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Ph.D.

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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.

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A wealth of experiments have been performed studying image encoding techniques as applied to non-time varying or single-frame images. However, to date little work has been done to apply these techniques to time varying images, with most of such works emphasizing various ad hoc redundancy reduction techniques. In this work, a computer based experimental system is implemented which makes more methodological studies of time varying images possible. Particular attention is devoted to obtaining very accurate inter-frame registration and uniform quantization of the images. Using this system, a selection of 35 mm movie film images are digitized and stored on computer magnetic tape in a format compatible with many other computing installations, providing a standard data base for future experiments. An often used model for describing picture data is the stationary Gauss-Markov model. In this work, the appropriateness of this model for describing time varying images is studied by comparing the autocorrelation functions as described by the model and as obtained by computation from the picture data. These results indicate that the autocorrelation function is best described by a function which is separable in the time dimension and nonseparable in the spacial dimensions. A number of DPCM communication systems are then studied as a vehicle for evaluating the effect of using the Gauss-Markov model. These results indicate that, for the sample images studied here, the estimated performance using the Gauss-Markov model is good when the model is a good fit to the first data point of the computed autocorrelation function. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

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