No description available.

Call number: LD2668 .T4 1958 W43

Infrared photometry has been used to determine the physical characteristics of cometary solids. Observations were made of the reflected and thermal parts of the spectra of seven comets. Two of these comets, Bowell and West, were nonperiodic; the other five, Chernyhk, Encke, Kearns-Kwee, Stephan-Oterma, and Tuttle, were periodic. Observations in the 3 μm region of the spectrum of Comet Bowell provide the first direct evidence for the presence of H₂O ice in a comet. This detection represents one of the strongest possible confirmations of Whipple's (1950) icy conglomerate model of cometary nuclei. The observations of the periodic comets have yielded the following picture of the dust in this type of objects: grains with a size distribution ranging from about 0.3 μm to 10 μm, and peaking around a few microns. These grains were made up of at least two components, a silicate material and an absorbing material. These characteristics are remarkably similar to those of the dust in nonperiodic comets. This indicates that the type of dust a comet ejects does not change with age, and supports the absence of large scale differentiation in cometary nuclei. Comet West is the first case of a splitting comet in which the fragments were observed to have differences in their dusty component. These observations suggest that the nucleus of this comet did not have an "onion skin" or layered structure but rather had pockets containing dust grains with different size distributions. Based on the results presented, the relation between cometary and interstellar dust, and the origin of comets are discussed.

No description available.

The operation of a grating lateral shear heterodyne interferometer as a wavefront sensor for atmospherically perturbed wavefronts is analyzed. A novel wavefront sensor design is given and its feasibility is proven by laboratory experiments. The applications in mind are closed-loop active optical systems for compensating atmospheric perturbations and open-loop atmospheric wavefront measuring device. The optical properties of the turbulent atmosphere are summed up and the resulting wavefront sensor requirements are given. Among them are the property of sell-referencing, high white light efficiency, independence of scintillation effects, and high spatial and temporal sampling rates. Then the general heterodyne grating shearing interferometer is introduced. A description of the phase measurement by the heterodyne process in the frequency domain has been derived. The heterodyne process is interpreted as convolutions of the signal with a pair of filter functions, which isolate a particular harmonic term of the signal and provide its phase. The representation of the convolutions in the frequency domain provides an elegant way to analyse the systematic errors of the heterodyning with general, non-sinusoidal signals. Also the random phase errors of the heterodyne process have been determined using Gaussian error propagation. An algorithm is derived to carry out the wavefront reconstructions from the measured differences on a square array of discrete data points. It is based on a modal expansion in complex exponentials, leading to a simple filtering operation in the spatial frequency domain. The algorithm provides unbiased reconstructions over the finite data set. It has minimal error propagation in a least squares sense. It is computationally efficient in that the number of operations required for a reconstruction is approximately proportional to the number of wavefront points, if a Fast-Fourier-Transform algorithm is used. Finally, a compact wavefront sensor design is described fulfilling the requirements posed by the turbulent atmosphere. It determines wavefronts at 24 by 24 points at a sampling rate of 60 Hz. A rms-wavefront error of better than λ/20 can be achieved with astronomical light sources of sixth stellar magnitude. Laboratory experiments proved the feasibility of the design.

We have conducted three surveys to try and locate distant star forming galaxies. The most general survey used deep 2μ images with optical CCD photometry to locate objects with peculiar SEDs. Using the IR data we should be able to locate rapidly star forming galaxies to z = 25. With a 3σ detection limit of 18.5 at K we have found no objects with z > 5 but we have found several blue objects at z < 4 in 16min² of sky. This suggests tha there is no extremely luminous early phase of galaxy formation. We have found several blue objects at z < 4 in 10min² of sky. Of particular interest is an object which has a flat SED from V to K but shows a strong spectral break between B and V and a weaker break at 5800Å. We suggest these may be Lyman limit and Lyman α forest absorption at z-3.8 in a galaxy forming ≈400M(⊙) year⁻¹ of stars. A large sample of galaxies (100 objects) selected to have similar properties (R – I < .5, B – R > 1) has also been found. From this sample it appears this possible high redshift star forming phase only contributes 1/10 of the metal present in disks or spheroids. We have also found 30 Lyman α emission line companions to 12 z = 3 quasars. These objects have Lyman α equivalent widths (50Å) and luminosities (V = 24) consistent with galaxies forming ≈100M(⊙) year⁻¹ of stars. Also, 2 of the quasars have 8 companions and may be in cluster environments. A final survey analyzed optical to IR SEDs of luminous blue radio galaxies at z > 1. In these objects we find SEDs indicative of star formation rates between 10 and 100M(⊙) year⁻¹ but interpretation is difficult due to the AGN component of the sources. While these data seem to suggest a significant star forming phase taking place in galaxies at z ≈ 3-4, interpreting this result is difficult since we cannot determine if we are observing disk or spheroidal populations. In the case of the quasar companions and the radio galaxies, consideration of their dense environments and current epoch morphology suggest that these may be spheroids but these galaxies may not be typical of galaxies in general.

