***Construction and utilization of lunar observatories.***

Roger Angel

Steward Observatory, University of Arizona

Testimony for the hearing on Lunar Exploration

US Senate Committee on Commerce, Science and Transportation,

November 6th 2003

I am an astronomer at the University of Arizona, where big ground-based
telescopes and their mirrors are made. Steward Observatory is now
completing the mirrors for the Large Binocular Telescope, which will
become the single largest telescope in the world.

In September this year I chaired a meeting sponsored by the National
Academy of Science's Space Studies Board, looking at future needs and
technologies for large optics in space. Our group of represented
astronomers, Earth scientists and several government agencies who share
a common interest in very large telescopes. Optics bigger than the 2.4 m
Hubble and the planned 6 m James Webb Space Telescopes would be valuable
for astronomical research, for environmental studies and for defense.
The different uses lead to different telescope configurations (single
dishes, arrays), wavelengths of operation (from ultraviolet to
millimeter), and different optimum locations in space. But we found
strong common interest across the agencies in developing technologies to
make and control very big optical systems to exquisite,
diffraction-limited quality and in the infrastructure to construct,
deploy and service very large optical systems in space.

Optics for Earth imaging and defense need to be near the Earth, and
geosynchronous orbits are especially valuable. For astronomy, operation
in low Earth orbit, like Hubble Space Telescope, has the huge, proven
advantage of astronaut access, but has limit performance because of the
constant cycling in and out of sunlight. The major drawback for low
Earth orbits is that deep infrared observations are not possible,
because they require a cryogenically cooled telescope, far from the
Earth to get away from its radiated heat, and permanently shaded from
sunshine. Looking at the future balance of large telescopes on the
ground and in space, very cold telescopes in space enable science that
is completely impossible from the ground. We see this trend already with
the recent successful WMAP and SIRTF telescopes, and the planned James
Webb telescope, all cryogenic telescopes operating very far from Earth.

Let me mention two major astronomical goals for a much bigger, \~ 20 m
cold telescope in space. One is the search for life on any warm,
Earth-sized planets around nearby stars like the sun. We expect these
will be found with smaller telescopes, but we have no idea if they will
have life. On Earth, abundant life chemically transformed the atmosphere
already 2 billion years ago. A big telescope or array could detect such
transformations by spectroscopy. Another goal will be to see starlight
that has been on its way towards us through most of time. Our
understanding is that the big bang created a uniform gas of just
hydrogen and helium, and that after this cooled off the universe was
completely dark and without form for hundreds of millions of years.
Gravity slowly pulled the gas together into lumps until at some point
these exploded into the first massive, brilliant stars, and started to
make the elements like carbon and oxygen and iron from which the Earth
and life are made. We know a lot about the big bang, because it was so
bright we can see it easily, now cooled off to become radio waves. First
seen from New Jersey, these were recently mapped out from Antarctica and
by WMAP. Today we can only speculate about the first stars, but their
light will now be in the form of faint heat waves. A very big, very cold
telescope in space that stares for a year at the same spot could likely
detect them find out when they formed and what they were like.

Big cryogenic telescopes in space present a conundrum. We can see how to
build, maintain and improve them, based on experience with the Hubble
Space Telescope and in building ISS. But how do we do this if the
telescope must be operated far from Earth?

There are free orbits far from Earth, towards or away from the sun,
where a spacecraft will stay put and not drift away like SIRTF. Most
thinking so far at NASA has focused on operation in the one a million
miles beyond Earth, where WMAP was and the Webb telescope is to go.
Servicing a big telescope would likely involve ferrying new instruments
or even the whole telescope back and forth to a more accessible orbit,
but still ¼ million miles away.

The Moon's south pole is an alternative location. The pole is down in
the Shackleton crater where the sun never shines and cryogenic
temperatures prevail. If there were a Moon base on the crater's edge,
this would be convenient for construction and maintenance. The Moon has
no atmosphere, so light from the stars would have the same pristine
quality as in free space. Only the sky's southern hemisphere would be
observable, but this is not a major astronomical limitation.

The lunar south pole is a good place for a lunar base, independent of
any telescope. The craters are believed to contain water ice, more
valuable than gold for the base^1^. Also, because the Moon's spin axis
is not tilted like the Earth's there are no seasons and the crater rim
has small areas of nearly eternal sunshine, simplifying problems of
maintaining electric power and temperate living conditions^2^.
Furthermore, the area is of intrinsic scientific interest: the adjacent
South-Pole-Aitken basin is the oldest and deepest impact crater on the
Moon, and has been flagged for study in the recent NRC study^3^.

