Hashima Hasan

Galactic bars with central mass concentrations : three-dimensional dynamics

The stellar dynamics at the transitions between a strongly barred and rotating potential and a spherical and dense central mass concentration is investigated in a model potential. Three different degrees of concentration of the central mass are studied. The main families of three-dimensonal periodic orbit are found by numerical means. The structure of phase space is presented by bifurcation diagrams and surfaces of section. A large region of instability develops near the inner Lindblad resonance (ILR) due to the strong bar potential. In such a mass configuration the radial ILR is typically near the corresponding vertical ILR, consequently the instability of orbits is radial as well as vertical, helping the heating of disk stars into a bulge

The imaging performance of the Hubble Space Telescope

Problems with the HST instantaneous imaging performance and pointing performance are discussed. Optical tests have clearly demonstrated that the HST suffers from spherical aberration. The top level specification was that 70 percent of the energy be focused in a 0.1 in. radius, but the present, and close to optimum, focus setting gives only about 16 percent. The pointing control system also is having problems with the results that the spacecraft achieves a stability of about 0.007 rms in quiescent periods, falling short of specification which requires that such performance be maintained for 24 hr. The finite guidance sensors are not guiding well on faint stars. There is a loss of sky coverage at high Galactic latitude, especially for the Wide-Field/Planetary Camera. The consequences of these defects for the scientific program are examined.

Fitting Multimodal Lognormal Size Distributions to Cascade Impactor Data

A procedure is described for fitting multimodal lognormal size distributions to cascade impactor data. The method was applied to published cascade impactor measurements for ammonium and nitrate that were previously analyzed by procedures that do not assume a functional form for the size distribution. An evaluation of the lognormal fitting procedure by Monte Carlo simulations demonstrated that unbiased parameters and reliable estimates of their uncertainties require correct specifications of measurement precision and impactor characteristics.

TELESCOPE IMAGE MODELLING (TIM)

The Telescope Image Modelling (TIM) software has been designed to simulate a polychromatic image of a large number of point sources with known colors, as recorded on a pixellated detector after reimaging through an optical system of known characteristics. Though developed primarily for images produced by the Hubble Space Telescope and its scientific instruments, it is general enough for wider application. The software is modular and produces monochromatic point spread functions and modulation transfer functions at intermediate stages. Details of known optical aberrations of the system, microroughness scatter from the telescope mirrors, telescope pupil aberrations, pointing jitter, photon noise and detector defects can all be included.

In-flight performance of the Faint Object Camera of the Hubble Space Telescope: II

This paper gives an update on the performance of the Faint Object Camera--launched with the Hubble Space Telescope--since the last report two years ago. The primary camera, the f/96 relay, continues to work well, but the f/48 relay has recently developed serious problems. The stability of the f/96 relay has been very good with the only change being a small apparent decrease in UV sensitivity. Preliminary results for the f/48 DQE are presented. In-orbit UV flat fields have been obtained and the f/96 objective prisms and polarizers have been calibrated.

Effect of mirror microroughness scattering on the Hubble Space Telescope point spread function

The effect of internal light scattering caused by the microroughness of the Hubble Space Telescope primary mirror was measured and compared with theoretical models. It was found that the effect was much smaller than predicted and would not be a problem even in the UV.

Collimating the Hubble Space Telescope

Collimation techniques for the Hubble Space Telescope before and after the 1993 December servicing mission are discussed. As a first step we convert actuator positions of the HST secondary mirror into tilts and decenters, by means of a conversion matrix. Theoretical estimates of wavefront errors introduced by known amounts of tilts and decenters are related to each other by means of a conversion matrix. Several sets of stellar images taken with the Faint Object Camera over a two year period are analyzed and aberration coefficients derived. A residual coma of approximately 1/19th wave at 632.8 nm is found in the images. We report here the secondary mirror move made, after the refurbishment mission in 1993 December, to remove the coma and the analysis of the images taken after the move to verify the collimation.

Investigating a self-consistent galactic potential with central mass concentration

The evolution of a barred galactic potential. containing a central. mass concentration is examined by means of a self-consistent 2-D JV-body simulation. It is found that the bar weakens as the central. mass grows and eventually dissolves, in agreement with earlier orbital. studies of this problem.