128 research outputs found
Status of CHIPS: A NASA University Explorer Astronomy Mission
In the age of Faster, Better, Cheaper , NASA\u27s Goddard Space Flight Center has been looking for a way to implement university based science missions for significantly less money. The University Explorer (UNEX) program is the result. UNEX missions are designed for rapid turnaround with fixed budgets in the 15 million US dollar range. The CHIPS project was selected in 1998. The CHIPS mission passed the Design Verification Review in April 2001 and is now proceeding into implementation with a launch in mid-2002. Many lessons have already been learned from the CHIPS UNEX project. The 2000 paper discussed the early issues surrounding the use of commercial satellite constellations and the politics of small satellites using foreign launchers. The difficulties of finding a spacecraft in the UNEX price range were highlighted. The advantages of utilizing Internet technologies from the earliest phases of the project through communications with the spacecraft on orbit were discussed. The 2001 paper will discuss the implementation status of CHIPS, the first of this new class of NASA mission, and the lessons learned. The current state of the program will be summarized and the project’s plans for the future will be charted
Diagnosing space telescope misalignment and jitter using stellar images
Accurate knowledge of the telescope's point spread function (PSF) is
essential for the weak gravitational lensing measurements that hold great
promise for cosmological constraints. For space telescopes, the PSF may vary
with time due to thermal drifts in the telescope structure, and/or due to
jitter in the spacecraft pointing (ground-based telescopes have additional
sources of variation). We describe and simulate a procedure for using the
images of the stars in each exposure to determine the misalignment and jitter
parameters, and reconstruct the PSF at any point in that exposure's field of
view. The simulation uses the design of the SNAP (http://snap.lbl.gov)
telescope. Stellar-image data in a typical exposure determines secondary-mirror
positions as precisely as . The PSF ellipticities and size, which
are the quantities of interest for weak lensing are determined to and accuracies respectively in each exposure,
sufficient to meet weak-lensing requirements. We show that, for the case of a
space telescope, the PSF estimation errors scale inversely with the square root
of the total number of photons collected from all the usable stars in the
exposure.Comment: 20 pages, 6 figs, submitted to PAS
Fabrication of the DESI Corrector Lenses
The Dark Energy Spectroscopic Instrument (DESI) is under construction to
measure the expansion history of the Universe using the Baryon Acoustic
Oscillation technique. The spectra of 35 million galaxies and quasars over
14000 square degrees will be measured during the life of the experiment. A new
prime focus corrector for the KPNO Mayall telescope will deliver light to 5000
fiber optic positioners. The fibers in turn feed ten broad-band spectrographs.
We describe the DESI corrector optics, a series of six fused silica and
borosilicate lenses. The lens diameters range from 0.8 to 1.1 meters, and their
weights 84 to 237 kg. Most lens surfaces are spherical, and two are challenging
10th-order polynomial aspheres. The lenses have been successfully polished and
treated with an antireflection coating at multiple subcontractors, and are now
being integrated into the DESI corrector barrel assembly at University College
London. We describe the final performance of the lenses in terms of their
various parameters, including surface figure, homogeneity, and others, and
compare their final performance against the demanding DESI corrector
requirements. Also we describe the reoptimization of the lens spacing in their
corrector barrel after their final measurements are known. Finally we assess
the performance of the corrector as a whole, compared to early budgeted
estimates
Computational investigation on CO2 adsorption in titanium carbide-derived carbons with residual titanium
We develop a new approach for modeling titanium carbide derived-carbon (TiC-CDC) systems with residual titanium by the generation of modified atomistic structures based on a silicon carbide derived-carbon (SiC-CDC) model and the application of weighted combinations of these structures. In our approach, the original SiC-CDC structure is modified by (i) removing carbon, (ii) adding carbon and (iii) adding titanium. The new atomic scale carbide-derived carbon (CDC) structures are investigated using classical molecular dynamics simulations, and their pure CO adsorption isotherms are calculated using grand canonical Monte Carlo simulations. The system of TiC-CDC with residual titanium is modeled as weighted combinations of pure carbon CDC structures, CDC structures with titanium and a TiC crystalline structure. Our modeling is able to produce both structural properties and adsorption isotherms in accordance with experimental data. The fraction of different models in the systems successfully reflects the structural differences in various experimental TiC-CDC samples. The modeling also suggests that in partially etched TiC-CDC systems, the titanium that may be accessible to CO gas at the transitional interface may provide significant interaction sites for CO and may lead to more efficient overall gas adsorption
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BigBOSS: The Ground-Based Stage IV BAO Experiment
The BigBOSS experiment is a proposed DOE-NSF Stage IV ground-based dark energy experiment to study baryon acoustic oscillations (BAO) and the growth of structure with an all-sky galaxy redshift survey. The project is designed to unlock the mystery of dark energy using existing ground-based facilities operated by NOAO. A new 4000-fiber R=5000 spectrograph covering a 3-degree diameter field will measure BAO and redshift space distortions in the distribution of galaxies and hydrogen gas spanning redshifts from 0.2< z< 3.5. The Dark Energy Task Force figure of merit (DETF FoM) for this experiment is expected to be equal to that of a JDEM mission for BAO with the lower risk and cost typical of a ground-based experiment
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