Many technical, engineering and infrastructure issues remain to be
explored. The Moon provides a platform on which to build big structures,
but it also comes with gravity and weight, albeit at 1/6^th^ of the
Earth's value. Freely-orbiting telescopes avoid the need for bearings
and drives. On the Moon magnetic levitation on superconducting bearings
might simplify the task of turning the telescope around during each
month to track the stars. We would need to make sure the telescope
optics were not spoiled by vibrations or dust and condensed gas from the
base.

Gravity could be turned to an advantage for a special telescope to look
back to the very faint first stars, which will be all over the sky. From
the Moon's pole the infrared sky is darkest overhead, and we can look
there uninterrupted at the same unchanging patch of sky for the years
needed for the study. Thus a specialized telescope for this work could
be fixed in place looking straight up. If desired, very high resolution
images of the same patch could be made with multiple such telescopes
laid out as an interferometer, again with no big moving parts. We may
even be able to use a trick to make a telescope mirror looking straight
up by spinning a thin layer of reflecting liquid in a big dish. A 6-m
diameter telescope of very high quality has been built like this very
inexpensively in Canada^4^. No matter the exact shape of the bowl, the
liquid surface takes the shape of a perfect telescope mirror. Bigger
diameters can't be used on the Earth because the spinning makes a wind
that ruffles the surface. But with no wind or air on the Moon, a 20 m or
larger mirror might be made simply this way. A cryogenic liquid with
evaporated gold coating would be used.

More details of zenith-pointing telescopes and their scientific
potential are give in the attached white paper. While it would do some
jobs really well, a fixed telescope would not satisfy the many
astronomical goals which need access over a good part of the sky. For
example, the few nearby stars where we can hope to study Earth-like
planets are randomly distributed all over the sky. But it could do great
science and give a basis of experience at the base for building a
fully-steerable big telescope.

![](media/image1.jpeg){width="3.0in" height="2.7395833333333335in"}In
conclusion, based on astronomical goals and telescope engineering
constraints, the lunar pole deserves to be taken seriously as an
observatory site for large cryogenic telescopes, along with remote free
orbits. I hope that both options will be evaluated in considering the
future of human spaceflight beyond near-Earth orbit.

**References**

1\. Vondrak, R. R. and Crider, D. H. Ice at the Lunar Poles. *American
Scientist* (2003)

2\. Bussey, D. B. J., Robinson, M. S., Spudis, P. D. Illumination
Conditions at the Lunar Poles 30th Annual Lunar and Planetary Science
Conference, Houston (1999)

4\. Cabanac, R. A., Hickson, P. and de Lapparent, V. The Large Zenith
Telescope Survey: A Deep Survey Using a 6-m Liquid Mirror Telescope in
*A New Era in Cosmology*, eds Metcalfe, N. and Shanks, T. ASP Conference
Proceedings **283**. p 129 (2002)

3\. NRC New Frontiers in the Solar System: An Integrated Exploration
Strategy. Space Studies Board (2002)

5\. Page, T and Carruthers, G. R. Distribution of hot stars and hydrogen
in the Large Magellanic Cloud. Ap. J. **248**, 906-924 (1981)

**Attachment**

**A deep field infrared observatory at the lunar south pole**

White Paper for the Space Studies Board (SSB) and the Aeronautics and
Space Engineering Board workshop on \"Large Optics in Space"

Roger Angel

Steward Observatory, University of Arizona

September 6, 2003

## Abstract

Our understanding of the early universe has been revolutionized by deep
optical fields imaged with the Hubble Space Telescope (HST) and analyzed
spectroscopically with larger ground-based telescopes. Much deeper
fields in the infrared could reach still further back in time, to the
era of first star formation at redshift z\~25. But spectroscopy will
require very long integrations with a very large cryogenic telescope
located above the atmosphere.

The Moon is an ideal location for such a telescope. Provided the field
is chosen near the south ecliptic pole, it could be viewed continuously
by a fixed, cryogenic, zenith-pointing telescope or interferometer at
the lunar south pole. The unique advantage of the Moon is its
combination of gravity and no atmosphere. A zenith pointing mirror can
be made simply by spinning a cryogenic liquid and (vacuum) coating it
with metal. The pole is a practical location because there is
nearly-continuous sunshine for solar power, yet cryogenic cooling
requires only simple shielding. There is also water ice in the
permanently dark craters. Given a manned polar base, telescopes with
primary mirrors as large as 20 m and with exquisite, diffraction-limited
accuracy could be constructed. With auxiliary beam-steering optics, a
continuous observation of the ecliptic pole could be maintained for
years. It would have 3 times higher resolution and reach 100 times
fainter than the James Webb Space Telescope (JWST).

