Optimizing the fine lock performance of the Hubble Space Telescope fine guidance sensors

This paper summarizes the on-orbit performance to date of the three Hubble Space Telescope Fine Guidance Sensors (FGS's) in Fine Lock mode, with respect to acquisition success rate, ability to maintain lock, and star brightness range. The process of optimizing Fine Lock performance, including the reasoning underlying the adjustment of uplink parameters, and the effects of optimization are described. The Fine Lock optimization process has combined theoretical and experimental approaches. Computer models of the FGS have improved understanding of the effects of uplink parameters and fine error averaging on the ability of the FGS to acquire stars and maintain lock. Empirical data have determined the variation of the interferometric error characteristics (so-called 's-curves') between FGS's and over each FGS field of view, identified binary stars, and quantified the systematic error in Coarse Track (the mode preceding Fine Lock). On the basis of these empirical data, the values of the uplink parameters can be selected more precisely. Since launch, optimization efforts have improved FGS Fine Lock performance, particularly acquisition, which now enjoys a nearly 100 percent success rate. More recent work has been directed towards improving FGS tolerance of two conditions that exceed its original design requirements. First, large amplitude spacecraft jitter is induced by solar panel vibrations following day/night transitions. This jitter is generally much greater than the FGS's were designed to track, and while the tracking ability of the FGS's has been shown to exceed design requirements, losses of Fine Lock after day/night transitions are frequent. Computer simulations have demonstrated a potential improvement in Fine Lock tracking of vehicle jitter near terminator crossings. Second, telescope spherical aberration degrades the interferometric error signal in Fine Lock, but use of the FGS two-thirds aperture stop restores the transfer function with a corresponding loss of throughput. This loss requires the minimum brightness of acquired stars to be about one magnitude brighter than originally planned

Program Notes: The Newsletter of Literary Managers and Dramaturgs of America, volume 2, number 2

Contents include: a report of the LMDA Columbia Conference, Performaturgy, an announcement of the 1987 LMDA Conference, books by members, excerpts of Dr. Howard Stein\u27s keynote at the LMDA Symposium, a response to Terrance McNally\u27s article, Samurai dramaturgy, an open letter to Terrence McNally, an excerpted letter to the American Laboratory Theatre from Francis Fergusson, and life after literary management. Issue editors: Richard E. Kramer, Laurence Maslon, and Jeffrey Lawson.https://soundideas.pugetsound.edu/lmdareview/1003/thumbnail.jp

Potential Impacts of Energy Price Changes on Consumer Food Costs

This paper documents actual energy inputs for selected agricultural products for all activities from production to home consumption. It then estimates the potential food cost impacts of several alternative energy pricing schemes. The findings of this analysis indicate that modest energy price changes are unlikely to have significant impacts on food costs, contrary to some beliefs

Computational study of C-C activation of 1,3-dimesitylimidazol-2-ylidene (IMes) at ruthenium:The role of Ligand bulk in accessing reactive intermediates

Density functional theory calculations have been employed to model phosphine substitution in Ru(PPh3)3(CO)(H)2 to form Ru(IMes)(PPh3)2(CO)(H)2 (1mono) and Ru(IMeS)2(PPh3)(CO)(H)2 (1bis), as well as the novel C(aryl)-C(sp3) intramolecular bond activation of the IMes ligand in 1bis. The computed ligand exchange energies show that 1bis, is unstable with respect to displacement of IMes by PPh3 and will thus re-form 1mono over time. PPh 3/IMes substitution also leads to a significant labilization of the PPh3 ligand trans to hydride, a result of increasing steric encumbrance upon the introduction of the bulky IMes ligands. The energetics of intramolecular C-C and C-H activation have been computed for both 16e Ru(IMes)n(PPh3)3-n(CO) and 14e Ru(IMes) n(PPh3)2-n(CO) species (n = 1 or 2) and indicate that the introduction of a second IMes ligand does not significantly promote the actual C-C activation step. Instead the need to have two IMes ligands present in the metal coordination sphere before C-C activation can occur is linked to the promotion of PPh3 loss in 1bis, which makes the formation of unsaturated species such as Ru(IMeS)2(CO) particularly accessible.</p

Ability of N-Heterocyclic Carbene Ligands to Promote Intermolecular Oxidative Addition Reactions at Unsaturated Ruthenium Centers