This dissertation describes the techniques involved in using a fiber optic coupled spectrometer to do multiple object spectroscopy of astronomical objects. The Medusa spectrograph, with optical fibers fixed in a focal plane aperture plate, was used to study clusters of galaxies via velocity distributions. Some relevant problems in the study of the structure and dynamics of clusters of galaxies are outlined as motivation for building a multiple fiber spectrometer. The history of fiber optic spectroscopy in astronomy is presented along with an outlook for the future. The results and experience gained from the Medusa spectrograph are used to design a second generation instrument. The MX Spectrometer uses optical fibers which are positioned remotely under computer control. These fibers are optically matched to the telescope and spectrograph optics to achieve optimum performance. The transmission, flexibility, and image scrambling properties of step-index silica fibers allow efficient reformatting of multiple objects into the spectrometer entrance aperature. By allowing spectra of 32 objects to be obtained simultaneously, the MX Spectrometer will make an order of magnitude increase in the quantity of spectroscopic data that can be recorded with a large telescope. Mechanical, control, and optical elements of the MX design are discussed. Telescope and fiber parameters influencing the design of the fishermen-around-the-pond mobile fiber head are detailed. Results of testing the stepper motor driven fiber positioner probes are described. The algorithm for controlling the motion of 32 positioners in the telescope focal plane without collisions is outlined. Detector performance and spectrometer efficiency are considered for both the Medusa and MX systems. The use of a Charge Coupled Device (CCD) array detector provides increased quantum efficiency, dynamic range, and stability, as well as allowing digital sky subtraction. Microlenses and their use in correctly coupling optical fibers to the telescope and spectrograph are discussed. In particular, the pupil-imaging technique for microlens matching to fibers is introduced.

The magnetic rotator model has long been the favored explanation for coherent photometric modulations in the DQ Herculis class of cataclysmic variables. However, to date, all evidence supporting this model has been of the indirect variety. Unlike their synchronously rotating cousins, the AM Herculis objects, DQ Herculis stars have not yet been discovered to emit polarized radiation. Therefore, in light of this crucial lack, the evidence used to place these objects in the magnetic cataclysmic variable category has been strictly circumstantial, based primarily on the coherence of the photometric periodicities. In this work, time-resolved spectroscopy of four long-period DQ Herculis stars is performed. In addition, two of the same objects are observed with a new, sensitive circular polarimeter. Chapters II and III describe these observations and the results of each. To summarize, coherent variations in the wavelength of emission lines were found with the spectroscopic observations. A model is put forth, explaining this phenomenon as being due to varying illumination from a bright spot on the primary. This, of course, adds strength to the magnetic rotator model. Secondly, circular polarization was definitely found in one object studied, and possibly in a second. Therefore, for the first time, there is direct evidence of the magnetic nature of these binaries. In Chapter IV, the model of the rotating bright spot illuminating the disk is explored in further detail, including modeling with a minicomputer. Afterward, a problem brought out by the low polarization coupled with large amplitude photometric variations and a cool spectrum is investigated, namely, is it possible to produce large amounts of cyclotron radiation without producing large amounts of circular polarization? The results tend to show that, for a large emitting area, the answer is yes. Chapter V is a summary of the rest of the work.

The work of this thesis is to investigate new techniques for Integral Field Spectroscopy (IPS) to make the most efficient use of modem large telescopes. Most of the work described is aimed at the FMOS for the SUBARU 8m telescope. Although this is primarily a system for Multiple Object Spectroscopy (MOS) employing single fibres, there is an option to include a multiple-IFS (MIPS) system. Much of this thesis is therefore aimed at the design and prototyping of critical systems for both the IPS and MOS modes of this instrument. The basic theory of IFU design is discussed first. Some particular problems are described and their soludons presented. The design of the MIPS system is described together with the construction and testing of a prototype deployable IFU. The assembly of the pickoff/fore-optics, microlens array and fibre bundle and their testing are described in detail. The estimated performance of the complete module is presented together with suggestions for improving the system efficiency which is currently limited by the performance of the microlens array. The prototyping of the MIPS system is supported by an extensive programme of testing of candidate microlens arrays. Another critical aspect of the instrument is the ability to disconnect the (IPS and MOS) fibre input which is installed on a removable prime focus top-end ring from the spectrographs which are mounted elsewhere on the telescope. This requires high-performance multiple fibre connectors. The designs of connectors for the MOS and IPS modes are described. Results from the testing of a prototype for the MOS mode are presented. This work is supported by a mathematical model of the coupling efficiency which takes into account optical aberrations and alignment errors. The final critical aspect of FMOS which has been investigated is the design of the spectrographs. The baseline system operates in the near-infrared (NIR) but an additional visible channel is an option. Efficient designs for both the visible and NIR systems are presented. The design of the NIR spectrograph presents challenges in the choice of materials for the doublet and triplet lenses employed. The choice of material and the combinations in which they can be used are described. This thesis shows that all these critical aspects of FMOS have good solutions that will result in good performance of the whole instrument. For the multiple IFU system, the prototype demonstrates acceptable performance which can be made excellent by the use of a better microlens array. The multiple fibre connector prototype already indicates excellent performance. Finally, the spectrograph designs presented should result in high efficiency and good image quality.

A thesis submitted to the Faculty of Science in fulfillment of the requirements of the degree of Doctor of Philosophy School of Computational and Applied Mathematics University of the Witwatersrand September 20, 2012 / In this thesis, we analyse reasons for poor image quality on the Southern African Large Telescope (SALT) and we analyse control methods of the segmented primary mirror. Errors in the control algorithm of SALT (circa 2007) are discovered. More powerful numerical procedures are developed and in particular, we show that singular value decomposition method is preferred over normal equations method as used on SALT. In addition, this method does not require physical constraints to some mirror parameters. Sufficiently accurate numerical procedures impose constraints on the precision of segment actuator displacements and edge sensors. We analyse the data filtering method on SALT and find that it is inadequate for control. We give a filtering method that achieves improved control. Finally, we give a new method (gradient flow) that gives acceptable control from arbitrary, imprecise initial alignment.