# Rationale

The 6-m JWST will obtain the first deep images of the high redshift
universe, taking advantage of the very low zodiacal sky background in
the 2-5 μm spectral region. But even much larger future ground
telescopes will be incapable of spectroscopic follow up, because of
thermal emission and absorption by the atmosphere and telescope. Larger
telescopes in space will be necessary, cryogenically cooled and used
with spatially-multiplexed spectrometers in very long integrations. As
an example, spectra of the first stars formed at very high redshift may
take a few years to record, even with much larger aperture than JWST.

If a cryogenic telescope is required to point at different targets
around the sky, the best location is a gravity-free environment far from
the warm Earth, such as L2. It would be difficult, though, to operate
and maintain to diffraction-limited accuracy a 20 m telescope a million
miles from earth. But for a telescope to be dedicated to deep field
spectroscopy, access to the whole sky is not necessary. A single
extragalactic field chosen near one of the ecliptic poles, where the
infrared sky is darkest, is sufficient.

For such a task, the lunar south pole is a location closer to home with
unique advantages. Here the south ecliptic pole lies very close to the
zenith, because the Moon's spin axis is tilted only 1.5 degress to
ecliptic pole. The precession period is 18 years. Cryogenic operating
temperature needed for infrared observations can be obtained by locating
the telescope in a permanently dark crater. Alternatively, a simple
perimeter radiation shield ![](media/image2.jpeg){width="3.625in"
height="2.716666666666667in"}would suffice, because the sun and Earth
are always close to the horizon. To track the monthly rotation of the
field, only the instrument and small auxiliary optics need be moved.

# A liquid mirror primary mirror telescope

Zenith-pointing telescope mirrors of liquid mercury have been made up to
6-m diameter, spinning at a few revolutions per minute in the 1-g
gravity field of Earth (Cabanac, Hickson and de Lapparent, 2002). The
dish holding the liquid is made of stiff, lightweight composite panels,
shaped to within a fraction of a millimeter of the final figure. The
liquid when spinning at the correct speed is only 1 mm deep. The
existing telescopes are at mid-latitudes, and include compensation for
field motion at near the sidereal rate of 15 arcsec/second. They also
have to deal with vibrations and wind acting directly on the liquid
surface. Despite these difficulties, images of better than 1 arcsec are
obtained.

The south-pole lunar environment is much more favorable for liquid
mirror telescopes. There is no wind and very little seismic disturbance.
A 20 m diameter f/1 mirror would require rotation at 2 rpm. The bearing
might use cryogenic, superconducting magnetic levitation to ensure
freedom from vibration. A tripod would be erected above the mirror to
support secondary and beam-steering optics. The slow monthly rotation of
the south ecliptic pole field on the sky (15 milliarcsec/sec) would be
compensated by rotation of the instrument and corrector optics. Mercury
is not suitable for a cryogenic telescope as it freezes at 234K.
Dimensional changes on freezing and cooling would spoil the accurate
figure of the liquid phase. What is needed is a cryogenic liquid of very
low vapor pressure, to avoid evaporation loss over years of operation.
An example is 1-butene, which is liquid at 90K with a vapor pressure of
10^-7^ torr. To obtain high reflectivity, after reaching equilibrium in
the rotating dish the liquid (which is quite viscous when cold) would be
coated like a glass mirror with evaporated metal layer. Given a low
emissivity coating (1% in the thermal infrared), a telescope at 90 K
will reach the zodiacal background at wavelengths shorter than 8 μm.

# Sensitivity and scientific potential

For imaging in the 0.8-8 μm spectral range, sensitivity is limited by
photon noise in the optical and thermal zodiacal light. The scaling is
as D^2^/√t where D is telescope diameter and t is integration time.
Sensitivities are listed for the JWST with D=6 m and t=100,000 sec or
1.16 days (http://www.stsci.edu/jwst/science/sensitivity/). For the
lunar telescope with D=20 m and t = 100 days, the improvement will be by
a factor 100. The 10σ point source sensitivity in the range 1-4 μm will
then be \~ 1.5x10^-11^ Jy, and from 5-8 μm about 3x10^-10^ Jy.

