Anticipated Performance And Future Plans

In this section, we estimate an expected count of zodiacal light observed by WIZARD based on the table provided in [6]. The table represents an annually averaged brightness

Figure 3. Side view of the CCD system.

with low spatial resolution. Kelsall et al. [2] reported a detail model of the zodiacal cloud based on COBE infrared observations. We assume the dust distribution of the visible zodiacal light is the same as that of the infrared emission, and employ this model as a visible zodiacal cloud model. We use a volume scattering phase function given by Hong [7], and adjust the absolute brightness referring to Levasseur-Regourd and Dumont [6]. As a result, we can simulate not only the absolute brightness in the visible band, but also the minute spatial structures, which are absent in the original table.

Taking into account the photon and read-out noise, we estimate an observed intensity, which includes the zodiacal light, by the new CCD system at Mauna Kea, Hawaii (4200 m). An optical depth for the diffuse light is assumed to be 0.10, and the brightness of the airglow and scattered light by the Earth's atmosphere are assumed to be 30 and 12 S100 respectively, at the zenith. These are the typical values deduced from our previous observations. The brightness of airglow and scattered light are extrapolated by the van Rhijn function and Dumont's formula (see e.g. [4]). Integrated star brightness is assumed to be 2O5io0, which is independent of the galactic coordinate. The observed time is set to the end of astronomical twilight (19:08 HST on December 22).

The dots in Figure 4 show the anticipated brightness of the night sky observed by the new CCD system with an exposure time of 5 min. It should be noted that these are the results of a simulation with 1.35'x 1.35' spatial resolution. In the case where the pixels are combined to give same resolution as IRAS ZOHF (20'x20'), the noise was reduced to ~ 1/V220 (lines in Figure 4). It is apparent that even a five minute exposure will allow us to detect asteroidal dust bands near the ecliptic plane (bump structures around f} ~ —1°) and the north-south asymmetry of zodiacal light brightness. This new observation system will also yield information on the brightness of airglow

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