A discussion is presented of the advantages of the Moon for X-ray astronomy. It is concluded that large area detectors connected to long focal length telescopes will provide superior signal to noise ratios and resolution compared to any high energy photon observatories that can be practically placed in Earth orbit

Gorenstein, Paul

English

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N89 - 1581  6 
ENERGY ASTRONOMY FROM A LUNAR BASE 
Paul Gorenstein 
Harvard-Smithsonian Center for Astrophysics 
Cambridge, Massachusetts 02138 
Introduction 
Astronomical investigations in the x-ray (0.2 to 100 keV) and gamma-ray (10 - 1 to 104 GeV) 
regions of the electromagnetic spectrum were made possible by spacecraft above the absorption of the 
Earth’s atmosphere. For the future, x-ray and gamma-ray telescopes that are  considerably larger and 
more massive than those currently under development for the next generation of satellite 
experiments are  envisioned. Indeed, the collecting area of these instruments is limited by the size 
and weight capacity of current spacecraft rather than by any intrinsic difficulty in constructing 
larger detectors or telescopes. The virtually unlimited “real estate” of the lunar surface would 
accommodate instruments with considerably more capability than is possible on an  Earth-orbiting 
platform. Consequently, a lunar base offers investigators the best opportunity to achieve the 
ultimate potential in high-energy astronomy. 
First-generation high-energy astronomy observatories on the Moon will consist of telescope 
and detector components that  are  manufactured on Earth and aligned in situ. As a lunar 
manufacturing capability evolves, it  may become possible to fabricate the heavy telescope 
components by using the Moon’s low gravity and good vacuum. 
The unlimited space of a lunar base as compared to a n  Earth-orbiting station offers the 
following advantages. 
1. Large space: Many large-collecting-area x-ray and gamma-ray instruments can be 
accommodated a t  the same time. 
2. Stable baseline: Long focal length between telescopes and detectors can result in 
configurations with very high resolving power. 
3. Long duration: Inactive instruments need not be removed to make room for other telescopes 
needed. 
4. Assembly of large instruments: Manned activity on the lunar surface is less difficult than 
extravehicular activities from a space station. Using common surveying techniques, lunar 
astronauts can align various components of a telescope system, change the configuration for other 
measurements, or incorporate improved technology. 
5 .  Service: Sufficient space is available for a large supply of consumables, replacement parts, 
and dedicated computers. 
6. Expandability: Telescope systems, particularly those of a modular type, can be developed 
incrementally in reserved spaces. 
7. Observing: Continuous 14-day coverage of cosmic sources is possible. 
https://ntrs.nasa.gov/search.jsp?R=19890006445 2020-03-20T04:56:05+00:00Z
The drawbacks of a lunar base - the cost of transporting cargo to the Moon and the difficulties 
of adapting to the day-night temperature extremes - would be ameliorated as transport capability 
improves and as more experience is gained on lunar surface operations. There is an  intrinsic problem 
in that the absence of a magnetic field on the Moon results in a cosmic-ray background that is four 
times higher compared to near-Earth orbit. Although this increased background is disadvantageous 
to some x-ray and gamma-ray detector systems, it is not a major impediment for focusing x-ray 
telescopes or for gamma-ray telescopes which measure the arrival directions of individual photons. 
These telescopes are capable of discriminating against a diffuse background on the basis of the 
source's position. 
X-Rav and Gamma-Rav Instruments 
Three generic classes of instruments are optimum for deployment on a lunar base. They are  
systems with one or more of the following characteristics. 
1. Very high throughput: Collecting area of 105 cm2 or more 
2. Very long focal length: Very high angular resolution when used in conjunction with 
occulting aperture masks 
3. Broad sky coverage, 2n sr: Studies of temporal variability in many objects simultaneously, 
including burst sources, flares, and transients, as well as monitoring the activity cycles of many 
galactic and extragalactic sources 
Photon fluxes (but not necessarily energy fluxes) of sources in the x-ray and gamma-ray bands 
tend to be relatively low compared to those of optical and radio-emitting objects. Hence, high 
throughput is a prerequisite for detailed astrophysical studies that go beyond merely detecting and 
cataloging sources. Investigators have stated that collecting areas exceeding 105 cm2 are  needed to 
satisfy their requirements in the next century. The objectives of such instruments include imaging, 
spectroscopy, and measurements of temporal variability. 
