Prepared by:

Jeffrey N. Cuzzi

SETI Program Office

Ames Research Center

 

Samuel Gulkis

SETI Project Scientist

Jet Propulsion Laboratory

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Radio astronomical investigations of great scientific interest can be carried out with the wide range of SETI antenna systems presently under discussion. This range includes both the SETI programs planned for the near future with existing antennas, and larger ground-based or spaceborne antenna systems that might be built in the future.

The effect of SETI technology on radio astronomy can be broadly broken down into two classes. In the first class we have the extension or improvement of existing microwave technology:

1. Receiver design : A class of receivers to be developed for SETI is characterized by near optimum noise figures (~10 K at a room temperature waveguide flange), broad instantaneous bandwidth, and octave bandwidth tuning ranges. This technology will probably be rapidly adopted by radio observatories so that the possession of such receivers will not make SETI systems unique, but would be a SETI spin-off.

2. Collecting area : Eventually, a SETI receiving system may vastly surpass radio astronomy facilities, existing or projected, in collecting area. (Compare the VLA at twenty-seven 25-m antennas to even two 1 00-m antennas.)

In the second class of SETI impact we have the development of a new generation of signal processing facilities:

3. Data Processing Hardwar: On the basis of SETI requirements, it is possible to predict the general properties of such hardware. To make a microwave search tractable, it will be necessary to utilize fully the entire receiver bandwidth while retaining high spectral resolution: a processor of 10⁶ channels. An integrating spectrometer with such characteristics is an impressive scaling-up of present radio astronomy technology, but a significant development is called for when we admit that we have no a priori knowledge of the nature of SETI signals. Then, we require a fully flexible data processing system that can measure all properties of a signal (e.g., frequency distribution, time structure, polarization) continuously in real time. The ability to fully characterize radio signals offers hope for recognizing and rejecting various kinds of interference. If the processor is eventually to be used with an antenna array, the ability to operate several array subsets or several array beams simultaneously would be very useful.

4. Data Acquisition Management : The management of a 10⁶ channel signal processor and the extraction and sorting out of various kinds of scientific data in real time represents another breakthrough area. The following kinds of data are some that need to be managed, preferably simultaneously:

• [150] Time averaged spectrum (10⁶ channels; recognize and extract spectral lines for astronomy; Doppler compensate for space motion of observatory)
• Dynamic spectrum (10⁶ channel with high time resolution; search for Doppler patterns perhaps characteristic of ET transmitters; recognize interfering signal patterns; e.g., solar bursts in sidelobes, satellite transmitters, etc.)
• Polarization (four Stokes parameters in 10⁶ channels needed for signal recognition; i.e., a weak, narrow, unpolarized signal would probably be an unknown spectral line rather than an ET signal; an ET signal might be polarization modulated)
• Dispersion removal (an ET signal might consist of broadband pulses that would be dispersed in the interstellar medium)

Thus, this second class of SETI impact would represent a major new way of handling data. It would permit astronomers to engage in survey projects of a scope that has only been attempted a few times in the past, and then only with a large dedication of scientific manpower. In this capacity, a SETI system would likely be unique for a considerable period of time.

As SETI activities widen in scope and increase in sensitivity, the utility of SETI facilities for radio astronomical investigations will surely increase. In particular, aspects (2-4) above present enormous potential for improvement. It is also apparent that immediate SETI efforts utilizing currently achievable advances along the lines of (1) and (3) will yield new results of significant radio astronomical interest. This complementary document discusses specific scientific benefits that would arise from SETI efforts. This treatment is by no means exhaustive. For instance, serendipity is a vital factor attendant to any major leap in instrumentation. It is, however, impossible to discuss benefits that derive from new and unexpected discoveries. As in the case of the 200-in. Hale Observatories telescope, there will surely be many that derive merely from each significant increase in collecting area. In addition, extensive sky and frequency coverage with high frequency resolution (several Hz, or ~0.001 km sec^-1 at 1.5 GHz), wide instantaneous bandwidth (~300 MHz), and possible sensitivity to pulsed signals will surely result in new discoveries of scientific importance.

Remaining within the domain of foreseeable scientific benefit, we present here likely applications of several near-and far-term SETI systems to radio astronomy. Three different scales of system complexity are represented: an optimally equipped single 26-m antenna, the equivalent of the full Cyclops array of 1026 antennas each of 100-m diameter, and an intermediate case.

