PDS_VERSION_ID        = PDS3
RECORD_TYPE           = STREAM
OBJECT                = TEXT
  PUBLICATION_DATE    = 2007-09-26
  NOTE                = "Geometry information and description of the contents
                         of the GEOMETRY directory."
END_OBJECT            = TEXT
END


Contents of the GEOMETRY directory
----------------------------------
The GEOMETRY directory contains 2 images, bug_lab_goniometer.jpg and
geometry_sketch.jpg. bug_lab_goniometer.jpg is a labeled image of the
instrument used to acquire the archive data. geometry_sketch.jpg is a sketch
used to explain the geometry of the instrument (see below).


GEOMETRY Information
--------------------
The geometry of the observations requires some explanation (refer to
geometry_sketch.jpg in this directory). Our system records incidence and
emission angles as positive or negative. The emission arm is fixed in one
plane while the incidence arm rotates in its own independent plane which can
rotate in azimuth, also positive or negative. Because of this, the same
geometry can be measured using different combinations of i, e, and az. For
example, i = +30, e = -40, az = 0 and i = -30, e = -40, az = 180 have the
source and detector in the same exact positions. In the measurements here,
you will often find two congruent measurements, such as one at i = +60,
e = +30, az = -20, phase angle = 88.5 and another at i = -60, e = +30,
az = 160, phase angle = 88.5. In this case, the relative geometry is the
same, but the arms have switched sides of the table. We calculate phase angle
based on the relative positions of the two arms. We made twenty-seven
identical or congruent geometry measurements throughout a sample run to check
the fidelity of our data. In our 3D plots of the data, we assumed that
scattering was mirrored across the principal plane (azimuth = 0 to 180 line).

Our samples are illuminated at 5 different incidence angles
(i = 0, 15, 30, 45, and 60 deg) and emission angles from (absolute) 0 to 80
deg in 10 deg or finer increments. We vary azimuth from (absolute) 0 to 160
deg in varying steps, depending upon the incidence angle. When i is small
(<30 deg or so), we take larger azimuth steps between measurements than when
i > 30 deg. This is because even large changes in azimuth do not
significantly change the phase angle when i is small. In essence, we are
making our measurements at approximately equal solid angles around the
scattering hemisphere.