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From figure 8, we see that if we have a pointed system but use a room temperature telescope with an emissivity of 1, this would be the limiting case. With a reasonable number like a 1-percent emissivity, this curve is raised by a factor of 10, which would move the solid curve into the middle. This implies that, if we have an emissivity of 1 percent, then, as long as detectors do not improve over what they are now, we could make do with an uncooled telescope. On the other hand, if detectors improve as much as they theoretically can (1017), the telescope would become the limiting feature at very long wavelengths even if it were operating at 20° Kelvin with an emissivity of 1 percent.

It is quite obvious to me that we do not want to restrict ourselves to these narrow field-of-view telescopes. Figure 9 shows that for the wide field case, if we have a telescope of 300° Kelvin, the telescope is already a limiting feature even with an emissivity of 10. Therefore, we are required to decrease the temperature of the telescope to some reasonable number like 20° if we want to look with a wide field-of-view system. This means that the cryogenics must be developed not only to be used for the detector itself but also to be able to take advantage of space.

There are some other obvious specialized areas that need to be developed. Materials and, in particular, filters need to be developed for the long wavelengths. At the present time, filters are a “cookbook” kind of a business

out beyond 20 microns. Spectroscopy has to be developed; the problem is special for the infrared because of the detector limitation; hence, we have to take advantage of new techniques. For example, Fourier spectroscopy rather than the more conventional techniques is already being advocated vigorously.

There is also the whole, broad problem of imagery. Dr. Henize pointed out in his paper how important it is to have a picture. When we made the survey at 2 microns, we had to proceed star by star. This is actually a very awkward process. I think it is clear that, if we can develop imagery so that we can make pictures at 2, 10, and 20 microns, this would be an extremely important development. Finally, there is the problem of instrumentation contamination from the spacecraft. By "contamination" I mean that, as the satellite or rocket goes up, there is junk associated with it. Although it is small, it is hot enough to give totally spurious results in the infrared. This problem is one which has to be worked on. It is a technical problem, but it is the kind of problem that is going to be annoying in developing space telescopes.

All of these special problems are unimportant in comparison with the need for an emphasis on detectors and associated cryogenics. I think that this field of investigation, plus keeping open minds so that we are willing to grow with this field, is what is required.

High-Resolution Measurements in Space Astronomy and the Requirements for Diffraction-Limited Optics

Lyman Spitzer, Jr.
Princeton University

There are three topics covered in this paper. First, I will describe some of the astronomical research that might be carried out at high resolution. I interpret high resolution as including not only high spatial resolution but also high spectroscopic resolution, since the two are related technically and an instrument suitable for one is likely to be suitable for the other. Second, I will discuss the critical technological areas where more research and development needs to be done before we can forge ahead and construct the optimum instruments required for this high resolution. Third, I will treat very briefly the instrumental goals; i.e., the large space telescope that we have been talking about for some years and the intermediate steps that might be visualized.

Astronomical Research

The research we wish to conduct is, of course, the subject of primary interest to the astronomical community. Let us begin by posing a number of salient questions. What sort of astronomical observations can be made? Why are astronomers interested in high resolution instruments? At Princeton, we have completed several reports analyzing what could be done with such space instruments. Although the astronomical community does not think with one mind, we have at least begun to plan. Some points are still being debated; a number of others have been agreed to by astronomers with different viewpoints. We are gradually generating a reasoned document that can serve as the basis of a national plan for space astronomy. The enthusiasm of the space astronomers must be

tempered by the experience of those in ground-based astronomy. This experience must be taken into account in any long-range plan.

All of us who have had any contact with the subject are convinced of the enormous importance of scientific data that could be obtained with the telescopes of the future. I shall discuss only two typical problems, chosen from a much more extensive list, that could be explored with high spatial resolution, by which I mean a resolving power of approximately 0.03 arc second.

One is in the field of galactic structure. With increased resolution, much more information should be obtainable. One question that might be studied in detail with such higher resolution is the diameter of the very small galactic nuclei that have been discovered at the center of Seyfert galaxies. The chief astronomical result of the last Stratoscope II flight was to set an upper limit to the diameter of the nucleus in one of the brighter Seyfert galaxies. These results are now being used by one of my collaborators at Princeton to construct a model for these fascinating objects, a model consistent with this upper limit. This model is based upon the general point of view that collisions between stars are responsible for all the activity occurring in these galactic nuclei. Since this picture is by no means definite, the higher resolution offered by the space telescope can begin to provide unambiguous answers concerning the true physical processes of the galactic nuclei. A brilliant Russian astronomer, Ambartsumian, has suggested that wholly new physical principles must be involved to explain what is going on in these galactic

nuclei. He suggests that a state of matter unfamiliar to physicists, except perhaps with the most powerful atom-smashing machines, may be responsible for phenomena at the center of these nuclei. Whether or not we accept this hypothesis depends in part upon the results that would be achieved with higher resolution studies.

