EUVE SURVEY OBSERVATIONS OF THE MOON
          J.S. McDonald (1) and G.R. Gladstone (1,2)
      (1) Center for EUV Astrophysics, 2150 Kittredge St.,
    University of California, Berkeley, California 94720, USA 
                (2) Space Sciences Laboratory,
    University of California, Berkeley, California 94720, USA


ABSTRACT

   Owing to the geometry of the six-month all-sky survey phase of the mission,
EUVE has several times observed the Moon.  The Sun is far too intense to be
observed directly by EUVE's highly sensitive detectors, so solar EUV variations
can be most directly observed by EUVE in lunar reflection.  Observations of
the Moon can also tell us of the composition and texture of the lunar surface
as well as the nature of the local EUV background radiation.  Preliminary
survey images and analysis are presented.


1. INTRODUCTION

   The Extreme Ultraviolet Explorer [1] is executing detailed studies of local
white dwarf stars, coronally active stars, and even distant active galaxies.
Owing to the geometry of the six-month all-sky survey phase of the mission,
the satellite has also several times observed the Moon.
   The microchannel plate imaging detectors developed by Berkeley engineers
and astronomers, while capable of detecting faint extreme ultraviolet (EUV)
radiation from Sun-like stars many parsecs away, cannot directly observe the
nearest coronally active star, the Sun.  Bright solar EUV radiation, originating
in the million-degree corona, would destroy the sensitive photon-counting 
detectors and filters.  For this reason, EUVE observes solar radiation through
its effects on local sky background levels and lunar reflectivity.
   Solar variations in the EUV can be seen in varying levels of local back-
ground in the diffuse atmosphere present at the 528 km orbit of EUVE.  These
variations can be more directly observed in the reflection from the Moon's
surface.  Since EUVE scanned the Moon twice each month of the all-sky survey,
solar EUV variations can be monitored over the half-year period.  No other
solar monitoring satellite is currently operating in the wavelength range
spanned by these detectors.
   Observations of the Moon in the phases seen during the survey can tell us
of the composition and texture of the lunar surface.  Lunar EUV emission most
likely results from reflected solar radiation, but it has been proposed that
a component of this emission may be L- and M-shell x-ray fluorescence from
various elements on the lunar surface [2].  EUV observations may then result
in mapping of the surface distribution of certain elements.  Spectrophotometer
measurements made by Mariner 10 in 1974 were at wavelengths greater than 550 A
and the ROSAT x-ray satellite made observations at wavelengths shorter than
120 A, so EUVE fills a large information gap.
   The dark side of the Moon in a phase less than full can also tell us of the
nature of interplanetary and interstellar EUV background levels.  The ROSAT
observations found the dark side much darker in x-rays than the surrounding
sky, indicating that the x-ray background is largely extra-solar in origin.
EUVE observations provide similar information on the background from 50 to
740 A.  Its lunar eclipse observation of December 9-10, 1992, may yield
similar information on the EUV background.  When empty sky measurements are
subtracted from the eclipsed Moon image, the resultant flux can be measured,
provided there is no intrinsic EUV flux seen from the Moon during the eclipse.


