- Email: [email protected]
Astronomical photometry from the moon
~)
Adv. Space Res. Vol. 14, No. 6, pp. (6)99-(6)104, 1994 Copyright © 1994 COSPAR Printed in Great Britain. All rights reserved. 0273-1177/94 $6.00 + 0.00
Pergamon
ASTRONOMICAL PHOTOMETRY FROM THE MOON Hugh S. Hudson lnstitute for Astronomy, University of Hawaii, Honolulu, HI 96822, U.S.A.
ABSTRACT The Moon would be an excellent platform for photometric astronomical observations. This paper discusses such observations,emphasizing time-seriesphotometry of oscillatingstars (asteroseismology), of faint gravitating bodies (microlensing), and of the interplanetary medium. To prepare for the deployment of major new telescopes and instrumentation on the surface of the Moon, I suggest that smaller "site-survey" instruments be put in place as soon as possible. Each application suggested can derive great benefits from small site-survey instruments established relativelysoon, and each would ultimately need extensive arrays of large instruments. INTRODUCTION The art of photometry is one of the classic fields of astronomy, and one for which access to space makes a major technical difference because of the effects of atmospheric extinction and seeing. Thus far in the history of space astronomy there has been only one major instrument optimized for photometric observations, namely, the High Speed Photometer of the Hubble Space Telescope /17/. By contrast, there have been innumerable experiments simply exploiting the advantages of space for opening new wavelength ranges, such as X-ray astronomy. One can distinguish here between classical photometry, which is the measurement of the brightnesses of the stars and other discrete astronomical bodies, usually in broad spectral bands and with relatively infrequent sampling; and time-series photometry, which is the measurement of the temporal fluctuations of the brightness of a given astronomical object. Modern detectors and data systems now make time-series photometry over entire ensembles of objects easily possible (/6/). The diffuse sky background, due to sources ranging from the zodiacal dust to unresolved distant galaxies, also represents a photometric target of real interest. It is not faint: the surface brightness of the dark sky is the equivalent of a few 10th-magnitude stars per square degree. The heliospheric contributions to the diffuse sky brightness may vary in interesting ways clue to solar or magnetospheric plasma activity, and probably also due to the dynamics of the zodiacal dust cloud itself. Thus the diffuse sky brightness is a worthy object in its own right for study with time-series photometry. We can imagine instrumentation that would provide orders-of-magnitude improvement in astronomical photometry. In doing this we have in mind approaching as closely as possible to the ideal: to observe simultaneously all objects in the sky, with time resolution and sensitivity limited only by the inescapable effects of photon statistics. Clearly tradeoffs are necessary within any range of resources, but within a given optimization one can still envision tremendous improvement. The photon statistics limit depends upon the size of the optics, and at some point other natural limitations will come into play - - on Earth, it is seeing. On the Moon we do not know what the natural limits will be, except to note that the upper limits on the variability of the zodiacal brightness (determined partly by other diffuse interplanetary emissions) are on the order of 5%, as measured by the Helios zodiacal-light photometers ( / 1 1 / , / 1 0 / ) . (6)99
(6)100
H.S. Hudson
In improving astronomical photometry systematically by large factors, it is likely that most or all astronomical objects will turn out to be variable. The Sun exhibits many different types of variability in the range AI/I = 10-s~ 10-3, on a variety of time scales (/12/). The known mechanisms include magnetic activity, convection, rotation, and oscillations. Variations due to similar physics can be expected for many stars, with different and sometimes more extreme effects elsewhere on and off the main sequence. The Moon is clearly an excellent site for astronomy of all kinds, with no obvious disadvantages save that of the cost of transportation and operations there. This article gives a view of astronomical photometry in the lunar context, with comments about early "site survey" instruments as well about ambitious observing programs that might be pursued in the future. The development of lunar infrastructure, including permanent habitation as well as extensive robotic exploration, would be a necessary condition before such sophisticated astronomical observations could be undertaken. PHOTOMETRIC SCIENCE Photometric measurements inform us about the physical mechanisms that make astronomical objects vary in brightness. There is an extraordinary variety of these mechanisms, ranging from the extremely precise resonances of stellar oscillations to the diffuse vagaries of the emissions from the outer solar corona, the solar wind, and the interplanetary medium. Illustrating the breadth of this technique - - with modern observational and data-analysis technology - - it would be possible by direct photometry to detect planets orbiting distant stars (/1/). Among several interesting possible modern developments of astronomical photometry, consider: Asteroseismologv The extension of time-series photometry to the "micromagnitude" range (i.e., variability measured in parts per million of the average source brightness), in the mHz frequency range, will allow us to study stellar interiors via observation of the global oscillations. Such observations have exquisite precision because of the high stability of the oscillations - - resulting in narrow resonance lines in the power spectrum of the solar brightness and long-term observations should show interesting time variations of the parameters of the oscillations. The development of helioseismology has led (for example) to the measurement of the solar internal rotation rate, and to the discovery that the differential rotation seen at the photosphere extends radially inward through the convection zone. The asteroseismology of main-sequence stars at present consists mainly of solar observations (but see also/3/), although many oscillating stars of other types are known. An asteroseismology observatory capable of studying many stars will be necessary if we are to take the next scientific steps in studying stellar evolution by analysis of interior structure and dynamics; this will require obtaining statistically significant samples of stars of different ages (/2/). -
