Non-linear Redundancy Calibration

For radio interferometric arrays with a sufficient number of redundant spacings the multiplicity of measurements of the same sky visibility can be used to determine both the antenna gains as well as the true visibilities. Many of the earlier approaches to this problem focused on linearized versions of the relation between the measured and the true visibilities. Here we propose to use a standard non-linear minimization algorithm to solve for both the antenna gains as well as the true visibilities. We show through simulations done in the context of the ongoing upgrade to the Ooty Radio Telescope that the non-linear minimization algorithm is fast compared to the earlier approaches. Further, unlike the most straightforward linearized approach, which works with the logarithms of the visibilities and the gains, the non-linear minimization algorithm leads to unbiased solutions. Finally we present error estimates for the estimated gains and visibilities. Monte-Carlo simulations establish that the estimator is indeed statistically efficient, achieving the Cramer-Rao bound.Comment: 9 pages, 5 figures. Accepted for publication in MNRAS. The definitive version will be available at http://mnras.oxfordjournals.or

A search for the 55 MHz OH line

The OH molecule, found abundantly in the Milky Way, has four transitions at the ground state rotational level(J = 3/2) at cm wavelengths. These are E1 transitions between the F+ and F- hyperfine levels of the Lambda doublet of the J=3/2 state. There are also forbidden M1 transitions between the hyperfine levels within each of the doublet states occuring at frequencies 53.171 MHz and 55.128 MHz. These are extremely weak and hence difficult to detect. However there is a possibility that the level populations giving rise to these lines are inverted under special conditions, in which case it may be possible to detect them through their maser emission. We describe the observational diagnostics for determining when the hyperfine levels are inverted, and identify a region near W44 where these conditions are satisfied. A high-velocity-resolution search for these hyperfine OH lines using the low frequency feeds on four antennas of the GMRT and the new GMRT Software Backend(GSB) was performed on this target near W44. We place a 3-sigma upper limit of ~17.3 Jy (at 1 km/s velocity resolution) for the 55 MHz line from this region. This corresponds to an upper limit of 3 X 10^8 for the amplification of the Galactic synchrotron emission providing the background.Comment: 5 pages, 4 figures, accepted for publication in MNRAS. The definitive version is available at www.blackwell-synergy.co

Királis felismerés vizsgálata termoanalitikai és szerkezeti kémiai módszerekkel = Thermoanalytical and structural chemical investigations of chiral recognition

Kutatásaink segítségével a reszolválások hatékonyságát befolyásoló fizikai-kémiai folyamatatokba nyertünk betekintést több modellvegyület-rendszer esetében. Származék-származék reszolválások, ill. ún. holland reszolválások termékeinek termoanalitikai és szerkezeti kémia vizsgálatát végeztük el. Kísérleteket végeztünk akirális hozzátétnek a reszolválhatóságra gyakorolt hatásának vizsgálatára is. Megállapítottuk, hogy az alkalmazott modellvegyületek esetében a kiralitás centrumtól távol lévő és a funkciós csoportokat nem érintő szerkezeti változtatások jelentősen befolyásolják a reszolválás hatékonyságát, melyet a gyenge másodlagos kölcsönhatások a szilárd fázisú szerkezetek stabilitására gyakorolt szinergikus hatásával valamint a termodinamikai egyensúly elérését megelőző folyamatok hatásával, azaz a királis felismerés ill. szilárd fázis kialakulásának kinetikájával magyarázunk. | Based on the performed research we gained insight into the physico-chemical processes influencing the effectiveness of optical resolutions for several model systems. We performed thermoanalytical and structural investigations of the products of derivative-derivative resolutions, and so-called Dutch resolutions. Experiments were performed to determine the influence of achiral additives on resolvability. We established that for the examined model compounds structural changes distant from the chirality centre and not effecting the functional groups have a pronounced effect on the effectiveness of the optical resolution. This is accounted for by the synergic effect of weak, secondary interactions on the stability of solid phase structures as well as by the processes leading to the thermodynamic equilibrium, i.e. the kinetics of the chiral recognition and the formation of the solid phase