine the properties of
dark energy by obtaining a cosmologically deep supernova (SN) type Ia Hubble
diagram. Operated at L2, its science instrument is a 1.65m space telescope,
featuring a grism-fed near-infrared (NIR) (0.85-1.7micron) survey
camera/spectrometer with a 0.12 square degree field of view. During its
two-year primary mission, Destiny will detect, observe, and characterize ~3000
SN Ia events over the redshift interval 0.4<z<1.7 within a 3 square degree
survey area. In conjunction with ongoing ground-based SN Ia surveys for z<0.8,
Destiny mission data will be used to construct a high-precision Hubble diagram
and thereby constrain the dark energy equation of state from a time when it was
strongly matter-dominated to the present when dark energy dominates. The
grism-images simultaneously provide broad-band photometry, redshifts, and SN
classification, as well as time-resolved diagnostic data for investigating
additional SN luminosity diagnostics. Destiny will be used in its third year as
a high resolution, wide-field imager to conduct a multicolor NIR weak lensing
(WL) survey covering 1000 square degrees. The large-scale mass power spectrum
derived from weak lensing distortions of field galaxies as a function of
redshift will provide independent and complementary constraints on the dark
energy equation of state. The combination of SN and WL is much more powerful
than either technique on its own. Used together, these surveys will have more
than an order of magnitude greater sensitivity than will be provided by ongoing
ground-based projects. The dark energy parameters, w_0 and w_a, will be
measured to a precision of 0.05 and 0.2 respectively.Comment: Contains full color figure
Infrared analysis of polyethylene wear specimens using attenuated total reflection spectroscopy
Attenuated total reflection infrared spectroscopy was used to analyze ultrahigh molecular weight polyethylene wear test specimens. Three different specimens were analyzed. One specimen was gamma irradiated to a dose of 5.0 MRad, another to a dose of 2.5 MRad, and the final specimen was unirradiated. There was no conclusive evidence of chemical changes (i.e., unsaturation or oxidation) in the surface regions of any of the polyethylene samples. Therefore, it was concluded that the gamma irradiation sterilization procedure shoud not alter the boundary lubricating properties of the polyethylene
Suburban Poverty and the Health Care Safety Net
Examines trends in the number of suburban poor, their access to health care, and efforts to improve safety-net services in the suburbs of five cities. Explores causes of limited safety net capacity, community strategies, and policy implications
Community Efforts to Expand Dental Services for Low-Income People
Examines low-income patients' limited access to dental care, as well as state and community efforts to expand services, including changing Medicaid and State Children's Health Insurance Program reimbursement rates and licensing laws for preventive care
Public Health Workforce Shortages Imperil Nation's Health
Examines from a community-based perspective the scope of the shortages in the public health workforce; contributing factors such as inadequate funding, salaries, and benefits; and strategies for training, recruiting, and retaining public health workers
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