An artist's conception of a high-throughput x-ray telescope system on the lunar surface is 
shown in figure 1. The instrument shown is the large-area modular array of reflectors (LAMAR), 
which consists of arrays of independent modules with imaging x-ray telescopes and detectors. 
Although the LAMAR is shown as a compact unit mounted in a single large pointing system, in 
practice, the array is more likely to be in a modular pointing system to facilitate service and insertion 
of gratings and crystals for spectroscopy and polarization measurements. A small version of the 
LAMAR with about 103 cm2 of collecting area a t  an  energy of 1 keV is being developed as a Space 
Shuttle experiment (ref. 1). This sketch (fig. 1) could represent other instruments as well, including 
coded-aperture devices for hard x-rays and low-energy gamma rays and a system of spark chambers 
for gamma rays with energies greater than 100 MeV. In contrast to the isolation afforded by a lunar 
base, it seems virtually impossible to accommodate and maintain all of these large systems in the 
close confinement of an Earth-orbit platform simultaneously for long periods without encountering 
mutual interference. 
The angular resolution of a conventional x-ray telescope is limited by optical tolerances to 
perhaps 0.1 second of arc. Any further improvement would require a different approach, such as one 
based on occultation involving a long, stable baseline between apertures and detectors rather than on 
accuracy in the polishing process. For example, two 2-mm apertures separated by a 20-km baseline 
define a direction to within 0.02 arcsec. For wavelengths shorter than 0.4 nm, diffraction through 
2-mm apertures will be below the level of 0.02 arcsec. It is possible to specify a variety of 
46 
long-baseline instruments on the order of these dimensions that can achieve higher angular 
resolution over a limited angular range than is currently achievable by the best focusing telescopes. 
One approach is shown in figure 2. The instrument is a variation of the scanning modulation 
collimator of the High-Energy Astronomy Observatory (HEAO-1 and HEAO-3) experiment (refs. 2 
and 3) and is considerably larger. Rising or setting of sources caused by lunar rotation provides the 
scan. There is a 20-km baseline between two “picket fence” collimators that a r e  presumed to be 
located near the equator of the Moon. The collimators are  repetitively open for a distance of 2 mm and 
opaque for 6 mm. A lO4-cm2 moderate-resolution x-ray telescope, e.g., a subset of the LAMAR 
experiment (fig. 11, is behind the second collimator and serves as a detector. 
The telescope, as compared to a nonimaging detector, eliminates background and confusion 
from multiple sources. If a point source is in the field of view, the modulated intensity of the image as 
a function of time is a series of perfect triangles as the Moon rotates. If the source has finite structure 
on a scale between 0.01 and 0.1 arcsec, it  can be derived from a deconvolution of the shape of the 
modulation. Intrinsic time variations in the source are  corrected by monitoring the source with a 
portion of the detector array that is outside the collimators. If the source can be tracked for 1” of lunar 
rotation, or 2 hours, as it rises or sets, there would be sufficient counts for studying the structure of 
the faintest extragalactic sources detected by the Einstein Observatory (HEAO-2). The objective of 
such studies is the structure of the central regions of quasars and other active galactic nuclei, includ- 
ing the existence ofjets projecting from them. Resolving multiple images due to gravitational lensing 
effects is another objective. 
The picket fence collimators have to be precisely periodic. For each degree of tracking, the 
distant one would either have to be 320 m high (at a 20-km distance) or have to move along a 
320-m vertical track a t  a precisely known rate slightly different than that of the Moon’s rotation. 
