No description available.

No description available.

Thesis--Columbia University, 1900.

QC 351 A7 no. 38 / A brief description is given of the various types of image tubes presently used in astronomical research and a review is presented of the past applications of image tubes to direct astronomical photography. A detailed laboratory evaluation of the Carnegie image tube is summarized and photographs at the telescope are presented to confirm and extend the results obtained in the laboratory. Iris photometry of stellar images can be carried out on Carnegie tube photographs with about the same accuracy as is obtained by normal photographic techniques. Compared to unaided plates the image tube typically requires about 1/15 the exposure time to record stellar images of a specified threshold magnitude. When exposures are made to near the sky limit, however, the Carnegie tube cannot record stars as faint as can be recorded with an unaided plate. When exposed at a given focal length telescope, the limiting magnitude of an image tube record is about 1 magnitude brighter than that of an unaided photograph. Primarily two characteristics of the Carnegie tube, an over-all mottled sensitivity pattern and a light- induced background, are found to be responsible for the loss in limiting magnitude of a Carnegie tube record. The mottle pattern is characterized by an rms variation in sensitivity of ±1.3 percent. It modulates the photographic record of the night-sky radiation and seriously affects the signal -to -noise ratio of the threshold images. The additional background produced by the light- induced background of the image tube generally amounts to 25 percent of the night-sky radiation on a sky-limited photograph. In order to record the same sky-limited magnitude on a Carnegie tube plate and an unaided plate, the image tube record must be exposed at a longer focal length telescope. The exposure time required by the image tube is then about 1/2 to 1/3 that of the unaided plate. Because of the higher scale of the image tube photograph in such a case, however, the effective gain provided by the image tube over the unaided plate is generally somewhat larger than the relative exposure time. The photography of extended objects is found to be particularly affected by the nonuniformities of the image tube. Besides reducing the over-all signal-to-noise ratio of the image tube record, the generalmottle pattern and additional discrete patches and ripples in sensitivity of the image tube tend to mimic low contrast features of galaxies and nebulae. The rather subjective effects of the nonuniformities can be significantly reduced by using telescopes with moderately long focal lengths, so that the seeing image is then large in comparison to the nonuniformities. The photography of astronomical sources through narrowband interference filters has been found to be a particularly promising application of the Carnegie image tube. Preliminary tests reported in the present study include the photography of supernova remnants, planetary nebulae, galaxies, and reflection nebulae. The basic quality criterion for comparing the image tube to unaided photographic emulsions is argued to be the detective quantum efficiency. Typical values of the gain over unaided emulsions provided by the Carnegie tube are calculated to be in the range 10 to 20. It is emphasized, however, that because of the variety of requirements in specific research areas and because of the several unique characteristics of a given image tube, no single figure of merit may be defined that will predict the usefulness of an image tube in all applications. It is suggested that the resolution of a detector should not generally be combined into the calculation of a single figure of merit but should be considered as a separate quality criterion. Certain problems with the Carnegie tube (and other image tubes as well) potentially limit its usefulness in specific research areas. Besides the problems already mentioned, other problems include low resolution, geometrical distortion, the complexities of analyzing the final record (as compared to an unaided photograph), and the limited field of the image tube. Each of these characteristics can be highly significant or entirely inconsequential in different applications.

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.

No description available.

Thesis (Ph. D.)--University of Massachusetts Amherst, 2009. / Open access. Includes bibliographical references (p. 179-184). Print copy also available.

QC 351 A7 no. 20 / The control system described in this thesis was designed to automatically guide the position of photographic plates during long exposures of astronomical objects. The system senses the position of the guide star which is being used as a position reference, and moves the photographic plate in two orthogonal directions in order to maintain a constant position of the image of the astronomical object on the photographic plate. The system was designed to have a bandwidth of 35 radians per second in order to track the worst case image excursions due to atmospheric turbulence and telescope motion. The closed loop transient response of the system was determined from considerations of final image resolution of the photographic plate. Due to the necessity of guiding on stars of unknown brightness the system incorporated an automatic gain control loop within the forward control loop and was made unconditionally stable by high order feedback. In operation on the Steward Observatory telescopes, the system has made a significant increase in photographic plate resolution over manually guided plates.

Charge-Coupled-Device (CCD) cameras have opened a new world in astronomy and other related sciences with their high quantum efficiency, stability, linearity, and easy handling. Nevertheless, there is still noise in raw CCD images and even more noise is added through the image calibration process. This makes it essential to know exactly how the calibration process impacts the noise level in the image. The properties and characteristics of the calibration frames were explored. This was done for bias frames, dark frames and flat-field frames at different temperatures and for different exposure times. At first, it seemed advantageous to scale down a dark frame from a high temperature to the temperature at which the image is taken. However, the different pixel populations have different doubling temperatures. Although the main population could be scaled down accurately, the hot pixel populations could not. A global doubling temperature cannot be used to scale down dark frames taken at one temperature to calibrate the image taken at another temperature. It was discovered that the dark count increased if the chip was exposed to light prior to measurements of the dark count. This increase, denoted as dark offset, is dependent on the time and intensity of the prior exposure of the chip to light. The dark offset decayes with a characteristic time constant of 50 seconds. The cause might be due to storage effects within chip. It was found that the standard procedures for image calibration did not always generate the best and fastest way to process an image with a high signal-to-noise ratio. This was shown for both master dark frames and master flat-field frames. In a real world example, possible night sessions using master frame calibration are explained. Three sessions are discussed in detail concerning the trade-offs in imaging time, memory requirements, calibration time, and noise level. An efficient method for obtaining a noise map of an image was developed, i.e., a method for determining how accurate single pixel values are, by approximating the noise in several different cases.