-
Microlensing Following Paczynski's original suggestion/16/that gravitational lensing could be studied via timeseries photometry, several major observational programs have begun (/8/). The prospects for major astronomical advances in this area are quite bright, but wholesale production of data will be necessary: according to one estimate (/14/) the object of search would be a 0.3-magnitude increase, over a time interval of a week to a month, in one star out of 2.5 million at any given time! These are extremely difficult search parameters, and the ambitious ground-based search programs now in development may not achieve their goals (which include a wide variety of targets: the galactic halo, dark matter, gamma-ray bursters, binary stars have all been mentioned). Lunar deployment under stable and unvarying conditions seems like an appropriate development for this kind of observation, which will require large amounts of high-quality data. The optimization of observations for microlensing aims at longer sample intervals and wider fields of view than asteroseismology. Both optimizations, however, should generate a large amount of serendipitous discovery of low-level variability in objects of many types.
Astronomical Photometry from The Moon
(6)101
HeUospheric Remote Sensing The study of heliospheric dynamics via fluctuations of the diffuse background light (/11/) has had a promising beginning. The diffuse light can come from Thomson scattering of sunlight by interplanetary or magnetospheric electrons, or from emission lines. The heliosphere contains shock waves, coronal mass ejections, and other phenomena of solar origin, plus the zodiacal cloud, comets, asteroids, and possibly other phenomena of non-solar origin. The Moon offers several distinct advantages for the observation of all of these features: lower background light, remoteness from the Earth's magnetosphere (at inferior conjunction), and a variable orientation with respect to the magnetospheric structures that are among the targets of the observations. Because the heliosphere is optically thin, tomographic observations that would permit a 3-dimensional reconstruction of the source geometry can be attempted (/10/). This of course requires multiple lines of sight (see,
e.g.,/13/). The magnetospheres of Earth and Jupiter, in particular, exhibit dynamical plasma phenomena of great complexity, for which imaging observations would be extremely helpful. Such objects would be natural diffuse--imager targets for lunar-based observatories. THE MOON The surface of the Moon offers little technical advantage over deep space as a platform for astronomical observations at optical wavelengths, except to provide a massive and stable "spacecraft" for mounting telescopes. This presupposes that the lunar environment itself does not interfere with photometry at some level. Lunar astronomical interference seems rather unlikely, but should be tested with a proper site survey. Solar interference, in the form of many kinds of radiation, is guaranteed, and this suggests a program of solar observation both on the surface of the Moon and in deep space (/13/). The real advantages of lunar observatories would come instead from the beneficial intervention by human beings that would be possible if permanent habitation of the Moon were achieved. The deployment and maintenance of the networks of telescopes and their communication links would be a natural occupation for the inhabitants. The programs proposed here should continue for many years, if not decades, and maintenance from the surface of the Earth would be more complex. A true lunar base would essentially offer simplifications of many of the most expensive aspects of astronomy in space: not just the spacecraft itself, but also launch costs for instrument mass, power, telemetry, repair, upgrading, and so forth. These advantages are unfortunately not available in the short term, and in fact early observatories on the Moon may be more expensive than similar installations put in nearby space on free-flying spacecraft. SITE-SURVEY INSTRUMENTS The initial scientific instrumentation on the Moon will have to be small and lightweight. In any case, a "site survey" would follow in established astronomical tradition and would be a costeffective means of demonstrating the feasibility of installing a major network of large-aperture lunar telescopes prior to initiating their actual development. This is a reasonable technical approach. The site-survey instruments would also produce scientific results of great significance, simply because of lack of previous space experiments with photometric optimization. The main technical motivation for a site-survey telescope on the lunar surface would be to determine the effects of interfering light sources: either particles near the Moon, diffuse sources due to residual gases or the magnetosphere, or the zodiacal dust. Interfering light from these sources will obviously be at an extraordinarily low level; nevertheless, it would still be prudent to employ more modest telescopes initially before more elaborate photometric instrumentation is proposed for development. The instrumentation necessary for an adequate site survey is generally available. CCD detectors have been demonstrated to have the necessary stability for even the precision demanded by the asteroseismology observations (/4/). The primary optics of a photometric telescope may not need to be diffraction-limited, since optimization for photometric precision may involve deliberately enlarged images (/5/). This helps with systematic problems in achieving precise photometry; the
(6)102
H.S. Hudson
"strip scan" approach (/15/) also has this advantage. We envision the following overall requirements for an astronomical site-survey instrument: • Telescope aperture as large as feasible (when combined with the need for minimizing the weight of the telescope, the need for a large aperture suggests the possibility of a deployable structure -- here h u m a n help would be efficient).
