![](media/image1.wmf){width="1.3284722222222223in"
height="1.0888888888888888in"}

Miniature Integrated Camera Spectrometer (MICAS) Validation Report

# Extended Abstract

MICAS is an integrated multi-channel instrument that includes: an
ultraviolet (UV) imaging spectrometer, two visible-wavelength imagers ,
and a short-wavelength infrared imaging (SWIR) spectrometer that was
flown as a technology demonstration on the New Millennium Deep Space 1
Mission (Table 1). MICAS has no moving parts, uses a shared
multi-wavelength long-focal length primary telescope and provides a
multi-temperature set of focal planes. with different remote-sensing
scales (i.e. radians/ mm). The MICAS optical layout is shown in Fig. 1.

*Background and History:* MICAS is a close derivative of PICS (Planetary
Integrated Camera Spectrometer) that was originally developed under NASA
PIDDP (Planetary Instrument Definition and Development Program) funding
in response to a Pluto Fast Flyby instrument feasibility demonstration
program (Beauchamp et. al. 1994). The program goal was to demonstrate
the practicality of an instrument package with a mass and power \<7kg
and \<7w that could meet the prime scientific objectives defined for the
Pluto Fast Flyby Mission. PICS and MICAS were developed by a consortium
of JPL, SSG, Univ. of Ariz., Rockwell, and the USGS. The additional
goal, self-imposed by this consortium, was that PICS would achieve the
sophisticated scientific capacity of similar instrument collections
flown on Voyager, Galileo, and Cassini (at 10 times mass, 10 times the
power, and 10 times the cost). More specifically this meant PICS
performance goals included, for instance, *high-resolution* imaging (as
on Mariners, Voyager, Galileo or Cassini; e.g.\~10rad IFOV) and
*imaging* spectroscopy in the short-wavelength infrared and ultraviolet,
comparable to NIMS (Near Infrared Mapping Spectrometer) on Galileo or
VIMS (Visible-Infrared Mapping Spectrometer) on Cassini.

PICS was developed, demonstrated, and tested to the level of a fully
functioning breadboard (Beauchamp, et. al. 1994). The most crucial new
technology key to PICS and MICAS is the monolithic SiC construction
(silicon carbide) used for the integrated structure, optical bench, and
optics. This material was essential to realize the very low mass and to
achieve combined thermal and optical stability requirements. This
material has 1) very high stiffness-to-mass ratio and the strength to
allow it to be thinned to reduce mass while retaining rigidity and
exceptional optical performance and 2) high thermal conductance and a
very low coefficient of thermal expansion giving very high thermal
stability. The monolithic SiC structure meant that the instrument would
expand or contract thermally as a unit and stay in alignment and focus
over wide temperature ranges; the low coefficient of expansion meant it
would expand or contract very little in the first place. This allowed 1)
design that could be assembled at room-temperature and would stay
aligned and in focus when cooled to cryogenic operating temperatures
(reducing the cost/complexity of integration) and 2) making optical
alignment immune to thermal gradients across an instrument with several
focal planes held at different temperatures.

To meet the exceedingly low power requirement, the PICS electronics
design that was developed consists of a simple single signal chain with
minimal controllability, serialized operation, and run at low
clock-rates to minimize power. State-of-the-art electronics packaging,
hybrids, and gate arrays were employed to minimize mass and size. The
PICS PIDDP effort also motivated (and partly funded) an improved
short-wavelength infrared detector, the Rockwell Science Center PICNIC
array (a refined NICMOS detector) that provided improved radiation
tolerance and lower read noise (c.f. Beauchamp, et. al. 1994).

