53,364 research outputs found

    Planets as background noise sources in free space optical communications

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    Background noise generated by planets is the dominant noise source in most deep space direct detection optical communications systems. Earlier approximate analyses of this problem are based on simplified blackbody calculations and can yield results that may be inaccurate by up to an order of magnitude. Various other factors that need to be taken into consideration, such as the phase angle and the actual spectral dependence of the planet albedo, in order to obtain a more accurate estimate of the noise magnitude are examined

    Aggregation of metallochlorophylls - Examination by spectroscopy

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    Nuclear magnetic resonance measurements determine which metallochlorophylls, besides magnesium-containing chlorophylls, possess coordination aggregation properties. Infrared spectroscopy reveals that only zinc pheophytin and zinc methyl pheophorbide showed significant coordination aggregation, whereas divalent nickel and copper did not

    Peripheral Metal Complexes: Chlorophyll “Isomers” with Magnesium Bound to the Ring E ß-Keto Ester System

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    Peripheral magnesium complexes of methyl pheophorbides a and b, bacteriomethyl pheophorbide a, bacteriopheophytin a, and protopheophytin a have been prepared and characterized by UV-vis, IR, ‘H NMR, and luminescence spectroscopy. In these compounds, Mg is bound to the peripheral p-keto ester system present in most chlorophylls rather than to the central tetrapyrrole cavity as in the chlorophyll proper. The ligand exchange reactions at Mg have been characterized, and the equilibrium between the peripheral complexes and their free ligands has been studied over the concentration range lo-’- M. AGO for complex formation has been determined by ’H NMR spectroscopy

    Two Populations and Models of Gamma Ray Bursts

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    Gamma-ray burst statistics are best explained by a source population at cosmological distances, while spectroscopy and intensity histories of some individual bursts imply an origin on Galactic neutron stars. To resolve this inconsistency I suggest the presence of two populations, one at cosmological distances and the other Galactic. I build on ideas of Shemi and Piran (1990) and of M\'esz\'aros and Rees (1993) involving the interaction of fireball debris with surrounding clouds to explain the observed intensity histories in bursts at cosmological distances. The distances to the Galactic population are undetermined because they are too few to affect the statistics of intensity and direction; I explain them as resulting from magnetic reconnection in neutron star magnetospheres.Comment: 25pp., WU-JIK-92-