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II Observations of the Zodiacal Light

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CCD imaging of the zodiacal light T. Mukai3 and M. Ishiguroa aGraduate School of Science and Technology, Kobe University, Nada, Kobe 657-8501, Japan

We review recent developments in ground-based observations of the zodiacal light at visible wavelengths. These developments are largely due to the introduction of refrigerated charge coupled device (CCD) detectors. CCD images show not only the global structure of the zodiacal cloud, e.g. its symmetry plane, but also faint structures, such as dust bands and dust trails with a brightness of a few 5io®, which were previously only seen in satellite data. We also briefly mention CCD spectroscopy of the zodiacal light, and emphasize the importance of laboratory calibration of the detector system, and application of state-of-the-art data reduction methods in the analysis of such faint and diffuse objects.


The IRAS, and later COBE, satellites initiated a new era in the zodiacal cloud sciences, starting in the 1980s (e.g. [1], [2], [3]). We have learned that the zodiacal cloud has a complex structure, including localized enhancement of interplanetary dust particles (IDPs) associated with asteroid families (dust bands), along cometary orbits (dust trails), and near planets (resonance rings or circumsolar rings). All of these new findings have come from satellite observations at infrared wavelengths. Since the late 1990s, new detector systems, such as cooled charge coupled device (CCD) cameras, have made it possible to detect faint structure in the zodiacal cloud at visible wavelengths from ground-based observation (e.g. [4], [5]). The resulting CCD images taken at high altitude sites have enabled us to study the structure of the zodiacal cloud in detail.

In this review, we will first mention the application of CCD detectors to the spectroscopy of the zodiacal light (ZL) in order to examine the dynamical behaviour of IDPs; we will then summarize recent results from CCD photometry of the ZL, including the Gegenschein.


Doppler shifts observed in sunlight scattered by IDPs (ZL) have been used to examine the dynamical behaviour of IDPs. Due to the low surface-brightness of the ZL, it is difficult to measure the Doppler shifts reliably. Two different experimental strategies have been adopted in previous work, as noted in [6], namely using: (i) a scanning Fabry-Perot interferometer to measure the profiles of Fraunhofer lines in the ZL (e.g. [7]), and (ii) a correlation mask radial velocity spectrometer to measure the line median or centre of gravity (e.g. [8]). It is demonstrated[6] that the line profiles in ZL spectra are important diagnostics of the dynamical behaviour of the IDPs. Consequently, they recommended observations "exploiting the optical advantages of the Fabry-Perot instrument in association with the 2D/high quantum efficiency of the charge coupled device (CCD) detector".

CCD spectroscopy for Doppler shift measurements was performed in 1995 at Mt. Haleakala (altitude 3000 m, Hawaii) using the Fabry-Perot etalon spectrometer developed by [9], with a CCD camera cooled to the temperature of liquid nitrogen (see [10]). Doppler shifts of the Mgll line at 5183.6 A in the morning ZL were detected. Although the resulting data, as shown in Figure 1, cited from [10], have a rather large scatter, those at a solar elongation angle, e, smaller than 40° confirm the previous conclusion that IDPs have prograde orbits, i.e. the Doppler shifts in the morning ZL occur on the red side. The error bar in the Doppler shifts is estimated as about ±0.lA in Figure 1, while that in e is ±2°. The Doppler shift values detected however, seem to be different from those reported by [8], and the data for e>40° appear on the violet side. No reasonable interpretation of these data has yet been presented.

Solar Elongation "Angle e (degree)

Figure 1. Doppler shifts of the zodiacal light as a function of solar elongation angle. The observed results are cited from [10] (morning ZL) and [8] (average of the morning and evening ZL). The computed results are cited from [6] for the morning ZL, and for a single grain in a circular Keplerian orbit.

Solar Elongation "Angle e (degree)

Figure 1. Doppler shifts of the zodiacal light as a function of solar elongation angle. The observed results are cited from [10] (morning ZL) and [8] (average of the morning and evening ZL). The computed results are cited from [6] for the morning ZL, and for a single grain in a circular Keplerian orbit.

It is generally realized that it is not easy to estimate the dynamical behaviour of IDPs from observed Doppler shifts. Each observed spectrum results from the combined scattering of sunlight by myriads of dust particles along a line of sight. These dust particles have different radial velocities relative to the observer, and also relative to the Sun. Con sequently, we detect a mixture of the Doppler shifts caused by different particles located at different heliocentric distances. Thus, the observed shifts are complex functions of the number density and spatial density of the IDPs, as well as of their scattering properties.

It was anticipated that CCD spectroscopy of the ZL might yield significant data on line profiles in the ZL spectra, allowing this complex situation to be disentangled. However, the spectra are extremely faint, especially at larger e, as shown by [10], and consequently very noisy, which prevents the line profiles from being studied in any detail. To measure Doppler shifts in the ZL, substantial improvement will be required in the next generation of CCD spectroscopic instruments.

3. CCD Photometry of the Zodiacal Light

To obtain a reliable 'snapshot' of faint and diffuse objects with a cooled CCD camera, we have to allow for many problems with the stability of the detection system, i.e. the linearity of the sensitivity of the CCD chips in the weak intensity region, dark current, read-out noise, and flat fielding (see e.g. [5], [11]).

Wavelength [A]

Wavelength [A]

Figure 2. The transmission spectrum of the filter used from the 1998 observations (dotted curve), where the emission spectrum of the airglow came from the CFHT Observers' Manual for the average zenith sky brightness.

Previously, we used an illuminated sheet of white acrylic plastic and/or a milk tank to take flat field images (reference frames). Although this method provided reasonable reference frames, the introduction of reference frames taken inside an integrating sphere has dramatically improved the quality of reference frames for flat fielding. The integrating sphere can supply uniform radiation with a controlled intensity to the CCD camera of interest, where the error in the absolute flux of the calibration radiation is less than 5%. This technique, coupled with applying a proper filter to reduce the contamination of airglow emission (see Figure 2), was the key improvement that led to success in finding the localized faint structures in the zodiacal cloud that are described below.

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