We illustrate these remarks with observations. Figure 10a shows 2
mm wavelength total power scans at different phases of the Moon. In these
observations, one scan passes through the subsolar point and the center
( EW direction at culmination); the other scan is made in
orthogonal direction. The figure shows the symmetric brightness
distribution around New Moon (age
0 days) and Full Moon (age
15 days). As shown in Fig. 10b and Fig. 10c, at
these phases the profiles
(East limb) and
(West limb)
are comparable so that the measurements at opposite limbs can be used to
construct a composite profile
(u), as done above (see
Figs. 3, 4). We find empirically that the difference of the profile
sections
(East) and
(West) is below
1
dB, and hence below the accuracy of the measurements, for observations
within
1 day of New Moon and Full Moon. Also, at these
phases of the Moon the unbalance of the profiles because of temperature
gradients across the Moon and limb darkening is smaller than the accuracy of
the measurements. From observed and synthesized scans we find that the
composite profiles are identical for measurements at New Moon and Full Moon
(see Fig. 10c). This allows a comparison of profiles taken at New
Moon and Full Moon, as done in Fig. 4. We find also that profiles
taken in u-direction or v-direction are similar within
1
dB when taken within
1 day of a certain phase, as used in
Fig. 7.
The brightness distribution at other phases of the Moon than New Moon and
Full Moon shows a more or less steep gradient and a more or less pronounced
crescent (Fig. 10a). The disadvantage of using for the analysis
other phases than New Moon and Full Moon lies in the fact that the available
models (Krotikov Troitskii 1964; Linsky 1966, 1973; Mangum 1993) for
calculation of the Moon's millimeter wavelength brightness distribution
T0
(Eqs. (17,18)) are not sufficiently precise, and
mathematically cumbersome, to produce reliable synthetic profiles
and
for comparison with observations.
The mm-wavelength radiation from the edge of the Moon is polarized in radial
direction by at 1.3 mm (Barvainis et al. 1988). This is a
small and systematic effect and need not be considered in the profile
analysis, even when using polarized feeds.
Acknowledgements
The holography data were kindly made available by D. Morris. The holography data of 1996 and 1997 were obtained in a larger IRAM campaign including the collaboration of D. Morris, J. Lamb, B. Lazareff, M. Carter, F. Mattiocco, and the staff of IRAM-Granada. D. Teyssier of IRAM-Granada made the new measurements used in Fig. 7; A. Karpov (IRAM) provided and operated the 350 GHz receiver. The data of Fig. 1 are taken from the ARGE-Krupp-MAN documentation of the telescope. We appreciated the discussions with D. Morris and B. Lazareff, and the pointed comments of D. Downes (IRAM) and the referee, R.E. Hills.
Copyright The European Southern Observatory (ESO)