72
73
SPECTRAL REFLECTANCE MAPPING: OBSERVATIONAL TECHNIQUES AND DATA REDUCTION
INTRODUCTION
The importance and relevance of visible reflectance spectra in determining the composition and studying the compositional variations of planetary surfaces has been discussed by several authors. With the return of lunar samples such remote sensing techniques hove become exceedingly important tools for extending the knowledge of the composition of o few landing sites to other areas of the moon.
There ore two approaches to interpretation of reflectance spectra in connection with the formation and modification of a planetary surface. The
first involves a direct determination of the surface composition in the ordinary sense of spectral analysis. In the case of visible reflectance spectra, the solid absorptions ore not at all well understood. A unique determination of composition is very difficult to achieve since so many variables concerned with structure and mineralogy of geologic materials control their reflectance spectra. Little progress has been mode in that particular direction o
The second approach is to map the oreal variations of the reflectance spectra in order to study the relationships between mappable spectral units and variations in albedo, morphology, and age. This is the approach token
in this investigation o
Normalized lunar relative reflectance curves obtained by McCord (1968) indicate that o minimum of three narrow-bond reflectance measurements between 0.4 microns and 0.7 microns are required to resolve each of the spectra I types. The centers of these bond posses were chosen at 0.4 microns,
74
0.52 microns and 0.7 microns. The mapping was done photoelectrically rather than photographically in order to permit measurement of small spectral variations (~1%) across the large albedo variations (50% - 200%). Such measurements ore not possible photographically because sufficiently precise and consistent calibrations of films over the required dynamic range cannot be achieved.
INSTRUMENTATION AND OBSERVATION
Observations were made at the bent-cassegrain focus of the Mount Wilson 24-inch telescope. A single beam of photoelectric photometer was used with an 18 ore sec aperture corresponding to a diameter of about 30 km at the sub-earth point on the lunar surface. The narrow band (o02\.--
width) interference filters centered at .4 microns, .52 microns and .7 microns were placed between the aperture and a cooled ITT FW 130 (S -20) photo- multiplier tube o The signals were recorded with a highspeed pulse-counting data system o
Lateral variations in the lunar reflectance spectrum were measured by scanning strips 130 ore sec in length and 18 ore seconds in width twice in each of the three filters. The telescope was moved continuously during each scan at the rate of 13 ore sec/sec. Samples were token along the scan after
integration periods of .35 seconds separated by .80 seconds of recording time.
This allowed an overlap of about 35% between consecutively sampled areas.
This choice of spacing was made to minimize aliasing and to maximize coverage. Nine samples were token per scan. Before and after each line was scanned a standard area in Mare Serenitatis (17.6°N, 19.4°E) was measured. The period between standard measurements was about 5 minutes.
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GENERAL METHOD AND DATA REDUCTION
The measurement of such reflectance varictions is complicated by scan position errors, short and long period atmospheric transmission errors, and statistical pulse counting errors o This section is intended to describe how
these effects are minimized and to provide a foundation for interpreting the resu Its.
At some time, t, and at a lunar phase angle, a. , a strip along the lunar surface reflects radiation in the direction of an observer. The width of the strip is controlled by the field of view of the instrument o This radiation can be represented by a function B(x1A.) which is the brightness averaged across the strip in the intervals x-x+dx and x.-x_+dx_, where x is the coordinate along the strip and A. is the wavelength of the radiation o
Without loss of generality, B(x1A.) can be written
B(x,A.) = G(x) C(x,A.) S(A) and (1)
C(x,A.)=l
0
where S(A) is the solar insolation, G(x) the albedo variations along the strip, and C(x,A.) the differential wavelength dependence along the strip.
Here A. is taken to be 0.52 microns, so that, the reflectance spectrum,
0
C(x,A.)1 is normalized at 0.52 microns. In general1 variations in C(x,A.) from area to area se Idem exceed 10% whereas G (x) can vary by factors of 2 or 3.
The transmission of the earth's atmosphere can be written as the sum of two functions
76 (2) A (A.1Z1t)
=
T (A.1z)+
V (A.1t)T (A.1Z) is a secu lor variation dependent on wave length and the thickness of the atmosphere traversed; V (A.1t) is a smaller 1 high frequency variation, the amplitudes of which are of the order of 2% under nominal conditions.
S orne periods of V (A., t) are longer than the integration times of the measurement 1 and thus its constitutes a major source of noise.
The instrument response is characterized by the spatial and spectral windows W(x) and F. (A.) where the index 1 i, refers to the ith filter.
I
W(x) has the form of a symmetric trapezoid and depends on the instrument aperture diameter and the scan rate; F. (A) is the transmission of the ith
I
filter (Figure 13). The jth sample along a scan taken in the ith filter can be written
CX)CX)
s ..
I 'I
=
-(X) .s
G (x1) C(x1,A.) T(A.,z) S(A)(3) • F. (A) W (j !:1 x - x 1) dA. dx 1
I
+
noise from V (A., t)To a high degree of precision (<0 .1% error for lunar areas), C (x ,A.) can be considered constant over the bandpass {--.-0 .31-l). Then equation (2) becomes
(4)
S . .
=
I,J
f
CX) T (A. ,z) S (>-.) F i (A) d /,CX)
r
G (x I)c.
(x I)w ( i
!:1 X - X I) dx Iu I
-(X)
+ noise
where C.(x) is the wavelength average of C(x,A.) over the bandpass of the ith
I
fi Iter. The unknown atmospheric dependencies in the first integral in equation (4) are eliminated by forming the ratio between the scan sample, S . . , and a
l , j
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100~---,
1-
z 80t- w
0cr::
w
Q_
60-
z
0(j)
40
t-(j)
-
~
(j)
z
<!
20
f-0::
I-
1