• Fully autonomous telescope operation, with direct high-bandwidth data links to Earth. • Sophisticated program control to permit flexible operation, exploring different domains among the tradeoffs and autonomous reaction to phenomena. • Telescope operation as extensive in time as possible, i.e. covering all lunar phases and long enough integration times on individual fields to give adequate sensitivity. These requirements would all be met reasonably well by a 1-meter prime-focus instrument (/5/), which would address asteroseismology as its scientific theme but would also study the variability of many (all) objects in specified fields. The parameters of this telescope are as follows: Table 1. A Site-Survey Telescope for Asteroseismology Aperture Focal ratio Focal length Focal plane Data rate Target rate Pointing Stability
1.1 m
f/lO 11 m 20 x 20 cm ~ 100 kbps One field per month 0.1 degree 0.1 arcsec/sec
The diffuse-light survey instrument would need to be quite different optically. It will need to observe most of the ~2~r sr of the sky visible from a given place on the lunar surface, while at the same time rejecting interference from direct viewing of the Sun and the Earth. It should have enough sensitivity to obtain a good signal-to-noise ratio on the zodiacal emission even in the antisolar direction; at the same time it should have adequate angular resolution to be able to deal with starlight as a background term for the diffuse photometry. It must also have time resolution (tens of seconds) adequate to follow moving disturbances in the solar wind and in the geotail. The solar-wind disturbances include rapidly-moving shock fronts and coronal mass ejections initiated by solar flares or other low-coronal disturbances (/9/). A site-survey instrument in this category would have an additional application of possible significance to manned habitation. Such an instrument can track coronal disturbance and significantly aid in the anticipation, if not forecasting, of the arrival of solar energetic particles. LARGE
INSTRUMENTS
The ultimate astronomical objective for lunar habitation would be the creation of observatories with large telescopes. Beyond the site-survey instruments, what facilitiescan we anticipate justifying lunar deployment? There is no reason at this point to compromise, since the development of full facilitieson the M o o n will come only much later in time. Feasibility is an issue that can be dealt with separately and in due time, considering the ultimate scientific return in the long run. In this respect one could consider the use of lunar materials and of lunar fabrication via autonomous (robotic) tools (/7/).In such a scenario, there would be no limit imposed by the infrastructure of transportation (i.e.,costly shipments by rocket from the surface of the Earth) to the establishment of extensive observing networks. The following list can therefore hardly be comprehensive, but I hope that it samples some of the range available for astronomical photometry.
Astronomical Photometry from The Moon
(6)103
Ordinary Telescopes To study the asteroseismology of stars in the open cluster M67, a network of 4-m class telescopes with CCD sensors has been organized/6/. The large aperture and continuous viewing permitted by such a network, plus the photometric advantage of the ensemble reference that an imaging sensor allows, all make this the state-of-the-art program in time-series photometry from the surface of the Earth. A (smaller) network of (larger) telescopes on the Moon would have substantial advantages over this approach. The photon-statistics limitation would require a dedicated array of large telescopes with panoramic detectors. For asteroseismology, we can follow the rule of thumb "one meter, one month, one micromagnitude at 10th magnitude" imposed by counting statistics and suggest fifteen-meter telescopes for asteroseismology surveys down to 15th magnitude.
