DESCRIPTION |
Online Reference: http://wundow.wustl.edu/clbin/clsearch.pl
Instrument Overview
===================
The Near Infrared Camera (NIR) uses a catadioptric lens with a
256x256 indium antimonide (InSb) focal plane array (FPA)
mechanically cooled to cryogenic temperature. The FPA operated
at 70 K plus-or-minus 0.5 K at the Moon and showed excellent
stability over the more than 500 hours of operation in space.
The lens design features all ZnSe refractive elements with a
relay to provide an external pupil for 100% efficient cold
shielding. This lens design was chosen for image quality and
focus stability.
Wavelength range was constrained by the optics and the InSb
response to between 1.0 and 5.5 microns. Six wavebands were
selected by the NASA advisory committee, all falling well
inside this range.
Programmable camera electronics allow 4 integration times, 5
bits of gain, and 8 bits of offset. Gain states are spaced
approximately evenly from 0.5 to 36 factors of voltage
multiplication. Offset is subtracted before gain is applied,
with a range from 0 volts to full well that can be set in 1/255
full well increments.
Scientific Objectives
=====================
The primary scientific objective of the NIR was to support
lunar mineral mapping investigations. Pole-to-pole NAIDR
observations with solar phase angles kept to less than 30
degrees at mid-latitudes were the predominant viewing
conditions during the two month systematic mapping phase of the
mission. The UVVIS and NIR cameras provided 100% coverage of
the lunar surface under 11 spectral bands ranging in
wavelengths from 415 to 2690 nm. Image resolution ranges from
100 meters/pixel at periselene (-28 degrees south latitude
during first month observations, +28 degrees the second month)
to 400 meters/pixel at the poles.
Calibration
===========
The radiometric calibration converts the digitized signal
received from the camera (DN value) into a quantity that is
proportional to the radiance reaching the sensor. The
sensitivity of the CCD focal plane array varies across the
field of view. The instrument response is sensitive to the
temperatures of the FPA, optics, and cyrocooler. During
mission operations it was discovered that a sufficient
cryocooler cool-down period was needed before temperatures of
the instrument became stable. NIR images at the start of an
observational pass over the Moon just prior to turning on the
cryocooler may be difficult to calibrate due to temperature
instabilities of the instrument.
The NIR camera was calibrated before launch. Laboratory
observations of a flat field under various operating
temperatures and camera operation modes provide information
about the sensitivity of the camera under expected spaceflight
conditions. During inflight operations, a variety of
calibration observations were made including Apollo Landing
site observations where laboratory spectra of returned lunar
samples have been measured.
Geometric calibration removes optical distortions of the
imaging system. The geometric distortion of the NIR camera has
been shown to be minimal (maximum optical distortion does not
exceed 3.0 pixels) and can be satisfactorily modeled by a 2nd
order polynomial.
For additional information on the geometric and radiometric
calibration of the Clementine imaging systems, contact the PDS
Imaging Node.
Operational Considerations
==========================
The pole-to-pole lunar observations provided scenes with a
broad range of viewing conditions, ranging from bright
observations near zero phase angle at the equator to very low
light-level observations at the poles. In order to properly
record an observation with an optimal signal-to-noise ratio it
is important to adequately fill the 8-bit (255 levels) dynamic
range of the A/D camera output. The integration time (exposure
time) and the gain and offset operating modes of the instrument
were adjusted to properly record each image. Lunar
observations were broken into 10 latitude bins. Each latitude
bin contained fixed gain and offset modes and integration times
for each camera/filter combination.
The Clementine orbit was designed to provide overlapping
coverage in both the down-track (~15% overlap) and cross-track
(~10% overlap at the equator) directions. The image overlap is
necessary to geometrically control images in cartographic
applications.
Operational Modes
=================
The NIR camera had three operating modes:
1. Four selectable image integration times (11, 33, 57, 95 ms)
2. Gain Mode. The gain mode represents the multiplicative
constant applied to the image data passing through the A/D
converter. Thirty two (5 bit) gain state settings were
available.
3. Offset Mode. The offset mode represents the additive constant
applied to the image data passing through the A/D converter.
There were 256 (8 bit) offset mode settings.
Camera Specifications
=====================
Detectors
---------
Focal Plane Array
Type : Amber PV InSb
Pixel format : 256x256
Pixel size : 38x38 microns
Non-operable pixels : less than 0.5%
FPA operating temp. : 70 K
FPA well capacity : 11.7 million electrons
Field of view : 5.6 deg. x 5.6 deg.
Pixel IFOV : 400 x 400 microrad
Point spread : greater than 50% energy in 30
micrometer slit
Electronics
-----------
A/D resolution : 8 bits
Frame rate : 7.1 Hz (single frame mode)
Integration times : 11, 33, 57, and 95 ms
Digitization gain : 0.5 to 36 X voltage multiplication
Offset control : 8 bits
Power : 13.0 W
Filters
-------
Filter
Wheel Spectral
Position Band
---------------------------------------------
A : 1100 nm (plus-or-minus 30 nm)
B : 1250 nm (plus-or-minus 30 nm)
C : 1500 nm (plus-or-minus 30 nm)
D : 2000 nm (plus-or-minus 30 nm)
E : 2600 nm (plus-or-minus 30 nm)
F : 2690 nm (plus-or-minus 60 nm)
Optics
------
Clear aperture : 29nm
Effective focal length : 96 mm
Cold stop : F/3.33, 6.0 mm diameter
Cold shield efficiency : 100%
Mechanical
----------
Mass : 1920 grams
Size : 10.4 cm x 11.5 cm x 36.5 cm long
Subsystems
==========
Cryocooler
----------
Type : Ricor K506B integral Stirling with
H-10 FPA temperature closed-loop
control electronics
Avg. power : 11.0 W steady-state
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REFERENCES |
Priest, R.E., I.T. Lewis, N.R. Sewall, H. Park, M.J. Shannon, A.G.
Ledebuhr, L.D. Pleasance, M.A. Massie, and K. Metschuleit, Near-infrared
Camera for the Clementine Mission, Proceedings of the Society of
Photo-optical Instrumentation Engineers (SPIE), 2475, pp. 393-404, 1995.
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