Evaluation of image tubes in astronomy
Brand, Peter W. J. L.
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It is to be hoped that the methods described in the first part of this thesis will be extensively applied to imaging devices and to photographic emulsions, since this is the only way in which an unambiguous figure for the performance of the detector may be derived. Lack of attention to such concepts has produced many pitfalls, notably the enormous (and imaginary) gains reported earlier for various types of image tube, which, when they were not realised in practice, produced some considerable despondency.The work of Shaw(60) on aerial films is an out- standing example of thorough application of Information Theory to the photographic process. His results are formally the same as those of Chapter I of this thesis.The results derived enable a prediction of signal-to-noise-ratio (accuracy) and information gain for the image tube under any conditions of exposure. The results also point to the necessity of knowing, for instance, the IMF of the spectrograph used with the detector, in order to fully exploit the storage capacity of the detector.Part II of this work has described the manner in which the Spectracon, as a particular example of an astronomical image tube, may be brought into routine use as part of the observational astronomer's instrument capability.Much is still to be achieved in this field. In particular, the emulsions are in no way comparable in standard (understandably, they are still a laboratory - rather than commercial - commodity) with the best astronomical emulsions. Similarly, such is the experimental complexity in building a tube, that to produce a flawless cathode and window ( and both are focal surfaces) is asking a great deal. But it is not asking for the impossible, and it is foreseeable that in time such difficulties will be eliminated.There remains the problem of the small area of the cathode. In effect, while the information storage capacity of the image tube is considerably greater than that of astronomical emulsions, its limited area rather nullifies this advantage in that, with suitable coding, all the information in the small cathode area could be spread onto one large astronomical plate. However, to do this the exposing intensity must be reduced, and this, because of the nonlinearity of the photographic process, reduces the efficiency.Thus the crucial advantage in using an image tube must be in exploiting its higher detecting efficiency. If programs consist in looking at small regions of spectrum (or of sky), then the advantages of the image tube are obvious. Some of this work has already been reported in a paper read to the 3rd Symposium on Photoelectronic Imaging Devices.