Development of ground-based lunar VIS/NIR spectral imager

In order to obtain 3dimentional spectral images of mare on the nearside of the moon, we have developed a ground based Lunar Spectral Imager. It focuses on the absorption band of 950 nm. The resolutions of the space and the wavelength are 9 km and 10 nm (VIS), and 20 km and 20 nm (IR). Introduction: The spectral analysis of mare materials is essential in order to clarify the lunar resurfacing process, in particular, the relationship between the distribution of volcanic emplacements and origins of the magma [1, 2]. In order to obtain 3dimentional spectral images of mare on the nearside of the moon, we have developed a ground based Lunar Spectral Imager. The Lunar Spectral Imager focuses on the absorption band around 950 nm, and the effective wave length is between 600 and 1600nm. The band is characterized by the iron contents of mare soils, which indicate the classification of basaltic rocks of mare lava [3, 4]. Specification of Lunar Spectral Imager: Fig. 1 shows the entire system of our ground-based Lunar VIS/Near IR Spectral imager. It is consisted of a Newton telescope (F5, focal length of 150 cm, Takahashi MT-300), a spectrometer (Genesia co. grating 200 g/mm), a visible camera (SBIG ST-7E, Si area sensor), a near infrared camera (SU Inc, SU320-1.7RT-D, InGaAs area sensor), and a viewing camera (Genesia co. based on Phillips co. ToUCam-Pro). Lights collected by the telescope focused on a slit mirror. Lights reflected by the mirror, which indicate the lunar surface except the position of spectral observation, are focused on the viewing camera. Lights passing through the slit are dispersed by the spectrometer, and then split into visible and near infrared by a dichroic mirror. The split lights are focused on the VIS camera and the near-IR camera, respectively. The visible image data can be obtained simultaneously with the infrared image data. Therefore, it is easy to correct the spectral data, including the atmospheric conditions, the phase angles of the lunar surface, and so on. Fig.1. Entire system of the Ground-based lunar VIS/near IR spectral imager. Purple, red, and green boxes are VIS camera, IR camera, and spectrometer, respectively. Yellow green pipe is a telescope. A viewing camera is not shown. Detailed specifications of each subsystem are summarized in table 1. The reflective mirrors of the telescope are coated by MgF2 in order to avoid the low reflectivity of the wavelength of 900 nm. The equatorial mounting of the telescope (Takahashi EM500 Tenma PC) is controlled remotely while confirming the observed regions using images of viewing camera. PC camera is utilized for the viewing camera in order to reduce the cost and the weight (< 100 g) of the entire imager. Spectral data and spatial data are recorded in each line and column of area sensors of VIS and IR cameras, respectively. Using viewing camera and remote control systems of the telescope, the imaging positions are shifted and then, 2dimentional spatial images and spectral data of each position can be obtained. CCD cameras can be on the markets and less expensive than the originally developed ones. Moreover, the total weight except the telescope and control PCs is within 4 kg. Thus, our imager is designed for the case of limited weight and cost. Lunar and Planetary Science XXXV (2004) 1595.pdf