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REQUIRED READING:
- Reuter et al. (2007) [REUTERETAL2007]
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      The LEISA description was was adapted from Reuter et al. (2007)
      [REUTERETAL2007], Reuter et al. (2005) [REUTERETAL2005] and the
      New Horizons website.
 
 
INSTRUMENT OVERVIEW
========
 
LEISA is a wedged filter infra-red spectral imager that operates in a
push-broom mode.
 
 
SPECIFICATIONS
--------
 
NAME:                    LEISA (Note 1)
DESCRIPTION:             Infrared Imager
PRINCIPAL INVESTIGATOR:  Alan Stern, SwRI
WAVELENGTH RANGE:        1250 - 2500 nm
FIELD OF VIEW:           15.9 x 15.9 mRad
ANGULAR RESOLUTION:      0.062 mRad
WAVELENGTH RESOLUTION:   6.5 nm/pixel (Note 2); 3.7 nm/pixel (Note 3)
 
Note 1:  Linear Etalon Imaging Spectral Array
Note 2:  Full spectral range
Note 3:  2100 - 2250 nm
 
 
DESCRIPTION
--------
 
LEISA operates at infrared wavelengths (it uses heat radiation), and its
etalon acts like a prism to bend different wavelengths of light by different
amounts so that each wavelength can be analyzed separately. Since different
molecules emit and absorb light at different wavelengths, analysis of the
different components of the light by LEISA can be used to identify the unique
'fingerprints' of these molecules. LEISA will be used to map the distribution
of frosts of methane (CH4), molecular nitrogen (N2), carbon monoxide (CO),
and water (H2O) over the surface of Pluto and the water frost distribution
over the surface of Charon. LEISA data may also reveal new constituents on
the surfaces that have not yet been detected.
 
 
Scientific Objectives
========
 
Hemispheric near-infrared spectral maps of Pluto and Charon at best
resolution exceeding 10 km/pixel
 
Hemispheric distributions of N2, CO, CH4 on Pluto at a best resolution
exceeding 10 km/pixel.
 
Surface temperature mapping of Pluto and Charon
 
hase-angle-dependent spectral maps of Pluto and Charon
 
 
 
Calibration
========
 
See Reuter et al. (2007) sections 5 &
 
 
Operational Considerations
========
 
LEISA images a scene through a wedged filter (linear variable filter,
Rosenberg et al., 1994) placed about 100 micrometers above a 256x256 pixel
Mercury Cadmium Telluride (HgCdTe) detector array (a PICNIC array). An image
is formed on both the wedged filter and the array simultaneously (there is
less than 5% spectral broadening by the f/8.7 beam). LEISA forms a spectral
map by scanning the FOV across the surface in a push broom fashion, similar
to that of the MVIC TDI channels. The frame rate is synchronized to the rate
of the scan, so that a frame is read out each time the image moves by the
single pixel IFOV. The LVF is fabricated such that the wavelength varies
along one dimension, the scan direction.
 
 
Detectors
========
 
The LEISA detector is a 1.2 to 2.5 micrometer HgCdTe PICNIC array, supplied
by Rockwell Scientific Corporation of Camarillo CA. The array is a 256x256
pixel array and each pixel is 40x40 micrometers^2 in area. However, several
modifications were made to the standard PICNIC array. The HgCdTe was grown on
a CdTe substrate using Molecular Beam Epitaxy (MBE) to provide good lattice
matching and low dark currents. The detector was bump bonded to a standard
PICNIC multiplexer and the resulting hybrid was mounted to molybdenum pad.
This process reduces mechanical stress induced during cooling to operational
temperature. It is estimated that the assembly can safely undergo at least
1000 thermal cycles.
 
See Reuter et al. (2007) for more detector details.
 
 
Electronics
========
 
See Reuter et al. (2007) section 4.0.
 
 
Filters
========
 
The filter, supplied by JDSU Uniphase/ Optical Coating Laboratories Inc. of
Santa Rosa CA, was made in two segments. The first, covering from 1.25 to 2.5
micrometers at a constant resolving power (constant delta-lambda/lambda) of
about 240, provides information primarily for surface composition mapping.
The second, covering from 2.1 to 2.25 micrometers at a constant resolving
power of about 560, uses temperature dependent changes in the spectral
structure of solid N2 near the alpha to beta transition at 35 K to provide
surface temperature maps. In both segments, a constant resolving power is
achieved by making the transmitted wavelength depend logarithmically on
position. The two segments were bonded together to form a single filter
element. This filter was, in turn, bonded into a holder and mounted such that
the filter surface is about 100 micrometers above the surface of the array.
The refractive index of the array is approximately 2.7 so that the total
optical path between the filter and photo-active area of the array is less
than 200 micrometers.  In this distance, the f/8.7 beam spreads about 0.5
pixel, so when the focus position is optimized between the array and filter
surface, the convolved image smear is about 0.04 pixel.
 
 
Optics
========
 
See Reuter et al. (2007) section 3.
 
 
Operational Modes
========
 
The array readout is performed in Read/Reset pairs. The accumulated charge is
read, then the CCD is reset and read immeadiately. After the integration time
has expired, the readout cycle is repeated. The difference between the Read
value and the previous Reset value is the charge accumulated durring the
integration time.  LEISA has two recording modes. In the Subtracted mode the
Reset value is subtracted from the Read value and the result is recorded. In
the Raw mode the Read and Reset values are both recorded.
 
[***N.B. We do not see the fourth axis appear in the single lrw_ image taken
so          far - Archive team]
 
              Prefix
              for FITS    Dimensions of
Mode          file        data FITS file
 
Subtracted    lsb_        (256 x 256 x N)
Raw           lrw_        (256 x 256 x N x 2)
 
N: number of image frames in the observation
 
 
Measured Parameters
========
 
Radiance.