Area Scanning Telescopes Relatively simple telescopes can make deep and comprehensive variability surveys via staring or strip-scanning /15/. A strip scan uses the rotation of the Moon to record a swath repeatedly. One telescope essentially would sample once per month; multiple telescopes viewing the same swath would sample correspondingly more frequently. Other optimizations involving scan patterns with different sampling could be considered, for example in optimized searches for microlensing phenomena of a given type. In general one would have the same kind of flexibility for a generalpurpose telescope, within the constraints of its optical layout. Such a telescope could hop from galaxy to galaxy in a programmed pattern in order to search for extragalactic supernovae, as is done by ground-based telescopes within their limitations.
Diffuse Photometry Telescopes (All-Sky Monitors) By analogy with an all-sky camera network in the Earth's auroral zones, an all-sky monitoring capability for heliospheric and magnetospheric diffuse light would be extraordinarily productive in terms of interplanetary plasma physics (K-corona) and the physics of the zodiacal cloud (F-corona). All-sky telescopes also have applications in extrasolar astronomy, especially in patrolling for transient phenomena. Permanent dark--sky sites in polar craters have been proposed for astronomical use, and they would be ideal for such an application. Reactive Observatories With unrestricted access to a large fraction of the sky, telescopes capable of reacting to transient astronomical would be extremely interesting. An observatory with this capability would probably involve broad spectral coverage, including high-energy (X-ray and 7-ray) instrumentation. The observatory would automatically detect a transient, assess its significance, call up the necessary resources and then make definitive observations. Not a single 7-ray burster source has yet been identified in terms of an optical or other counterpart object as of the time of writing this paper, with the possible exception of the extragalactic supernova remnant N49. This suggests that the real scientific development of subjects such as this may well lie in the future! CONCLUSIONS
The Moon would be a good place for the deployment of several types of observatories dedicated to astronomical photometry. The last few years have seen several remarkable new developments in applications of astronomical photometry, of which I have mentioned asteroseismology, microlensing, and heliospheric diffuse light in particular. These developments strongly suggest that classical fields of astronomy will flourish given access to space, which is the key to a substantial improvement in the systematic effects that limit Earth-based observations. To prepare for this potential new blossoming of astronomy on the Moon, site-survey telescopes capable of searching for the limiting systematic problems should be put on the surface of the Moon as
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H.S. Hudson
soon as programs permit, i.e. as soon as lunar exploration resumes. This paper has described a sitesurvey instrument capable of making great strides in asteroseismology as well as other photometric applications on objects bright enough to be accessible to one-meter aperture. As a CCD-based stable instrument with a large field of view, it would also have interesting applications to astrometry. Other small site survey instruments for other specialized purposes should be considered, especially in the area of heliospheric remote-sensing measurements. These diffuse-sky measurements would easily become the state of the art for this application, given the uniqueness of the lunar platform. If permanent lunar habitation ensues, over the next several decades, it would be scientifically attractive to create major lunar observatories dedicated to photometry. ACKNOWLEDGMENTS I would like to thank E. Rhodes for helpful commentary.
