Evaluación de los aprendizajes de los estudiantes

173 We conclude by revievring the results of the previous chapters and giving some thoughts and suggestions for the future of elect ronography.

The studies of NGC 5053 and ITGC 6366 provide a good example of

programmes in which elect ronography can be used to advantage. The ability to extrapolate photoelectric sequences clearly does save much large-

telescope time. For studies such as these, the utmost accuracy is not essential provided sufficient numbers of stars are observed. As long as

there are no systematic effects in the photometry, then the important features of the C-M diagrams will be apparent and from these a number of interesting properties of the clusters can be deduced, as has been the case for NGC 5053 and NGC 6366. The fact that elect ronography did not provide all the photometric data used on the two clusters was due to technical problems with the image tubes, rather than to any inherent shortcoming in the technique. It is pertinent to note here perhaps that, althou^ because of linearity, results can still be obtained from cathodes as bad as that used in Chapters 2 and 3, this should not be regarded (as sometimes happens) as an advantage of elect ronography, since the quality of the detectors is a feature of the technique itself.

The developments in data handling described in Chapter 4 are clearly only a first, but very necessary, step in achieving the efficient retrieval of substantial amounts of information from electronographs. We believe that Chapter 6 records the first substantial use of this kind of reduction scheme. While it has enabled the reductions to be completed in a reason­ able time, the restrictions already noted show that the system is still inadequate in many ways and efforts should be increased to develop the alternative approach, also given in Chapter 4, of using a more realistic profile (in this case the two-dimensional Franz profile). The application to multiple image groupings is particularly important as it employs the

174

lost by the more traditional methods of photometry. Testing the results from multi-image analysis presents a slifÿit problem in that, by the very nature of the stars observed, there will be nothing with which to compare the resultant electronographic magnitudes. One way around this will be to obtain special electronographs in which a group of stars is simulated by a number of exposures of one star, with the telescope moved slightly between exposures. Accurate timing of the exposures will then give the

expected relative magnitudes of the images in the group.

In Chapter 6 we employed electronography on a potentially very rewarding astronomical programme and one which would seem ideal for an accurate two-dimensional detector. In theory, the results should have enabled a complete survey of the cluster to be made with (if the current literature is to be believed) an accuracy normally associated with photo­ electric photometry. Ignoring the instrumental difficulties, the results indicate that only a partially complete survey was possible. This was due primarily to:

1) Image crowding problems which precluded the measurement of a number of stars. That a two-dimensional linear detector should be restricted in this way is clearly unacceptable.

2) Defects on the cathode which occasionally coincided with the positions of images. Even when this was not directly visible, its effects probab]y explain the cause of some measures having large residuals.

3) The fact that on G5 film the results indicate that an image must be approximately one magnitude above the plate limit before a measuring accuracy of 0.05 magnitudes is achieved. This set the limit of the photometry somewhat bri^ter than we had anticipated.

The results from the use of fine grain l4 emulsion were always

175 be used for this kind of work. However, as it takes approximately five

times the e3qposure to achieve the same visibility on l4 (we do not sub­

scribe to the * the-information-is-there-but-you-just-cannot—see-it * philosophy) this would have required exposures of about three hours for the M 5 observations. Obtaining many of these is not very practicable. Nonetheless, the results of Chapter 6 show that this kind of photometry

can yield useful information on the temperatures and gravities of globular cluster giants.

Stellar photometry is, of course, not the only field in which elect ronography is being used. Numerous studies of two-dimensional objects (galaxies, nebulae, etc.) are in progress, but here again it remains to be seen whether the full potential of the technique can be realised. In this regard, it seems at present to be Mackay (1976) versus the rest. Nhether the interpretation of this should be that

Mackay is the only one prepared to be truly objective about the situation must await further work. We are in fact beginning an investigation to

compare the capabilities of electronography with other techniques in the field of galaxy surface photometry* Surface photometry of galactic halos to less than 1^ of the sky brightness is now commonplace using modem photographic emulsions. The question is whether the knovm complications of electronography i.e. cathode sensitivity variations and emulsion problems, can be compensated for accurately enough, to allo\f photometry to this level. In collaboration with J, J. Lorre of the Jet Propulsion Laboratory we hope to apply their image processing techniques to some electronographs in the expectation that, at least in this case, the reduction procedures will not be found wanting.

As regards improvements in the accuracy of stellar photometry, we can but list some of the basic requirements that have become apparent from our experiences:

Reference

a) The quality of the basic materials, i.e. photocathode and emulsion, must be improved. An immediate improvement is no doubt technically possible, if those responsible are given sufficient encouragement and incentive by the users,

b) Conditions prevailing in most darkrooms are not adequate for process­ ing electronographs. The conditions required are closer to the

‘clean room* situation common in many scientific fields, than to the average observatory darkroom. At the very least, automatic tempera­

ture control of the chemicals, filtered washing water and, most ' 4

important, a dust—free drying area should be available.

c) An effort equivalent to that expended in the development and manu­ facture of the electronographic cameras must be put into devising further improved reduction schemes. While speed and simplicity are excellent ideals, in the end they are no substitute for accuracy.

In conclusion, we would suggest that the results of the thesis as a whole indicate that the use of elect ronography is advantageous for

certain projects, but that with present equipment and reduction techniques we are still a good way from the realisation of elect ronography as two-

dimensional photoelectric photometry.