Astronomical investigations of individual radio sources have achieved higher sensitivity levels than could be obtained with a 26-m antenna in a survey program. As a survey instrument, however, a 26-m SETI facility with an optimum front end compares favorably in sensitivity with surveys that have been done, but with the added advantages of higher spectral resolution, greatly expanded frequency coverage, and complete coverage of the visible sky (see Section I-2).

In considering potential programs, we will assume a minimal sensitivity system consisting of a 15 K system on a 26-m antenna. We will assume a dual-polarization receiver, although the same sensitivity can be achieved with a single-polarization system operating twice as long. In comparing various observations, we will compute the minimum detectable flux density from

where S = flux density, Jy; K_D = detection limit factor 5; K_R = receiver mode factor, for receiver switching, for autocorrelation spectrometer, if includes both on and off measurement; T_s = system temperature, K; D = antenna diameter, m; B = bandwidth, Hz; = integration time, sec; and = antenna aperture efficiency.

For studies of extended objects, such as the larger interstellar clouds, the antenna resolving power may not be an important factor. For each sky position, we have a minimum detectable brightness temperature of

where is the beam efficiency. A higher sensitivity can be achieved by averaging adjacent sky positions so that the effective beamwidth is larger.

A natural consequence of the SETI program will be a number of very sensitive radio source surveys over the frequency range 1.4 - 23 GHz covering all the visible sky. The sensitivity that will be achieved in the constant beamwidth surveys extends beyond the confusion limits for nearly all frequency intervals which would be observed with the 26-m telescope. Thus it will be possible to [152] generate radio source surveys over 6 sr of sky which are complete to the 0.3 Jy level² (or less) for any frequency desired in the range quoted above. For comparison, the NRAO "deep" survey at 5 GHz, carried out using the 43-m telescope, is complete to the 0.1 Jy level over only 6 x 10^-3 sr of sky (ref.1).

In particular, all-sky surveys at high frequencies have not been carried out in the past. The larger sample of sources that would be available through the SETI program would give greater statistical accuracy to the source counts and to the distribution of the sources within the different optical and radio classes that have so far been found. Surveys at very high frequencies will aid the study of the population of very young sources, and may even uncover new classes of sources. A uniformity of beam size will be especially useful in the intercomparison of surveys at different frequencies.

When ionized hydrogen gas recombines, radio spectral lines are emitted when the atom passes through states of high excitation (large quantum number n). For example, between 1.4 and 1.7 GHz, transitions between quantum levels 157 to 166 are observed. The phenomenon is most prominent in hot, ionized gas near early-type stars. A survey of 43 sources was conducted by Dieter (ref. 2) at 1.65 GHz, using the Hat Creek 26-m antenna. The sensitivity limit of this search, T_s = 140K, B = 10⁴ Hz (1.8 km sec^-1),varied between andJy, depending on the source observed. With an optimized 26-m SETI system, the same sensitivity will be reached in 1.5 to 15 min. However, if one considers averaging the results for all the recombination lines in this 300 MHz band, sensitivity is increased by a factor of 3.

If the simplifying assumption is made that the ionized gas is populated according to a Boltzmann distribution, combining a measurement of free-free continuum emission with a recombination line intensity allows us to deduce the electron temperature and density. This has already been done for 120 sources in the northern sky by Reifenstein et al. (ref. 3) using a recombination line at 5 GHz. A SETI-related all-sky survey using a 26-m antenna would not add significantly to this. However, it is known that in some sources at least, the population of energy levels does not follow a Boltzmann distribution. This effect, which is due to strong radiation fields and low collision rates, can be determined by measuring recombination lines at widely separated frequencies. For this reason, recombination line data from a SETI survey covering many frequencies would be useful.

As the electron temperature of an ionized region decreases, the recombination line intensity relative to the radio continuum increases. Thus, narrow recombination lines might be observed in the absence of radio continuum emission, although calculations for plausible situations suggest that very high sensitivity would be required (integration measured in days). If the electron density is also low, narrow lines could arise by stimulated emission. Weak narrow lines have been observed [153] in a number of instances (ref. 4) although whether the former or latter mechanism is responsible is not clear. Discovery of a strong, narrow line in an unexpected direction would be of high scientific interest. Ruling out the presence of such lines would be useful.