REFERENCES
1. Borucki, W.J., Scargle, J.D., and Hudson, H.S., 1985, Astrophys. J. 291, 852. 2. Brown, T. M. and Cox, A. N., 1987, in Stellar Pulsation, Lecture Notes in Physics 274, Eds A.N. Cox, W.M. Sparks and S.G. Staxrfield, Springer-Verlag, Berlin, pp. 415-418. 3. Brown, T.M., Gillilsnd, R.L., Noyes, R.W. and 1L~nsey, L.W. 1991, Astrophys. J.368, 599. 4. Buffington, A., Hudson, H.S., and Booth, C.H., 1990, Publ. Ast. Soc. Pacific 102, 688. 5. Burlington, A., Booth, C.H., and Hudson, H.S., 1991, Publ. Ast. Soc. Pacific 103, 685. 6. Gilliland, R.L., and Brown, T.M., 1992, Pubi. Ast. Soc. Pacific 104, 582. 7. Gorenstein, P., in M.J. Mumma and H.J. Smith (eds.), Astrophysics from the Moon, AIP 207, 382. 8. Greist, K., 1991, Astrophys. J.306,412. 9. Jackson, B.V., 1989, in J.H. WaJte, Jr, J.L. Butch and R.L. Moore, eds., Solar System Plasma Physics (Geophysical Monograph 54), 287. 10. Jackson, B.V., and Leinert, C., 1985, Yourn. Geophys. Res. 90, 10759. 11. Leinert, C., Harmer, M., Richter, I., and Pitz, E., 1986, Astron. Astrophys. 82, 328. 12. Hudson, H.S., 1988, Annual Reviews 26, 473. 13. Hudson, H.S., and Hildner, E., in M.J. Mumma and H.J. Smith (eds.), Astrophysics from the Moon, AIP 207, 584. 14. Max), S. and Paczynski, B., 1991, Astrophys. J. 374, L37. 15. McGraw, J.T., 1993, these Proceedings. 16. Paczynski, B. 1986, Astrophys. J. 304, 1. 17. Van Citters, G.W., Jr., Bless, R.C., Dolan, J.F., Elliott, J.L., Robinson, E.L., and White, R.L., 1988, in W.J. Borucki (ed.), Second Workshop on Improvements in Astronomical Photometry, NASA CP-10015, p. 1.
Adv. Space Res. Vol. 14, No. 6, pp. (6)99-(6)104, 1994 Copyright © 1994 COSPAR Printed in Great Britain. All rights reserved. 0273-1177/94 $6.00 + 0.00
Pergamon
ASTRONOMICAL PHOTOMETRY FROM THE MOON Hugh S. Hudson lnstitute for Astronomy, University of Hawaii, Honolulu, HI 96822, U.S.A.
ABSTRACT The Moon would be an excellent platform for photometric astronomical observations. This paper discusses such observations,emphasizing time-seriesphotometry of oscillatingstars (asteroseismology), of faint gravitating bodies (microlensing), and of the interplanetary medium. To prepare for the deployment of major new telescopes and instrumentation on the surface of the Moon, I suggest that smaller "site-survey" instruments be put in place as soon as possible. Each application suggested can derive great benefits from small site-survey instruments established relativelysoon, and each would ultimately need extensive arrays of large instruments. INTRODUCTION The art of photometry is one of the classic fields of astronomy, and one for which access to space makes a major technical difference because of the effects of atmospheric extinction and seeing. Thus far in the history of space astronomy there has been only one major instrument optimized for photometric observations, namely, the High Speed Photometer of the Hubble Space Telescope /17/. By contrast, there have been innumerable experiments simply exploiting the advantages of space for opening new wavelength ranges, such as X-ray astronomy. One can distinguish here between classical photometry, which is the measurement of the brightnesses of the stars and other discrete astronomical bodies, usually in broad spectral bands and with relatively infrequent sampling; and time-series photometry, which is the measurement of the temporal fluctuations of the brightness of a given astronomical object. Modern detectors and data systems now make time-series photometry over entire ensembles of objects easily possible (/6/). The diffuse sky background, due to sources ranging from the zodiacal dust to unresolved distant galaxies, also represents a photometric target of real interest. It is not faint: the surface brightness of the dark sky is the equivalent of a few 10th-magnitude stars per square degree. The heliospheric contributions to the diffuse sky brightness may vary in interesting ways clue to solar or magnetospheric plasma activity, and probably also due to the dynamics of the zodiacal dust cloud itself. Thus the diffuse sky brightness is a worthy object in its own right for study with time-series photometry. We can imagine instrumentation that would provide orders-of-magnitude improvement in astronomical photometry. In doing this we have in mind approaching as closely as possible to the ideal: to observe simultaneously all objects in the sky, with time resolution and sensitivity limited only by the inescapable effects of photon statistics. Clearly tradeoffs are necessary within any range of resources, but within a given optimization one can still envision tremendous improvement. The photon statistics limit depends upon the size of the optics, and at some point other natural limitations will come into play - - on Earth, it is seeing. On the Moon we do not know what the natural limits will be, except to note that the upper limits on the variability of the zodiacal brightness (determined partly by other diffuse interplanetary emissions) are on the order of 5%, as measured by the Helios zodiacal-light photometers ( / 1 1 / , / 1 0 / ) . (6)99