We report the results of density functional calculations on the reactivity of a series of coordinatively unsaturated mixed phosphine/N-heterocyclic carbene complexes of ruthenium of the type Ru(CO)(IR)3-n(PH 3)n, where n = 1-3 and R = H (1,3-imidazol-2-ylidene) and R = Me (1,3-dimethylimidazol-2-ylidene). The oxidative addition reactions of H2 and CH4 and the C-C bond activation of C 2H6 have been studied. For all three processes, substitution of PH3 by IH results in minimal changes in the reaction energetics. In all cases H2 oxidative addition is barrierless and is downhill by around 120 kJ/mol. With CH4 activation barriers of around 75 kJ/mol are computed and the reaction is approximately thermoneutral. With C2H6 activation barriers increase to around 260 kJ/mol and the reaction is disfavored by about + 35 kJ/mol. Introduction of the IMe ligand disfavors oxidative addition, especially for the C2H6 reaction, and this trend is linked to increased steric bulk of the IMe ligand compared to IH. Computed Ru-PH3 and Ru-IR bond strengths and CO stretching frequencies indicate that PH3/IR substitution does create a more electron-rich metal center, and yet this does not facilitate oxidative addition with these Ru species. A fragment analysis reveals that, as expected, PH3/IH substitution enhances the Lewis basicity of the metal reactant. However, a more important effect is a reduction in Lewis acidity, and this factor lies behind the similar reaction energetics computed for analogous PH3- and IH-containing species.</p

Computational studies of intramolecular carbon-heteroatom bond activation of N-aryl heterocyclic carbene ligands

Density functional theory calculations have been performed on intramolecular C(aryl)-X bond activation reactions in model N-heterocyclic carbene (NHC) complexes of the type Ru(NHC)(PH3)2(CO), where NHC = 1-(C6H4-2-X)imidazol-2-ylidene (I(o-C 6H4X), X = H, CH3, F, OH, NH2, OCH3, and CF3). In all cases C(aryl)-X activation is found to be thermodynamically favored, and the largest barrier to reaction is computed to be +21.3 kcal/mol when X = CH3. As C(aryl)-CH3 bond activation has been observed experimentally for a Ru-NHC complex (Jazzar, R. F. R.; Macgregor, S. A.; Mahon, M. F.; Richards, S. P.; Whittlesey, M. K. J. Am. Chem. Soc. 2002, 124, 4944), this suggests that a wide range of heteroatom-substituted N-aryl NHC ligands may be susceptible to intramolecular bond activation and potential ligand degradation. The computed exothermicity of C(aryl)-X activation follows the trend X = NH2 &lt; CH3 &lt; H &lt; OH ≈ OCH3 &lt; CF3 &lt; F, while the barriers vary as X = H &lt; F &lt; OH ≈ OCH3 &lt; CF3 &lt; NH2 &lt; CH3. Both series reflect the promotion of C(aryl)-X activation by the formation of stronger Ru-X bonds in the product. However, the ability of heteroatom ligands to stabilize the Ru(O) reactants through chelation can disfavor C(aryl)-X cleavage and explains the low exothermicity and high barrier associated with C(aryl)-NH2 activation. For X = OCH3 C(aryl)-O bond activation was found to be favored kinetically over O-C(alkyl) activation, although the latter process yields an extremely stable aryloxide product. The arrangement of coligands around the metal can significantly affect C(aryl)-X bond activation, and when X is a π-donor, this process is promoted by a trans CO ligand. These insights suggest not only possible ways to control unwanted C(aryl)-X activation in heteroatom-subtituted N-aryl NHC ligands but also factors that may promote such reactions in catalytic processes where this step is desirable.</p

Structure, Reactivity, and Computational Studies of a Novel Ruthenium Hydrogen Sulfide Dihydride Complex

Addition of 1 atm of H2S to [Ru(IMes) 2(CO)(EtOH)H2] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) affords the air-stable hydrogen sulfide dihydride complex, [Ru(IMes)2(CO)-(H 2S)H2] 2. Treatment of 2 with excess H2S leads to formation of the 16-electron bis-hydrosulfido complex, [Ru(IMes) 2(CO)(SH)2] 3. Preliminary studies show that 3 reacts with both H2 and CO in solution as well as in the solid state. Both 2 and 3 have been structurally characterized.</p