A second field where high resolution imagery is important and where results can be interpreted immediately in terms of physical understanding is in the structure of galactic clouds of gas or nebulae. Such objects include the bright Orion nebula, planetary nebulae, and, perhaps the most spectacular of all, the shells of gas that are emitted in explosions of a supernova, such as the Crab nebula, the Cygnus loop, and other regions where gases ejected initially at a velocity of many thousands of kilometers per second have been slowed down by interaction with the interstellar medium. There is some evidence from the highest resolution picture of these complex filamentary structures that magnetic fields may play a dominant role in the structure of these gas clouds. Photographs of these objects with 10 to 30 times higher resolution will enormously increase our knowledge of the processes taking place within them.

In considering high spectroscopic resolution, I have selected only two applications. By "high spectroscopic resolution" I mean the ability to resolve 0.1 angstrom. With the advanced type of spectroscopic instrument being planned, we can think of getting 0.1-angstrom spectral resolution on a star of 10th magnitude with an exposure of approximately one-half hour with a 120-inch spaceborne telescope. I may point out that, with a telescope providing high spatial resolution, the best way of getting high spectroscopic resolution may be to use a fairly wide entrance slit to admit all or most of the light, thus giving highest efficiency and greatest ease of acquisition, and then to use the sharp image and the guidance precision of the instrument to give the spectroscopic stability and resolution needed.

The first field in which high spectroscopic resolution can be used profitably is the study of the outer atmosphere of the stars. This region, which is generally in a state of violent activity, changing rapidly with time, is a predominant source of ultraviolet radiation and, in the case of the sun, has been extensively explored with space vehicles. With high resolution spectrophotometry from a large spaceborne telescope, we can hope to make similar observations on a wide variety of stars other than the sun and perhaps even for much colder stars and for such faint objects as protostars. These objects, at the extremes of the evolutionary scale, should hold very fascinating and important clues to the origin and fate of stars in general. To obtain high spectral resolution of these objects in the ultraviolet would be a fascinating field of research.

The second field, which is of particular interest to me personally, is high spectroscopic resolution in the ultraviolet for determining the composition and distribution of interstellar gas. With a few trivial exceptions, most of the atoms in interstellar space absorb only in the ultraviolet, an effect that cannot be observed from the ground. By going to the ultraviolet, we get an increase in sensitivity of three orders of magnitude; that is, we can detect interstellar gas with a density of only 1/1000 of that needed for observations in the visible or in the 21-centimeter line of radio astronomy. Thus, high resolution observations can open an essentially new field of research in interstellar studies. With a large space telescope, we could examine the physical nature of the vast halo of gas that is believed to surround the entire galaxy and which may be a dominant physical factor in its origin and evolution.

Critical Areas of Technology

There are critical technological areas that are in particular need of development in order to realize the potentialities of space telescopes. I have excluded most of the items generally needed for space research, such as

reliable components, large amounts of electric power, and data transmission, because these items are not unique to the present discussion. I have considered only the particular technological areas needed for high resolution astronomical purposes.

One such area is the primary mirror, which is of central importance for high resolution. The first problem is one of manufacturing. How do we make a large primary mirror with the desired properties, and, closely related to this problem, how do we test it? Evidently, if we cannot test it, we do not know whether it has been manufactured correctly. This problem has been solved successfully for the 36-inch mirror in Stratoscope II; the solution constitutes a very important result of that program. For the larger instruments, however, there are still a good many uncertainties.

Another problem in connection with the primary mirror is its thermal properties. While fused silica is much better than pyrex, there are new materials being developed, such as ultra-low-expansion quartz and Cervit, that have a thermal coefficient of expansion 10 times lower than fused silica. What are the required properties of materials applicable to large astronomical mirrors? In particular, what is the dimensional stability for thermal cycling and for vibration and aging? One field where development is urgently needed before much emphasis can be put on larger instruments is the testing of material properties in discs with sizes suitable for astronomical use; i.e., 40 to 60 inches. This includes not only manufacturing discs of the proprietary material and size to diffraction limits but also their thermal cycling and vibration testing as well.

Another central problem that must be resolved regarding a larger mirror is whether it should be monolithic, composed of a single disc, or segmented. Segmented mirrors would, I assume, be continually monitored, with all the segments being actively controlled by pulling and pushing them into a perfect figure. This is a very fascinating, challenging field of optical engineering in which more

information is needed for potential application to a larger instrument.