(6)100
H.S. Hudson
In improving astronomical photometry systematically by large factors, it is likely that most or all astronomical objects will turn out to be variable. The Sun exhibits many different types of variability in the range AI/I = 10-s~ 10-3, on a variety of time scales (/12/). The known mechanisms include magnetic activity, convection, rotation, and oscillations. Variations due to similar physics can be expected for many stars, with different and sometimes more extreme effects elsewhere on and off the main sequence. The Moon is clearly an excellent site for astronomy of all kinds, with no obvious disadvantages save that of the cost of transportation and operations there. This article gives a view of astronomical photometry in the lunar context, with comments about early "site survey" instruments as well about ambitious observing programs that might be pursued in the future. The development of lunar infrastructure, including permanent habitation as well as extensive robotic exploration, would be a necessary condition before such sophisticated astronomical observations could be undertaken. PHOTOMETRIC SCIENCE Photometric measurements inform us about the physical mechanisms that make astronomical objects vary in brightness. There is an extraordinary variety of these mechanisms, ranging from the extremely precise resonances of stellar oscillations to the diffuse vagaries of the emissions from the outer solar corona, the solar wind, and the interplanetary medium. Illustrating the breadth of this technique - - with modern observational and data-analysis technology - - it would be possible by direct photometry to detect planets orbiting distant stars (/1/). Among several interesting possible modern developments of astronomical photometry, consider: Asteroseismologv The extension of time-series photometry to the "micromagnitude" range (i.e., variability measured in parts per million of the average source brightness), in the mHz frequency range, will allow us to study stellar interiors via observation of the global oscillations. Such observations have exquisite precision because of the high stability of the oscillations - - resulting in narrow resonance lines in the power spectrum of the solar brightness and long-term observations should show interesting time variations of the parameters of the oscillations. The development of helioseismology has led (for example) to the measurement of the solar internal rotation rate, and to the discovery that the differential rotation seen at the photosphere extends radially inward through the convection zone. The asteroseismology of main-sequence stars at present consists mainly of solar observations (but see also/3/), although many oscillating stars of other types are known. An asteroseismology observatory capable of studying many stars will be necessary if we are to take the next scientific steps in studying stellar evolution by analysis of interior structure and dynamics; this will require obtaining statistically significant samples of stars of different ages (/2/). -
-
Microlensing Following Paczynski's original suggestion/16/that gravitational lensing could be studied via timeseries photometry, several major observational programs have begun (/8/). The prospects for major astronomical advances in this area are quite bright, but wholesale production of data will be necessary: according to one estimate (/14/) the object of search would be a 0.3-magnitude increase, over a time interval of a week to a month, in one star out of 2.5 million at any given time! These are extremely difficult search parameters, and the ambitious ground-based search programs now in development may not achieve their goals (which include a wide variety of targets: the galactic halo, dark matter, gamma-ray bursters, binary stars have all been mentioned). Lunar deployment under stable and unvarying conditions seems like an appropriate development for this kind of observation, which will require large amounts of high-quality data. The optimization of observations for microlensing aims at longer sample intervals and wider fields of view than asteroseismology. Both optimizations, however, should generate a large amount of serendipitous discovery of low-level variability in objects of many types.