Galactic Hydrogen - Within the general velocity limits of ±100 km sec^-1 and at a frequency resolution of about 10 kHz, neutral hydrogen has been very extensively mapped. Without repeating current survey work, there is interest in searching for narrow features, most likely to occur in absorption Knapp (refs. 5 and 6) has observed selected dust clouds with good velocity resolution (0.08 - 0.34 km sec^-1). Somewhat less than half of the clouds showed such features. This suggests that an all-sky survey with high frequency resolution will be useful as a technique for mapping cold dust clouds. The shapes of these absorption features can be analyzed for internal cloud motions, and their intensity can be compared with visual absorption in further studies of the gas/dust ratio in local cloud kinematics and cloud structures.

The study of high-velocity clouds, with bearing on theories of galactic structure, would benefit greatly from an all-sky 26-m survey. These clouds are widely distributed, and have velocities as large (in one case) as 400 km sec^-1, or a frequency shift of ~2 MHz. Most surveys have been more restricted in velocity space, and have covered only limited regions of the sky. In addition, many known high-velocity-clouds have narrow frequency half-widths, some being unresolved by existing surveys. Thus an all-sky, broad bandwidth, high spectral resolution survey would almost certainly turn up new and interesting results on these interesting objects.

Globular Clusters - Neutral hydrogen observations of globular clusters have been conducted, both to examine the content of the clusters themselves (ref. 7) and to study the nature of the intervening clouds (ref. 8). With the sensitivity of a SETI search, these data would be made complete.

Extragalactic Hydrogen - Recently, Fisher and Tully (ref. 9) made neutral hydrogen observations of 241 extragalactic systems in the David Dunlap Observatory (DDO) catalog. These are dwarf-like systems of low surface-brightness found on the Palomar Sky Survey. The observations were primarily with the NRAO 91-m telescope, and achieved a sensitivity (T_s = 50 K, = 240 sec,, ~0.55 Jy for a bandwidth of 100kmsec^-1 (4.73 x 10⁵ Hz). Altogether, 179 systems were detected. This led to re-examination of the Palomar Sky Survey to obtain a candidate list of more than one thousand systems, of which more than two thirds have been detected in neutral hydrogen. This suggests further productive searches can be done.

A highly optimized 26-m system compares favorably with the NRAO 91-m antenna. In favor of the 26-m system are a drastically lower system temperature, a sensitivity increase of from the use of two polarizations, a sensitivity increase of from using a spectrometer which does not clip the signal (as the NRAO autocorrelator does), and a somewhat higher aperture efficiency. Taking these factors into account, we can achieve the same sensitivity as Fisher and Tully did by integrating 8 min per point rather than 4 min per point. Another way to look at this is that if we [154] scan the sky at a sidereal rate ( ), we would have roughly half the sensitivity of Fisher and Tully, and detect 60 percent of their initial list. The usefulness of the survey drops off very rapidly with diminishing sensitivity. With one polarization, 38 percent of their sources would be detectable; if the effective time per position is reduced to 65^S, 21 percent of their sources would be detected.

There are a number of interesting possibilities for an all-sky survey. We would detect systems that are not optically visible, either because their surface brightness is intrinsically low or because they are obscured by material in our own Galaxy and, of course, we would measure their redshift directly from the radio line. Ultimately this would lead to a much better understanding of the mass function for galaxies, and the correlation of neutral hydrogen mass with system type.

Another very intriguing possibility is the detection of small condensations of neutral hydrogen within our own local group. In a search at the sidereal rate, our detection threshold would be, where D is the distance of the condensation in megaparsecs. (The Magellanic Clouds are at ~0.06 Mpc distance; M31 and M33 at 0.7 Mpc.) The detection of such material would, of course, be very exciting. Statistical studies of the redshifts would define the rest standard of the local group, and a measure of the "temperature" of the early universe from the dispersion in the local velocities. Even a nondetection would be useful in determining whether the local group is bound and whether there is a lower limit to the mass function of galactic systems.

This potentially exciting program puts severe constraints on the system. Absolutely every contribution to system noise would need to be minimized, and advantage taken of every opportunity to improve our statistics (e.g., dual polarization). The fastest observations would be at a sidereal rate (~200 days for the whole sky) and preferably, we would take longer. The minimum useful instantaneous frequency range to be examined (-3000 knm sec^-1 < v < 1000 km sec^-1) is 20 MHz, while 30 or 40 MHz would be desirable.

