Hasan, H. | Search NASA

NASA NTRS1996

Unstable Box Orbits in Cuspy Elliptical Galaxies

The aim of this work is to gain physical insight into the role played by a concentrated central mass in affecting the shape of elliptical galaxies, by examining its effect on the stability of box orbits which are the backbone of triaxial elliptical galaxies. Ample observational evidence is now available for the existence of a central mass concentration or central cusps in galaxies. The central mass is expected to cause orbital stochasticity and chaotic mixing of orbits, which could have ramifications on galactic evolution. We investigate here the interplay between potential cuspiness and eccentricity on the stability of axial orbits in a scale-free potential in a simple, preliminary attempt to characterize this effect.

galaxies central mass concentration box orbits↗

NASA NTRS1995

Stellar Orbits in Doubly Barred Galaxies

Stellar orbits have been computed in a two dimensional rotating potential representing a doubly barred galaxy and surfaces of section (s.o.s.) computed. Both bars are taken to rotate at the same pattern speed. Two cases have been considered. In the first case the secondary bar is aligned parallel with the primary bar while in the second case the bars are perpendicular to each other. In each case the effect of changing the mass of the secondary bar is studied. An examination of the s.o.s shows that the influence of a secondary bar on stellar orbits in a barred galaxy is to destabilize them and to weaken and eventually destroy both bars. A small secondary bar can retain its identity, but as it increases in mass, it will shrink in size, as will the primary. This behavior is interpreted as an interplay between the natural resonance frequencies of the bars and the appearance and outward movement of an outer ILR and, in the case of the parallel bars, also the inward movement of an inner ILR.

doubly barred galaxies↗

NASA NTRS1995

Stellar Orbits in Doubly Barred Galaxies

Stellar orbits have been computed in a two dimensional rotating potential representing a doubly barred galaxy and surfaces of section (s.o.s.) computed. Both bars are taken to rotate at the same pattern speed. Two cases have been considered. In the first case the secondary bar is aligned parallel with the primary bar while in the second case the bars are perpendicular to each other. In each case the effect of changing the mass of the secondary bar is studied. An examination of the s.o.s shows that the influence of a secondary bar on stellar orbits in a barred galaxy is to destabilize them and to weaken and eventually destroy both bars. A small secondary bar can retain its identity, but as it increases in mass, it will shrink in size, as will the primary. This behavior is interpreted as an interplay between the natural resonance frequencies of the bars and the appearance and outward movement of an outer ILR and, in the case of the parallel bars, also the inward movement of an inner ILR.

stellar orbits, doubly barred galaxies↗

NASA NTRS1993

Focus history of the Hubble Space Telescope: Launch to May 1993

Since the launch of the Hubble Space Telescope (HST) the secondary mirror of the telescope has been moved several times in order to collimate the telescope and also to define a position of best focus. In addition to these moves the focus position changes over time because of water desorption by the graphite epoxy in the metering truss. The authors report here the focus history of the telescope based on a knowledge of the mirror moves made and an analysis of desorption monitoring data obtained by the Faint Object Camera (FOC) in the F/96 mode and of the routine calibration data obtained by the Wide Field and Planetary Cameras. Focus values are extracted using two different methods. In the first method the distance between the center of the point spread function (PSF) and the shadows of the pads supporting the HST primary mirror are related to the focus error. In the second method an analytical formula for the PSF with variable aberration coefficients is fitted to the data. Focus positions derived from the two methods show good agreement. The data show that a desorption of about 83 microns has taken place since Aug. 16, 1990. The desorption has clearly not leveled off as expected from the trend of the earlier data. Long term variations of the secondary mirror position of approximately 3-15 microns from the 'best' focus position have been observed. Variations of the order of 2-5 microns over an orbital period have also been noted. Focus changes resulting from secondary mirror movements greater than approximately 5 microns changes the point spread function significantly and makes deconvolution and quantitative measurements difficult.

Hasan, H.↗

NASA NTRS1992

Titan: Evidence for seasonal change - A comparison of Hubble Space Telescope and Voyager images

A comparison of images of Titan obtained by the HST in August, 1990 with Voyager 1 and 2 images respectively obtained 10 and 9 years earlier has indicated a reversal of the seasonal hemispheric brightness asymmetry near 440 and 550 nm wavelengths; the northern hemisphere is in the more recent observations the brighter of the two, by about 10 percent. Titan's albedo pattern is therefore adequately explained by a seasonal model.

Caldwell, John↗

NASA NTRS1992

Deconvolution of Hubble Space Telescope images using simulated point spread functions

Presented is a study of the use of simulated point spread functions (PSF's) to deconvolve Hubble Space Telescope images. We concentrate on images from the Wide Field and Planetary Camera (WFPC) and examine the affect of position dependence of the PSF and the telescope focus position on deconvolutions. Comparisons will be made to what will be expected from WFPC 2, which will include corrective optics. Since PSF's can be simulated for any specific observation, with the added advantage of being noise free and the ability to subsample them, they may be more suitable for deconvolution than observed ones in some cases. And since finding a suitable observed PSF may be difficult, simulated ones may be easier to use.

Krist, J. E.↗