Astronomical Photometry from The Moon
(6)101
HeUospheric Remote Sensing The study of heliospheric dynamics via fluctuations of the diffuse background light (/11/) has had a promising beginning. The diffuse light can come from Thomson scattering of sunlight by interplanetary or magnetospheric electrons, or from emission lines. The heliosphere contains shock waves, coronal mass ejections, and other phenomena of solar origin, plus the zodiacal cloud, comets, asteroids, and possibly other phenomena of non-solar origin. The Moon offers several distinct advantages for the observation of all of these features: lower background light, remoteness from the Earth's magnetosphere (at inferior conjunction), and a variable orientation with respect to the magnetospheric structures that are among the targets of the observations. Because the heliosphere is optically thin, tomographic observations that would permit a 3-dimensional reconstruction of the source geometry can be attempted (/10/). This of course requires multiple lines of sight (see,
e.g.,/13/). The magnetospheres of Earth and Jupiter, in particular, exhibit dynamical plasma phenomena of great complexity, for which imaging observations would be extremely helpful. Such objects would be natural diffuse--imager targets for lunar-based observatories. THE MOON The surface of the Moon offers little technical advantage over deep space as a platform for astronomical observations at optical wavelengths, except to provide a massive and stable "spacecraft" for mounting telescopes. This presupposes that the lunar environment itself does not interfere with photometry at some level. Lunar astronomical interference seems rather unlikely, but should be tested with a proper site survey. Solar interference, in the form of many kinds of radiation, is guaranteed, and this suggests a program of solar observation both on the surface of the Moon and in deep space (/13/). The real advantages of lunar observatories would come instead from the beneficial intervention by human beings that would be possible if permanent habitation of the Moon were achieved. The deployment and maintenance of the networks of telescopes and their communication links would be a natural occupation for the inhabitants. The programs proposed here should continue for many years, if not decades, and maintenance from the surface of the Earth would be more complex. A true lunar base would essentially offer simplifications of many of the most expensive aspects of astronomy in space: not just the spacecraft itself, but also launch costs for instrument mass, power, telemetry, repair, upgrading, and so forth. These advantages are unfortunately not available in the short term, and in fact early observatories on the Moon may be more expensive than similar installations put in nearby space on free-flying spacecraft. SITE-SURVEY INSTRUMENTS The initial scientific instrumentation on the Moon will have to be small and lightweight. In any case, a "site survey" would follow in established astronomical tradition and would be a costeffective means of demonstrating the feasibility of installing a major network of large-aperture lunar telescopes prior to initiating their actual development. This is a reasonable technical approach. The site-survey instruments would also produce scientific results of great significance, simply because of lack of previous space experiments with photometric optimization. The main technical motivation for a site-survey telescope on the lunar surface would be to determine the effects of interfering light sources: either particles near the Moon, diffuse sources due to residual gases or the magnetosphere, or the zodiacal dust. Interfering light from these sources will obviously be at an extraordinarily low level; nevertheless, it would still be prudent to employ more modest telescopes initially before more elaborate photometric instrumentation is proposed for development. The instrumentation necessary for an adequate site survey is generally available. CCD detectors have been demonstrated to have the necessary stability for even the precision demanded by the asteroseismology observations (/4/). The primary optics of a photometric telescope may not need to be diffraction-limited, since optimization for photometric precision may involve deliberately enlarged images (/5/). This helps with systematic problems in achieving precise photometry; the
(6)102
H.S. Hudson
"strip scan" approach (/15/) also has this advantage. We envision the following overall requirements for an astronomical site-survey instrument: • Telescope aperture as large as feasible (when combined with the need for minimizing the weight of the telescope, the need for a large aperture suggests the possibility of a deployable structure -- here h u m a n help would be efficient).
• Fully autonomous telescope operation, with direct high-bandwidth data links to Earth. • Sophisticated program control to permit flexible operation, exploring different domains among the tradeoffs and autonomous reaction to phenomena. • Telescope operation as extensive in time as possible, i.e. covering all lunar phases and long enough integration times on individual fields to give adequate sensitivity. These requirements would all be met reasonably well by a 1-meter prime-focus instrument (/5/), which would address asteroseismology as its scientific theme but would also study the variability of many (all) objects in specified fields. The parameters of this telescope are as follows: Table 1. A Site-Survey Telescope for Asteroseismology Aperture Focal ratio Focal length Focal plane Data rate Target rate Pointing Stability
1.1 m
f/lO 11 m 20 x 20 cm ~ 100 kbps One field per month 0.1 degree 0.1 arcsec/sec
The diffuse-light survey instrument would need to be quite different optically. It will need to observe most of the ~2~r sr of the sky visible from a given place on the lunar surface, while at the same time rejecting interference from direct viewing of the Sun and the Earth. It should have enough sensitivity to obtain a good signal-to-noise ratio on the zodiacal emission even in the antisolar direction; at the same time it should have adequate angular resolution to be able to deal with starlight as a background term for the diffuse photometry. It must also have time resolution (tens of seconds) adequate to follow moving disturbances in the solar wind and in the geotail. The solar-wind disturbances include rapidly-moving shock fronts and coronal mass ejections initiated by solar flares or other low-coronal disturbances (/9/). A site-survey instrument in this category would have an additional application of possible significance to manned habitation. Such an instrument can track coronal disturbance and significantly aid in the anticipation, if not forecasting, of the arrival of solar energetic particles. LARGE
INSTRUMENTS
The ultimate astronomical objective for lunar habitation would be the creation of observatories with large telescopes. Beyond the site-survey instruments, what facilitiescan we anticipate justifying lunar deployment? There is no reason at this point to compromise, since the development of full facilitieson the M o o n will come only much later in time. Feasibility is an issue that can be dealt with separately and in due time, considering the ultimate scientific return in the long run. In this respect one could consider the use of lunar materials and of lunar fabrication via autonomous (robotic) tools (/7/).In such a scenario, there would be no limit imposed by the infrastructure of transportation (i.e.,costly shipments by rocket from the surface of the Earth) to the establishment of extensive observing networks. The following list can therefore hardly be comprehensive, but I hope that it samples some of the range available for astronomical photometry.