Emission from the hydroxyl radical has been observed (primarily at the frequencies of 1612, 1665, and 1720MHz) from a wide variety of sources. It is found extensively throughout dust clouds and appears in masers in such a variety of sources as HII regions, planetary nebulae, late spectral type stars, Wolf-Rayet stars, infrared sources, and supernova remnants. Within this wide range of objects, the OH excitation shows considerable variation, and Turner (ref. 10) has provided a classification scheme that is able to correlate the nature of the source with its OH spectral character for a large number of sources. Currently, Turner is reducing a survey of OH sources that covers about one-third of the galactic plane. It is essentially complete for a 1° strip, about 30 percent complete for the adjacent 1° strips, and has random coverage for selected objects in the rest of the sky. Altogether, about 2000 points were observed over the velocity range ±120 km sec^-1 relative to the galactic rotation velocity. However, larger velocities are known to occur so that the greater velocity coverage in a SETI survey would be of scientific interest.

[155] With a 26-m SETI system we would have roughly comparable sensitivity. An extension of the Turner survey to the whole sky and over an extended velocity range would be very useful. The data would be applicable to studies of the nature of OH excitation, studies of galactic kinematics, cloud structure studies, and in searches for star formation sites.

OH emission is generally polarized. A survey would therefore be most effective in a dual polarization mode, employing a data processing system that would yield all four Stokes parameters. The Zeeman effect has been proposed to account for the polarization of OH line emission. A 26-m SETI all-sky survey would observe a large sample of OH sources for apparent Zeeman patterns, yielding a useful probe of the local magnetic field strength in the Galaxy. The direct measurement of the magnetic field strength in regions of star formation will have an important bearing upon our understanding of the processes involved.

The presence of CH and its ion, CH+, in the interstellar medium has been known since their detection at optical wavelengths in 1937 at Mount Wilson. While the radio study of this molecule is still in its infancy, it promises to be an important tool for studying the interstellar medium, particularly because of the apparent sensitivity of its abundance to local density (ref. 11).

A 26-m SETI survey with the assumed system parameters (T_s = 15 K, K_R = 2) would achieve a sensitivity comparable to the current work (rms ~ 0.005 K, B = 10⁴ Hz) in approximately I hr of integration per sky position. However, useful information would still be obtained with integration periods of the order of several minutes, particularly in terms of locating the regions of most intense emission.

Formaldehyde is one of the most useful probes of interstellar clouds. Primarily by its transition at 4.83 GHz, it has been used to define the extent and distribution of interstellar clouds. The distribution of formaldehyde correlates well with the distribution of dust (refs. 12 and 13). Thus, it has been possible to study the kinematics of dust clouds in the solar neighborhood (ref. 14). Since the 4.83 GHz transition has sufficiently separated hyperfine components, it has been possible to use the hyperfine ratios to estimate the optical depth of the clouds. Because formaldehyde occurs in denser regions than neutral hydrogen, it is generally useful for the study of specific regions such as the galactic center and regions of star formation, and even for discovering major cloud complexes not optically visible (ref. 15).

Even the smaller radio astronomy facilities (e.g., Hat Creek 26-m) have an antenna beamwidth at this frequency ( 10 arcmin) which makes an all-sky survey impractical. However, an all-sky survey of medium resolution (30 arcmin) obtained by averaging adjacent records would be very useful in providing an overall picture of the distribution of dust clouds in the Galaxy, and [156] may, in fact, be more effective than current HI surveys in locating the spiral arms of the Galaxy (ref.16).

Interstellar ammonia has not been extensively studied, largely because the transitions are quite weak, a few tenths of kelvins. The most complete study is that of Morris et al. (ref. 17), which indicated that ammonia is fairly widespread in the interstellar medium. This molecule arises in regions having densities nH₂ > 10⁴ cm^-3. It also has the attractive feature of having a large number of transitions within a narrow frequency range (23.6 to 25.1 GHz), several of which are between metastable rotational levels. Thus, it would appear that ammonia is potentially as useful as CO as a probe of the temperatures of interstellar clouds. With an optimally equipped 26-m antenna, an all-sky survey at the ammonia frequency would readily detect many new sources.