For spectroscopy at resolution 1000, we envisage an integration of 1000
days. If the sensitivity is set by detector noise, as it is for JWST,
the sensitivity again scales as D^2^/√t for a factor of 300 improvement,
and we find 10σ limits of 5x10^-10^ Jy from 1-4 μm, and 6x10^-9^ Jy from
5-8 μm. The scientific potential of observations to this limit is
explored in the Appendix by Dan Eisenstein and Betsy Gillespie.

![](media/image1.jpeg){width="3.941666666666667in" height="3.6in"}**The
field at the south ecliptic pole**.

The telescope corrector gives access to fields up to 1.5° off--zenith.
This would allow for observation of the south ecliptic pole at any time,
and for observations of objects up to 3° from the pole, in a direction
depending on the Moon's 18 year precession.

Figure 2 shows the accessible field circled. The Large Magellanic cloud
(LMC) is to the right. This historical ultraviolet image was recorded on
the Moon over 30 years ago, by Apollo 16 astronauts John Young and
Charles Duke. (Page and Carruthers, 1981) The ecliptic pole at the
center of the circle is clear of absorbing dust in the LMC
(E~B-V~=0.05).

Because of the low zodiacal background at the ecliptic poles, they may
become already the most deeply observed regions of the infrared sky by
the time detailed observing plans are developed for the lunar facility.
Already deep near infrared sky surveys of the poles are planned for the
SNAP telescope.

**Logistical support at the lunar south pole.**

A liquid mirror telescope requires assembly of the dish and bearing, a
tower to support the secondary and auxiliary optics, filling with liquid
and spinning up, and vacuum coating. These steps might be difficult to
automate or conduct robotically, but would seem to be well suited as a
task for astronauts operating from a Moon base. In fact, the south pole
is a leading candidate site for such a base, because of the availability
of nearly continuous sunshine for solar power (Bussey et al, 1999) and
the likely presence of water ice in the dark craters (Vondrak and
Crider, 2003). It is also a place of intrinsic geological interest, the
South Pole-Aitken basin region being the the oldest and deepest impact
crater preserved on the Moon (NRC, 2002)

# Precursors and evolutionary developments

While the principles are well understood, much development will be
needed to reach the 20 m spinning mirror target. As a first step, a
remotely deployed cryogenic 1 m telescope might be deployed on the rim
of the Shackleton crater. Once a manned base is established,
construction techniques could be tested by erection first on the Moon a
smaller scale spinning mirror prototype, for example one of the current
6 m diameter.

Once a single 20 m dish was in operation, a still more powerful
observatory could be created by addition of duplicate 20 m elements,
linked interferometrically. The same field at the zenith is ideally
located for interferometric study, because the baseline linking the
elements is turned by the Moon's rotation about the line of sight. The
requirements for path correcting elements in the interferometric link
are minimal, and interferometric imaging by the Fizeau method will be
possible over a wide field of view (Angel, 2002).

**References**

Angel, J. R. P. Sensitivity of optical interferometers with coherent
image combination Proc SPIE **4838,** (2002)

Bussey, D. B. J., Robinson, M. S., Spudis, P. D. Illumination Conditions
at the Lunar Poles 30th Annual Lunar and Planetary Science Conference,
Houston (1999)

Cabanac, R. A., Hickson, P. and de Lapparent, V. The Large Zenith
Telescope Survey: A Deep Survey Using a 6-m Liquid Mirror Telescope in
*A New Era in Cosmology*, eds Metcalfe, N. and Shanks, T. ASP Conference
Proceedings **283**. p 129 (2002)

NRC New Frontiers in the Solar System: An Integrated Exploration
Strategy. Space Studies Board (2002)

Page, T and Carruthers, G. R. Distribution of hot stars and hydrogen in
the Large Magellanic Cloud. Ap. J. **248**, 906-924 (1981)

Vondrak, R. R. and Crider, D. H. Ice at the Lunar Poles. *American
Scientist* (2003)

