To further check for internal consistency in our reduction methodology, we compared ZL brightness maps at 5080A and 5300A and found excellent agreement between the two independently-reduced data sets. Therefore, we decided to combine the two data sets and produced in Figure 1 the merged isophotal contour map of ZL over the sky. This distribution clearly shows asymmetries between the morning (A — Aa > 180°) and evening (A — Aa < 180°) regions and between the north and south regions of the ecliptic. The distribution also shows small scale structure not seen in some earlier studies [e.g. 10].
We have also calculated ZL brightness using 3-dimensional optical models of zodiacal cloud . By comparing the overall morphology of the isophotal contours of the observed and calculated ZL distributions, we found the symmetry plane inclination i to be ~ 2° and the longitude of the ascending node fi to be ~ 80°. As seen in Figure 2, use of these symmetry plane parameters with a cosine model shows excellent agreement with the observed distribution, reproducing the observed southward shifts of the peak Gegenschein and of the morning side ZL cone.
A new reduction methodology for photometric observations of the night sky radiation is summarized and illustrated, a methodology with the following advantages: 1) Since all calibration parameters are determined from the same set of observations, an internal consistency is assured. This also reduces errors in the resulting brightnesses to about 10% or less. 2) Calculations using the QDM code enable us to determine the zenith distance dependence of the ADL brightness, even near the horizon. This permits the use of near-horizon observations without the heretofore very large uncertainties associated with the reduction of such observations.
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