Water shares with hydroxyl the importance of being an indicator of some unusual processes occurring in the extended envelopes of certain kinds of stars. Unlike hydroxyl, however, it is seen only in maser action. Because even small radio telescopes have quite small beamwidths at this frequency (22.235 GHz), the total sky coverage of all H₂O observations is quite small, and it is quite likely that the present set of known H₂0 sources is quite biased.

It appears that a SETI survey would make two important contributions. Because H₂0 maser lines are quite strong, a broad beam survey could locate intense H₂O sources that have not been included in the objects examined to date. Also, a sensitive targeted survey of selected stars would broaden the classes of stars examined and thus lead to a more exact understanding of the kinds of stars that are associated with H₂O emission.

Extragalactic Radio Sources - Here the main strength of the SETI program would be its ability to investigate all four Stokes parameters over a very wide range of frequencies (e.g., 1 to 25 GHz) in extremely fine frequency steps. In particular, all previous studies have been carried out using broadband, double-sideband receivers. The IF bandpasses of these receivers have rarely been as small as 20 MHz, and never smaller than 10 MHz. Hence, the fine-scale polarization structure of microwave spectra is completely unknown. It is quite likely that microstructure is present, arising from differential Faraday rotation and depolarization among several localized domains in these sources. At this time the study of magnetic field structure of extragalactic radio sources can be carried out for a very small number of extended sources at a few frequencies using polarization interferometry. SETI would allow statistical studies of inhomogeneities in the magnetic field structure of many unresolved sources.

[157] Galactic Radio Sources and Galactic Magnetic Field - Discrete continuum radio sources in our own galaxy which emit polarized radiation, such as supernova remnants, would be investigated in the same manner as the extragalactic sources.

The continuum background nonthermal emission originating in the galactic plane has been investigated at a number of wavelengths, all greater than 20 cm. At 21 cm the nonthermal emission in the galactic plane amounts to a brightness temperature of at least 1 K. If we assume a HPBW of 1° and a bandpass of 300 MHz, the available nonthermal flux is approximately 4 x 10^-17 W/m² . If the polarized flux is only 1 percent of the total, an optimized 26-m antenna would achieve a significant signal-to-noise ratio in each beamwidth solid angle while scanning at the sidereal rate. An all-sky survey of the SETI variety will automatically generate maps of rotation measure and intrinsic position angle for galactic nonthermal emission; these in turn would delineate structure of the longitudinal and transverse components of the galactic magnetic field and would extend spectral coverage to shorter wavelengths as well.

Intergalactic Magnetic Field in the Local Group - The all-sky survey, if carried out to high sensitivity, may be able to detect the Faraday effect due to the intergalactic magnetic field. This would be an extremely difficult measurement.

The time-averaged flux density of pulsars is generally quite weak: at 1.5 GHz it is 0.1 Jy. This may be compared with a minimum detectable flux for a 26-m SETI program (T_s = 15 K, = 200^s, K_R = 1, = 0.6) of 45 B^-1/2 Jy. So, pulsars start to become detectable at a bandwidth of 200 kHz, and at 20 MHz, we have good signal-to-noise for the stronger ones. The wider bandwidths can only be achieved, however, if the dispersion (differential time delay) is removed. Thus, the observation of known pulsars at estimated levels of sensitivity would not augment current knowledge significantly.

There is the possibility of detecting new pulsars but two difficulties are encountered. The first is that the flux of pulsars diminishes typically with the second to third power of the frequency. Detecting pulsars at 1 GHz would be at least twice as easy as at higher frequencies; a limiting sensitivity is reached approximately at that frequency where the galactic background dominates the receiver noise. However, previous surveys (though probably more sensitive) may be incomplete in sky coverage or dispersion measure range, so that the value of a 1.4 GHz survey bears further investigation.

The second problem associated with a pulsar search is that we must search two new parameter regions: pulse repetition rate and dispersion. However, dispersion removal techniques exist and would be readily implemented (ref. 18). The removal of dispersion effects is relevant not only to pulsars, of course, but to all pulsed signals, whether of natural or intelligent origin, and would be a valuable part of any projected data processing system.

At the Third Science Workshop on Interstellar Communication, a group of radio astronomers³ evaluated the Cyclops array as to its effectiveness in answering current astronomical questions.⁴ This complementary document presents those evaluations in a concise format, and in addition, attempts to evaluate the effectiveness of more modest SETI systems, of a scale intermediate between an equivalent to a full-scale Cyclops and a single antenna. This evaluation is tabulated in the charts presented at the end of this section.