Alternatively, the nearer collimator and telescope could be placed on an  elevator that  is situated in a 
crater (fig. 3). The elevation of these instruments above the crater floor is adjusted to maintain the 
line of sight to the distant collimator along a constant direction on the celestial sphere as the source 
rises or sets. There are potentially hundreds of interesting sources in a 1’ by 360” band of azimuth 
along the lunar equator. The amount of sky accessible to the instrument could be increased by 
providing a means of rotating the axis of the system off the lunar equator. In particular, the distant 
collimator is placed on a circumferential track around the near collimator-detector. Each 
320-m segment of track length adds another 1” band of sky. A semicircle, or about 66 km of track, 
would bring nearly the full celestial sphere within reach of the instrument. 
Although this particular instrument provides angular resolution in only one dimension, varia- 
tions of this configuration can be effective for two dimensions. For example, the Japanese satellite 
Hinotori obtained x-ray images of the Sun with a rotating modulation collimator (ref. 4). In analogy 
to that instrument, the two widely separated collimators of the lunar-based observatory are  slowly 
rotated about the same axial direction in space. Their rotation rates are synchronized to a common 
clock. The Fourier transform of the intensity modulations provides the high-resolution image. 
Obviously, many details need to be worked out, including the most practical means of rotating the 
collimators and determining whether they should be integral or modular. Simulation studies would 
clarify the resolving power of this configuration. 
The first reaction of someone habituated to working within the limited confines of a laboratory 
or a satellite experiment is to be intimidated by “fences” and platforms that can be elevated 320 m or 
tracks that are many kilometers long. However, these items are  not at all unusual to the transporta- 
tion and building construction industries. If a lunar base is developed, the necessary tools are  likely 
to already be present. Fukthermore, the problem of constructing tall fences and high elevators is 
eased by the low lunar gravity. 
47 
The preceding example is merely illustrative of many possibilities for achieving higher 
angular resolution with long-baseline observatories on the lunar surface. A narrow field of view, 
multiple-pinhole camera or Fresnel zone plates are other possibilities. A large high-energy gamma- 
ray telescope, for example, an  expanded version of the EGRET instrument of the Gamma-Ray 
Observatory (ref. 5) and a telescope under study for the Space Shuttle external tank (ref. 6) can 
function extremely well in the vacuum and the weak magnetic field of the Moon. A gamma ray is 
converted into an  electron-positron pair, which then travels in straight lines for long distances. The 
coordinates along their paths can be detected in spark or proportional chambers to define the incident 
gamma-ray direction with excellent precision. This is perhaps the only possibility for obtaining the 
precise positions needed for optical identification of high-energy gamma-ray sources near the galactic 
equator. 
The third general class of instrument is an  “all-sky detector.” Within the next decade, a great 
deal of additional information on the temporal behavior of many x-ray objects is expected from 
Japan’s ASTRO-C and the U.S. X-ray Timing Explorer (XTE) satellites. The ultimate instrument is 
one that combines the resolution of the small-area wide-field cameras of these satellites with the 
throughput of their large-area detectors, which have small fields of view. The instrument should be 
capable of resolving thousands of objects simultaneously and providing 104 cm2 of effective area for 
studying the temporal behavior of many objects without problems of excessive background or source 
confusion. An example of such a n  instrument for x-rays is shown in figure 4. It is based on concepts 
proposed by W. Schmidt (ref. 7) of the Max-Planck Institute and on the “lobster eye” camera of 
R. Angel (ref. 8) of the Stewart Observatory. This particular device is a hybrid in that it images in 
one dimension by focusing, whereas a circumferential, pseudorandom collimator with open bands 
along the axis of the cylinder acts as a coded aperture for angular resolution in the other dimension 
(ref. 9). 
Because individual photons are being detected from thousands of sources simultaneously with 
their position, energy, and arrival time information encoded to high precision, the quantity of data 
flowing from the instrument is enormous. A dedicated supercomputer would be needed for processing 
this data stream in real time and for extracting the essential results. Information on transient events 
is needed with minimum delay so that it can be transmitted to other observers. Indeed, the coordi- 
nates of a given object are needed a t  the lunar base so that other detectors, including optical and 
ultraviolet telescopes, can be pointed at  a source which is exhibiting gamma-ray or x-ray activity. 