# Appendix

# Notes on Ultra-deep field science from Dan Eisenstein and Betsy Gillespie

Current speculation is that the first stars were very massive (100 to
1000 solar masses).  Such stars have temperatures around 100,000 degrees
and radiate at the Eddington limit (10^40^\--10^41^ erg/s). This means
that about 90% of the energy is emitted in photons hard\
enough to ionize hydrogen and helium.  For example, we would predict the
Lyman alpha flux to be about 1 nJy at z=25 and R=1000 for a 100 M~~
star.  The result would scale linearly with the star\'s mass. The
equivalent photons for helium can\'t pass through the IGM, so the best
helium line is the HeII line at 1640 A, which would have a flux\
about 10% of the Lyman alpha line (i.e. 0.1 nJy for the above example).\
\
Unfortunately, it is unclear whether Lyman alpha photons from the
regions around these massive stars can propagate through the neutral IGM
that surrounds these first stars.  If not, then the Lyman alpha photons
are rescattered into a very low surface brightness sphere about 10\'\'
across.\
\
H is not rescattered and is similarly bright, about 1 nJy in our
example.  However, these photons must be observed at 15m where the
backgrounds are much less favorable.  Note that both the HeII line and
H are intrinsically thin, so one might benefit from going to higher
resolution than R=1000.\
\
The continuum itself from the first stars are very faint: about 0.0002
nJy per 100 solar masses at z=25.\
\
So, our conclusion is that a single first star is detectable only
through line emission and that the brightest line, Lyman , has
sufficient flux but may be rescattered by the IGM.  HeII and H remain
interesting, particularly if the star or star cluster is 1000 M~~
rather than 100.\
\
At z=10, the sensitivity to His fantastic.  10^40^ erg/s of line
emission yields about 20 nJy of flux, which is easily detected. In the
local universe, that corresponds about 0.01 solar mass per year of star
formation; at high redshift and hence lower metallicity, the detectable
star formation rate would be more like 0.001 solar mass per year.  One
sees all manners of galaxy and star cluster formation at z=10.\
\
One class of known objects that would be particularly interesting to see
in formation is globular clusters.  Local evidence suggests that
globulars must form very quickly, less than 10 Myr.  An instantaneous
burst at z=25 of 10^5^ solar masses with a normal IMF would be detected
in the continuum 10 Myr after the burst at 0.015 nJy (10) at 3.5m.\
In other words, one can detect young globular clusters in the continuum
at any relevant redshift.  At z=10, one could likely get spectroscopic
detections of the more massive and young cases.  In the local universe,
globular cluster formation is associated with large mergers; globulars
could be an interesting tracer of hierarchical galaxy formation.\
\
First supernovae are much brighter than first stars and are detectable
even at z=25 with this telescope.  Indeed, the predictions are that
supernovae from very massive stars at z=20 are quite bright, m=26, and
so will be detected by JWST.  With this telescope you would get
spectroscopic time-series.  More conventional supernovae, i.e. from
normal stars, should also be visible. \
\
As a speculative matter, if the neutral IGM does rescatter the earliest
Lyman alpha photons, these photons may still be detectable.  The action
of the IGM is to blur the emission onto scales of 10\'\' and 1000 km/s.
However, these regions contain only about 10^9^ solar masses and so the
emission will be highly clumped on these scales.  This means that the
background, when viewed on 10\'\' and 1000 km/s scales, will be highly
variable.  If 0.1% of all stars form as very massive stars at high
redshift, then the mean intensity today from this smeared background of
Lyman  photons is about 2% of the total extragalactic near-infrared
background.  The high redshift portion amounts to about 12 nJy per
square arcsecond, which is certainly detectable with the lunar
telescope.  The problem is distinguishing that emission from the other
98% of the background. However, the bulk of that emission is in
relatively compact sources (i.e. faint galaxies).  By looking between
the galaxies and by seeking small-scale variations in the spectral
dimension (e.g. by using the known redshifts of the galactic cores to
model the intergalactic spectra), one might be able to isolate these
earliest objects as low surface brightness narrow-band features.  This
would require the full data cube and excellent systematic control, but
it may be feasible. \
\
Please keep in mind that this last idea is not conventional wisdom; I\'m
not even sure it\'s appeared in the literature!\
\
After the universe is reionized, the intergalactic medium begins to
recombine.  This is a very low surface-brightness emission process. The
intensity is 0.013 nJy per square arcsec times ((1+z)/20)^1.5^ ^2^,
where Delta is the density of the gas relative to the cosmic mean. For a
1 sq arcsec patch, one could detect Delta\'s as low as 30. That\'s not
quite the cosmic web at =5, but it is still impressive.  One would be
making maps of the mid-density gas and correlating it with the star
formation of protogalaxies.  Note that the flux levels are similar to
the IR background arguments in the last section, so one would still
likely be having to model the foreground galaxies to remove their faint
wings.