The radio astronomical frontier, as envisioned by the speakers at the Third Science Workshop, was broken into topical areas and the needs of studies in these areas were evaluated in terms of instrumentation. That is, four dimensions of the instrument (1) sensitivity (total area), (2) spatial resolution (linear extent), (3) frequency range (minimum and maximum useful frequencies), (4) frequency resolution (i.e., how narrow)-have been considered as to whether they may have threshold or limiting values for useful work to be done in the topical areas. The method was simply to scale values of parameters given by the Workshop speakers (flux density, integration time, distance, etc.) to values characteristic of the use of more modest instruments. These parameters are related to each other by the inverse-square radiation law and the classical radio astronomy sensitivity equation,

where S is the minimum flux density detectable by a given system, T_s the system temperature,A the effective collecting area, t the integration time, B the system bandwidth (assumed here to be less than the "signal" bandwidth), and K a constant. Where no numbers were given by the Workshop speakers, no scaled values are presented here. Here we will deal primarily with variations in (1) collecting area, and to some extent (2) spatial resolution.

Not all of the evaluations presented at the Third Science Workshop are "topical areas" in the sense of scientific "questions." For instance, use of a SETI system as a VLBI terminal or for polarization studies are techniques and not current astronomical questions, and thus evaluations from these standpoints are distributed throughout the analysis by topical area.

[159] Before discussing the findings of this summary, we will briefly elaborate on "confusion," one of the constraints noted in the charts. At each frequency, and for each instrument, there is a confusion-limit to the minimum detectable flux density of a postulated discrete object. It is that flux density at which one finds at least one extraneous weak object in the main beam of the instrument (or one strong object in a sidelobe) and is this a function of spatial resolution. As there are more objects per steradian at lower fluxes, a smaller beam must accompany studies of weaker objects. A thorough treatment of confusion is beyond the scope of this brief discussion.⁵ However, using a likely frequency scaling relationship and a value of 10^-7 Jy as the confusion limit at 3 GHz (quoted by Kellerman for the Cyclops array) one may easily determine that confusion will be the limiting factor in many of the studies mentioned in the charts. Thus the resolution of the "nominal" SETI system will need to be increased for successful attainment of those quoted detections for which confusion is given as the limiting factor. This is easily done by the addition of "outrigger" array elements spaced at several array diameters from the main array. In the following discussion, proper use of outrigger antennas is presupposed.

In a general fashion, one may reach two interesting conclusions from even such a preliminary analysis as is given in the charts at the end of this paper.

First, it appears that the combination of sensitivity (surface area) and high resolution will make a large-scale system especially valuable to advances in our understanding of galactic phenomena, and in particular, to the following.

1. The interstellar medium : The "Hierarchy" of clouds (densities, temperatures, molecular abundances, kinematics, the role of magnetic fields in cloud collapse), especially if the frequency range to 25 -30 GHz is obtainable.

2. Stellar system formation and evolution : Protostellar systems, T Tauri variables, stellar winds in general, flare stars, evolved stars, peculiar broadband time variable stars, especially if a time resolution capability or frequency range to 30 GHz is provided. Knowledge of the structure and perhaps composition of the atmospheres and surfaces of the planets of our own solar system would be greatly enhanced.

3. Galactic structure : Small-scale distribution of hydrogen in our Galaxy and detailed mapping of nearby galaxies to an extent that would allow refinement of theories of galactic structure.

4. Pulsars : Both in the sense of enhanced knowledge of the pulsars themselves, and in the sense of the very useful tool for mapping the small-scale properties of the interstellar medium that they would then provide. The dispersion removal problem is more tractable by a factor of [160] (f_H/f_L)² at higher (f_H = 1-10 GHz) frequencies than at the frequencies (f_L~100 MHz) at which pulsars are strongest and are currently studied.

In addition:

5. Studies of extragalactic objects of low brightness (bridges between possibly associated objects, etc.) will also be greatly enhanced, especially if outrigger or similar (VLBI) resolution enhancing capability is provided, as will studies of "clumping" within galactic clouds.

6. Cosmological studies would be furthered by at least one important experiment; the accurate determination of the redshift-magnitude relation for normal galaxies out to Z = 1, with the possibility of other important observations (see chart).