Conclusions 
A lunar base is the optimum locale for the deployment of the ultimate high-throughput x-ray 
and gamma-ray instruments. The virtually unlimited space would permit the construction of 
telescope systems with a collecting area that is, a t  least, two orders of magnitude more than that of 
observatories currently in the planning stages for Earth orbit. A stable, very long baseline will also 
enable achievement of better angular resolution than is currently possible. 
References 
1 .  Gorenstein, P.: New Instrumentation for Space Astronomy, K. van der Hucht and G. S. 
Vaiana, eds., Pergamon Press (Oxford and New York), 1978, p. 237. 
2. Gursky, H., e t  al.: Astrophys. J. ,  vol. 223,1978, p. 973 
48 
I 
i 
Schwartz, D. H.; Schwarz, J.; Gursky, H.; Bradt, H.; and Doxsey, R.: Proceedings AIAA 
16th Aerospace Sciences Conference, 1978, p. 78. 
Oda, M.: Institute of Space and Astronomical Science Research Note 186, presented a t  
COSPAWIAU Symposium, Advanced Space Instrumentation in Astronomy, 1982. 
Hughes, E. B., e t  al.: IEEE Trans. Nucl. Sci., vol. NS-27,1980, p. 364. 
Koch, D.: Proceedings of the 19th International Cosmic Ray Conference, NASA CP-2376, 
Aug. 1985. 
Schmidt, W. K. H.: Nucl. Instrum. Methods, vol. 127,1975, p. 285. 
Angel, J. R. P.: Astrophys. J., vol. 224, 1979, p. 233. 
Gorenstein, P.; and Mauche, C.: High Energy Transients in Astrophysics, AIP Conference 
Proceedings no. 115, S. E. Woosley, ed., American Institute of Physics Press, 1984, p. 694. 
49 
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Figure 1.- Artist’s conception of a lunar-based LAMAR. As the Moon rotates, sources are maintained 
within the telescope fields of view by a pointing system. The collecting area of a lunar-based 
facility can be much greater than those of the next generation of Earth-orbiting x-ray and gamma- 
ray telescopes. 
50 
I’ PICK ET 
f T 
20 km 
- 1 
I MAGI NG 1-1 ANGULAR RESOLUTION 
?I DETECTOR 
b 0 . 0 8 ‘ 4  
FENCE” 
TIME - 
Figure 2.- Plan view of long-baseline system with very high angular resolution. Because flux 
transmitted through two “picket fence” collimators is modulated by lunar rotation, x-rays having 
wavelengths of 0.4 nm or less are  not diffracted significantly. Deconvolution of the modulations 
reveals the structure of the source in one dimension to a resolution of 0.02 arcsec or less. This 
resolution is far better than can be achieved by conventional x-ray telescopes Synchronized 
rotation of the two collimators about a common axis could, in principle, provide imaging 
information in two dimensions. 
51 
I 
I 
Figure 3.- Profile view of long-baseline system shown in figure 2. The elevation of the second 
collimator (short “picket fence”) and the detector is varied with respect to the floor of a crater so as  
to remain pointed a t  a source as  it rises or sets between angles 01 and 02. 
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OF POOR QUALW \ \ \ \ \ \  
MIRROR VANES 
TWO-DIMENSIONAL POSl 
SENSITIVE DETECTOR 
CODED APERTURE 
T lON 
Figure 4.- A large-area, wide-field x-ray camera system fixed on the lunar surface. The camera 
focuses in one dimension, as described in reference 7, and has a coded aperture for angular 
resolution in the other dimension. This instrument is capable of monitoring temporal variations of 
many sources simultaneously. The maximum angle of reflection, actually about lo, appears 
greatly exaggerated in the figure. 
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High-energy astronomy from a lunar base

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