Astronomical Photometry from The Moon
(6)103
Ordinary Telescopes To study the asteroseismology of stars in the open cluster M67, a network of 4-m class telescopes with CCD sensors has been organized/6/. The large aperture and continuous viewing permitted by such a network, plus the photometric advantage of the ensemble reference that an imaging sensor allows, all make this the state-of-the-art program in time-series photometry from the surface of the Earth. A (smaller) network of (larger) telescopes on the Moon would have substantial advantages over this approach. The photon-statistics limitation would require a dedicated array of large telescopes with panoramic detectors. For asteroseismology, we can follow the rule of thumb "one meter, one month, one micromagnitude at 10th magnitude" imposed by counting statistics and suggest fifteen-meter telescopes for asteroseismology surveys down to 15th magnitude.
Area Scanning Telescopes Relatively simple telescopes can make deep and comprehensive variability surveys via staring or strip-scanning /15/. A strip scan uses the rotation of the Moon to record a swath repeatedly. One telescope essentially would sample once per month; multiple telescopes viewing the same swath would sample correspondingly more frequently. Other optimizations involving scan patterns with different sampling could be considered, for example in optimized searches for microlensing phenomena of a given type. In general one would have the same kind of flexibility for a generalpurpose telescope, within the constraints of its optical layout. Such a telescope could hop from galaxy to galaxy in a programmed pattern in order to search for extragalactic supernovae, as is done by ground-based telescopes within their limitations.
Diffuse Photometry Telescopes (All-Sky Monitors) By analogy with an all-sky camera network in the Earth's auroral zones, an all-sky monitoring capability for heliospheric and magnetospheric diffuse light would be extraordinarily productive in terms of interplanetary plasma physics (K-corona) and the physics of the zodiacal cloud (F-corona). All-sky telescopes also have applications in extrasolar astronomy, especially in patrolling for transient phenomena. Permanent dark--sky sites in polar craters have been proposed for astronomical use, and they would be ideal for such an application. Reactive Observatories With unrestricted access to a large fraction of the sky, telescopes capable of reacting to transient astronomical would be extremely interesting. An observatory with this capability would probably involve broad spectral coverage, including high-energy (X-ray and 7-ray) instrumentation. The observatory would automatically detect a transient, assess its significance, call up the necessary resources and then make definitive observations. Not a single 7-ray burster source has yet been identified in terms of an optical or other counterpart object as of the time of writing this paper, with the possible exception of the extragalactic supernova remnant N49. This suggests that the real scientific development of subjects such as this may well lie in the future! CONCLUSIONS
The Moon would be a good place for the deployment of several types of observatories dedicated to astronomical photometry. The last few years have seen several remarkable new developments in applications of astronomical photometry, of which I have mentioned asteroseismology, microlensing, and heliospheric diffuse light in particular. These developments strongly suggest that classical fields of astronomy will flourish given access to space, which is the key to a substantial improvement in the systematic effects that limit Earth-based observations. To prepare for this potential new blossoming of astronomy on the Moon, site-survey telescopes capable of searching for the limiting systematic problems should be put on the surface of the Moon as
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soon as programs permit, i.e. as soon as lunar exploration resumes. This paper has described a sitesurvey instrument capable of making great strides in asteroseismology as well as other photometric applications on objects bright enough to be accessible to one-meter aperture. As a CCD-based stable instrument with a large field of view, it would also have interesting applications to astrometry. Other small site survey instruments for other specialized purposes should be considered, especially in the area of heliospheric remote-sensing measurements. These diffuse-sky measurements would easily become the state of the art for this application, given the uniqueness of the lunar platform. If permanent lunar habitation ensues, over the next several decades, it would be scientifically attractive to create major lunar observatories dedicated to photometry. ACKNOWLEDGMENTS I would like to thank E. Rhodes for helpful commentary.
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