However:

Studies of small, bright objects (QSO's, radio galaxies, etc.) are perhaps better suited to (and perhaps influence the design of) VLA-type instruments. Such instruments and objects are very nicely complemented in their capabilities and topics of study by any foreseeable SETI system.

Second, it appears that intermediate-scale SETI systems (10-30 percent of the magnitude of the full Cyclops array) are able to address practically all of the questions comprising our radio astronomical frontier as effectively as the full Cyclops. Longer integration times are of course required of more modest systems. The current questions listed by the Third Workshop speakers would, in most cases, be answered by such a "partial Cyclops" system within the useful lifetime of the instrument (decades).

These same questions would be answered by a system of scale comparable to the full Cyclops in less than a year. It is surely unimaginable that an instrument of this sensitivity would remain idle after answering all of our current questions; it has been the history of science that each answer becomes in its turn a new question.

Introduction to Chart

This chart represents, primarily, a summary and organization of the statements made by speakers at the Third Science Workshop on the uses of a proposed SETI system (full Cyclops) to radio astronomy. Scaling is given in the last two columns.

1. Pauliny-Toth, 1 I. K., Kellermann, K. l., Davis, M. M., Fomalont, E. B., and Shaffer, D. B., Astron. J., 77, 265 (1972).

2. Dieter, N., Ap. J., 150, 435 (1967).

3. Reifenstein, E. C., 111, Wilson, T. L., Burke, B. F., Mezger, P. G., and Altenhoff, W. J., Astron. & Astrophys., 4, 357 (1970).

4. Zuckerman, B., and Ball, J. A., Ap. J.,190, 35 (1974).

5. Knapp, G. R., Astron. J., 79, 527 (1974a).

6. Knapp, G. R., Astron. J., 79, 541 (1974b).

7. Knapp, G. R., Rose, W. K., and Kerr, F. J., Ap. J.,186,831 (1973).

8. Knapp, G. R., and Kerr, F. J., Astron. & Astrophys., 35, 361 (1974).

9. Fisher, J. R., and Tully, R. B., Astron. & Astrophys.,44,151 (1975).

10. Turner, B. E., J. Roy. Astr. Soc. Canada, 64,221 (1970).

11. Zuckerman, B., and Tumer, B. E., Ap. J., 197,123 (1975).

12. Myers, P. C., and Ho, P. T. P., Ap. J., 202, L25 (1975).

13. Myers, P. C., Ap. J., 198, 331 (1975).

14. Minn, Y. K., and Greenberg, J. M., Astron. & Astrophys., 22,13 (1973).

15. Hoglund, B., and Gordon, M. A., Ap. J.,182,45 (1973).

16. Simonson, S. C., 111. Astron. & Astrophys., 46, 26 (1976).

17. Morris, M., Zuckerman, B., Palmer, P., and Turner, B. E., Ap. J., 186, 501 (1973).

18. Linscott, l. R., Erkes, J. W., and Powell, N. R., Dudley Observatory Report No.10 (1975).

19. Colgate, S. A., and Noerdlinger, P. D., Ap. J.,165, 509 (1971).

20. von Hoerner, S., in Galactic and Extragalactic Radio Astronomy, Verschuur, C. L., and Kellerman, K. l., eds. (1974) (Springer-Verlag).

[171] 21. Refsdal, S., Stabell, R., and deLange, F.G., Mem. R.A.S.., 71, Part 3 (1967).

22. Petrosian, V., Third Science Workshop on Interstellar Communication. Held at Ames Research Center, NASA, Moffett Field, Calif. 94035, Sept. 15 and 16, 1975.

² 0ne Jansky = 10^-26 W/m² -Hz

³ A list of those who spoke on the use of a SETI system for radio astronomy is given in Section III-15.

⁴ Parameters of the full Cyclops array: 3.2 km clear aperture diameter, 10 km array diameter, 1026 antennas at 100 m diameter. At 1 GHz, single antenna efficiency = 60 percent, system temperature = 20 K, array beamwidth = 7'', single antenna beamwidth= 13' (Bracewell, Third Science Workshop). Note that the choice of these specifications for a "full" Cyclops array is essentially arbitrary, and is made for convenience. A Cyclops system may in fact consist of any number of antennas. In fact, it is the number required to detect signals of extraterrestrial intelligent origin.

⁵ See, however, reference 20 for a thorough discussion that includes the likely frequency dependence.

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