Finally, with high resolution work as in all types of space astronomy, we need improved efficiency of the primary optics. The coatings are presumably subject to degradation produced by air, waste products from the spacecraft, and energetic particles. The extent of this degradation must be known. There are some programs underway to measure the optical degradation of various coatings for short periods in manned missions. Another exceedingly important problem is scattered light, in the vicinity of the spacecraft, that directly affects all astronomical research.

There is also the problem of positioning accuracy. With the success of Stratoscope II in obtaining an rms pointing accuracy comparable with that needed for a large space telescope, an important milestone in this field has been achieved. I am sure, however, that other problems will remain, and both the error sensor and the actuators that move the image around will require further development before a pointing mechanism can be optimized.

Another area that I believe is crucial for the high resolution telescopes of the future concerns the detectors. At Princeton, we have had great enthusiasm for several years for integrating television tubes. I think it is clear that, while film has advantages for an instrument with a wide field of view, for most research programs the wide field is not as important as the high quantum efficiency provided by the photoelectric devices. An extremely important aspect of imaging with electronic readout (as with television) is the opportunity of getting away from all the inherent difficulties associated with photographic plates in space, including fogging by energetic particles, problems of storage and development, etc. It is our opinion that space astronomical research of the future will depend in a very central way upon integrating television camera tubes; thus, the development of effective tubes should play an essential role in space astronomy. An

improvement in the detector efficiency by an order of magnitude has the same astronomical effect, as far as the photon count is concerned, as increasing the aperture of the telescope by a factor of 10.

An extension of these imaging techniques into the far ultraviolet wavelengths less than 1100 angstroms, where image tube faceplates cannot be used, is an important research goal. Similarly, an extension of imaging techniques into the infrared is essential for the large telescope of the future. Although the specific technological developments needed for infrared astronomy have been described in a previous paper, let me reemphasize the importance of increasing the sensitivity against noise of infrared detectors and the importance of developing the cryogenic equipment needed for operation in the satellite environment.

An additional area of technology in connection with detectors is the development of gratings needed for high spectral resolution. Gratings in the ultraviolet have been used for many years in the physics laboratories, where few physicists have been concerned with their efficiency. By "efficiency" we mean how much of the light that hits the gratings actually ends up where we want it. In the physics laboratory, we simply expose the ultraviolet spectroscopic plate as long as necessary to get the proper image density. There is no specific need to take many exposures; hence, the length of one exposure does not matter particularly. On the other hand, with a space telescope, the overall efficiency of the instrument determines the amount of data that can be gathered; hence, the efficiency of the grating is of crucial importance. Very little has been done to improve the efficiency of ultraviolet gratings. Development of efficient gratings in the ultraviolet is probably one of the most important aspects of supporting technology for large telescopes.

One important field not germane to high resolution telescopes but essential to the permanency of very large instruments is manned maintenance. I agree with Dr. Henize

that the primary role of the astronauts is probably not to maintain and operate a space telescope on a moment-to-moment basis, but rather to keep the telescope working on a longer basis; i.e., to maintain it, to update it, to change it, to repair it, and to do all the things that are very difficult to imagine being automated. The greatest uncertainty at the moment in the plans for the large spaceborne telescope is just how man can best be used in connection with such a powerful long-range instrument.

There is a wide range of scale and complexity in the ways that man might be used for the maintenance of a large instrument. At one extreme, we can think of a minimum program where the astronaut floats around the instrument, taking out black boxes and replacing them with others, and then floats back to his capsule, and goes away. This is something we have explored at Princeton in some detail. From the standpoint of the instrument itself, we have convinced ourselves that to take out black boxes and to put others back in, even for a very precise high-resolution instrument, makes very good sense, provided all agree that astronauts can do this on an extravehicular-activity (EVA) basis. On the other hand, it is not clear to us that this is the way that the astronaut developments are going to proceed. It may be that the difficulties with EVA are too great and that this is not an effective way to use astronauts. Perhaps a more efficient method would be to have them go into a pressurized instrument compartment and operate in a shirtsleeve environment to make whatever changes are necessary. Or perhaps we should go to the ultimate in manned maintenance and think of taking the telescope into a large hangar when maintenance is needed. The hangar would then be closed and pressurized, and clean-room facilities would be introduced. After going through suitable air showers, the astronauts would emerge into the hangar and proceed to make whatever repairs are needed.

Somewhere in this range of possibilities lies the optimum way of using man with a

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