2. THE DATA

   Because the three scanning telescopes aboard EUVE point at right angles
to the antisun line, the Moon is observed during survey when it lies at 90
degrees to the Sun, i.e., at first or last quarter.  The fourth telescope,
pointed directly down the shadow cone, sees the Moon only when it appears
within the Earth's shadow, i.e., during a lunar eclipse  (as during the night
of December 9-10, 1992).  Thus, during six months, thirteen observations of
the Moon were collected.
   The spacecraft rotation of three times per 96 minute orbit, or once during
the half-hour period it is on the dark side of Earth, translates to approxi-
mately 12.5 minutes of arc per second.  Any source will, therefore, spend
a maximum of 24 seconds per orbit in the scanner's 5 degree field of view.
Fixed stars are scanned once per orbit for five to thirty days depending on
their ecliptic latitude.  Due to the Moon's orbital motion, however, it is
seen during only six or seven orbits at each first and last quarter.  This
amounts to about 170 seconds of observation per lunation, but the quarter
Moon is brighter than most EUV sources in the sky.  This is sufficient for
imaging and photon statistical calculations.  All first quarter observations
(for example) can also be added up to form a brighter image of the scanned
Moon.
   A computer "Pipeline" of 25 individual data reduction programs reduces
the raw telemetry from the orbiting observatory as it is received at the
Center for EUV Astrophysics (CEA) in Berkeley from EUVE via the Goddard
Space Flight Center.  Using spacecraft pointing information and accurate
timing of the arrival of each photon in the detectors, the photons are
remapped to their proper right ascension and declination coordinates.  This
process results in a map of the 5 degree wide, 360 degree long, strip of
sky scanned during a particular orbit.  Individual scans at one per orbit
are accumulated into all-sky maps as standard procedure during the survey.
The 24 seconds of lunar observation are extracted from this map.  These
data are then reduced further and may be enhanced by data reduction programs
such as IRAF (Image Reduction and Analysis Facility) or IDL (Image Display
Language) on the CEA computer system of workstations and servers, which
includes a 1 Terabyte optical disk data storage system.  Since the Moon is
a moving target, adding images is not as simple as accumulating photon right
ascension and declination events into a single image.  IRAF is used to shift,
and sometimes rotate (depending on the position of the Moon in its orbit
relative to that of EUVE), individual lunar images for addition.  Accurate
ephemeris data for the Moon relative to EUVE is useful for removing the
effects of parallax, as well.


3. DISCUSSION

   Fig.1 depicts the first quarter Moon across the entire wavelength range
observed by EUVE.  The central regions of the microchannel plate detectors,
the best for imaging, have been isolated in this image.  Effects of adding
seventeen images from different observations and across a wide wavelength
range have served to obscure surface features that may be represented.
Perhaps most notable is that background levels across the dark side of the
Moon appear indistinguishable from regions elsewhere in the image.  This
confirms that most EUV background radiation originates in regions that are
closer than the Moon, in the Earth's tenuous upper atmosphere.
   In Fig. 2, the Moon is depicted in first quarter phase within the 160-240
A aluminum/carbon filter band of EUVE.  In these medium EUV wavelengths,
some surface features become more evident, such as the Sea of Serenity on
the upper right limb of the Moon and other darker regions toward the southern
limb.  These data indicate a lower EUV albedo for lowland material.  Pre-
liminary measurements of the EUV albedo across the Moon in this wavelength
range are around 0.15%, lower than originally anticipated, but consistent
with earlier measurements [3].  In this narrower wavelength range as compared
with the previous image, we see, once again, that the dark side of the Moon
exhibits no significant brightness difference from surrounding EUV background.
However, fewer than 3000 photons are displayed in this image.  Further studies
are planned to examine the intrinsic sky background using the lunar dark side.


4. CONCLUSIONS

   Preliminary studies of these data have shown that the brightness of the
Moon varies little from observation to observation.  Further analysis is
necessary to make any inferences regarding solar flux.  Also, early results
show that the lunar albedo closely matches the relative reflectivity of
minerals found on the Moon's surface.  Once again, further studies during
the spectroscopic phase of the EUVE mission are being conducted to confirm
current results regarding the presence of x-ray fluorescence in the data.
At the time of this writing, the Deep Survey/Spectrometer observation of
the December lunar eclipse is still under investigation.  For such a nearby
and familiar object, the Moon has proven a wellspring of interesting scientific
results for the EUVE mission, as it has for other recent observing spacecraft.


REFERENCES

1. S. Bowyer and R. F. Malina, in Extreme Ultraviolet Astronomy, Pergamon
	Press, New York, pp. 397-408 (1991).
2. B. C. Edwards, W. C. Priedhorsky and B. W. Smith, GRL, 18, 2161 (1991).
3. Wu H. H. and Broadfoot A. L., JGR, 82, 759 (1977).


FIGURE CAPTIONS

Figure 1:  Last quarter Moon across whole wavelength range of EUVE.
Figure 2:  First quarter Moon in the 160--240  aluminum/carbon filter
		band of EUVE.