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The Moon Mineralogy Mapper (M3) instrument is an imaging spectrometer. It generates images of moon surfaces in long narrow strips and spreads them like a rainbow in push-broom fashion. The acquired images are formed into a multi-colored map of the lunar surface. As the spacecraft orbits the moon, M3’s rectangular photodetector quickly records these narrow strip images in a 260-color spectrum. These images are stored in computer memory and transmitted back to earth by radio several times per day. The data is processed and the images are combined to form a map aggregate of the lunar surface.
Sunlight that is reflected from the lunar surface enters the M3 instrument and is imaged by a three mirror telescope (fore-optic). A slit at the entrance to the spectrometer keeps other light out, so the spectrometer can only see this part of the lunar surface. The spectrometer disperses the blue to near infrared light (wavelengths of 400 to 3000 nanometers) onto 260 rows of the detector. This forms 260 images of the ground in a gradient of colors.
M3 sees a 24 degree field-of-view and can make an image that is 40 kilometers wide on the moon’s surface. This is imaged onto 600 detector pixels, with each pixel representing 67 meters on the surface. The circumference of the moon is 10,930 kilometers. With overlap, it takes more than 274 image swaths to completely map the moon.
The spectrometer optics were designed to be small and light weight. They were also designed to produce no distortion in either spatial or spectral direction. That means the color sample is the same for all pixels in a row and the ground image is the same for all pixels in a column.
The M3 photodetector array comprises the Teledyne 6604A mercury-cadmium-telluride sensor chip assembly. A set of filters is placed directly in front of the detector to eliminate unwanted light that was introduced by the grating.
Due to the large volume of data that is generated by this imaging spectrometer, and the desire to make a full mineralogical map of the lunar surface, the instrument was designed to operate in two modes. Target Mode provides full resolution for specific targets of interest and Global Mode which averages 2 pixels in the spatial direction and 2 to 4 pixels in the spectral direction and reduces the amount of data by a factor of 12. The priority for operations was to first obtain the full lunar surface in Global Mode, then obtain the most interesting regions using Target Mode.
 
M3 Instrument - Flight Configuration Pictures
This is the rear view of the M3 instrument in Flight Configuration with Optical Bench Assembly (OBA) and Instrument Electronics Assembly (IEA) connected.
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Front, right view.
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Left, front view.
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Left side.
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Left, top view.
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Rear, left view.
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Rear, right view.
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Top view.
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Links:
http://aviris.jpl.nasa.gov/
Home page for AVIRIS, an imaging spectrometer similar to M3.
http://speclab.cr.usgs.gov/aboutimsp.html
USGS tutorial on imaging spectroscopy, citing AVIRIS as the premier example currently in use.
http://rst.gsfc.nasa.gov/Sect13/Sect13_9.html
AVIRIS and other Imaging Spectrometers (from The Remote Sensing Tutorial).
Optics for Compact, High-Performance Imaging Spectrometers [ PDF, 191 kB ]
Article on Offner design.
Low-Distortion Imaging Spectrometers [ PDF, 124 kB ]
Article on Offner design combined with "pushbroom" method.
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