An essential requirement that drove directly the PICS design was that it
be "self-sequenced" that by its very nature its operation would result
in a conflict-free timeline. This would save enormously on the
traditional science planning and sequence integration processes that are
such a costly and time-consuming component of planetary mission
operations. Traditionally the hour-or-so around closest approach is
jammed full of conflict because many investigations place their highest
priority demands on this period. The requirement for a conflict-free
timeline, combined with restrictions on PFF data rate and power, led to
an approach in which the major observations were serialized. Sharing the
long-focal-length multi-wavelength primary telescope among the cameras
and the imaging spectrometers led to a situation whereby the sequence of
prime observations was naturally spread out in time: UV emission
spectroscopy would be acquired days before closest approach (C/A); the
prime global SWIR map acquired \~ 1 hour before C/A; and the
high-quality, high-resolution global imaging centered at C/A.

*MICAS Technology Contribution:* MICAS as selected for technology flight
demonstration for the New Millennium (NMP) Deep Space 1 Mission (DS1)
was a direct derivation of PICS with only minimal design changes.
Several aspects of the instrument should have been substantially revised
to address the different target requirements, in particular, the
electronics were designed for a very different operating environment.
Severe DS1 budget and schedule constraints precluded addressing these
liens.

Table 1 shows the basic characteristics of MICAS. The four channels
include the single-octave UV and SWIR imaging spectrometers as they were
designed for PICS (although the UV grating was blazed for slightly
longer wavelength). Two new detectors were also key elements, the new
technology contribution: the Rockwell NICMOS-based PICNIC infrared
array, which as mentioned above, was developed under the original PICS
PIDDP program, and the JPL 256x256 Active Pixel Sensor (APS) imaging
array, a very low power (\~10mw) CMOS-based device in which
amplification and readout electronics are embedded within each pixel.

The principal MICAS technology (also the major NMP DS1 development
investment) is the integrated low-mass SiC multi-temperature,
multi-focal plane structure, optical bench, and multi-wavelength optics.
Using methods and processes developed by SSG, both hot-pressed SiC (for
the overall structure and optics) and composite SiC/SiC (for the SWIR
spectrometer structure) were incorporated. The optics was fabricated
from hot-pressed SiC forms that were cladded with silicon and then
diamond turned. The optical and thermal designs of MICAS have novel
elements as well. The SWIR focal plane uses a \"direct-drive\" radiative
cooler; the optical configuration using off-axis diamond-turned aspheres
and off-axis toroidal gratings enable higher through-put with improved
point spread function performance. The goal for multi-wavelength optical
performance (e.g. providing acceptable transmission from ultraviolet
through near infrared: \~50-5000 nm) was accomplished with a series
optical coatings: Pt for the primary mirror (best compromise for full
wavelength range UV-SWIR), SiC for the UV alone, Al for combined VIS and
SWIR, and Au for SWIR alone. The triangular structure with externally
mounted optical elements provided easy access to all focal planes.

*MICAS In-flight Testing:* In-flight testing of the full set of MICAS
channels was deferred multiple times during the first several months of
DS1 flight. Originally, a major component of the validation was to
observe the Earth-Moon system during the first few weeks after launch.
This was to provide the entire set of channels with extended targets
critical for many elements of the demonstration and validation; the loss
of this opportunity adversely affected the early in-flight
characterization of MICAS reducing its subsequent readiness for the
Braille encounter. The cancellation of the Earth-Moon sequence stemmed
from startup problems in getting the DS1 ion engine started and running
smoothly. Once the engine was operating properly, the decision was to
defer MICAS characterization even further in order to get the ion engine
lifetime operating test underway and to insure the mission performance
to reach the asteroid.

Early in the DS1 flight OPNAV data were collected with the VISCCD
channel of star fields and it became clear that MICAS was plagued by
stray light. The stray light was due to reflected sunlight entering the
aperture. This was caused by 1) location of MICAS on the DS1 S/C in a
position that allowed scattered sunlight to reach the back of the sun
shade and thereby into the aperture and 2) use of a reflective material
in fabricating the occultation port in the sun-shade (leaving a
reflective surface on its the inside edge). This stray light was
pervasive, severely affecting optical navigation.

*Validation Results:* Validation criteria for the MICAS integrated
multi-wavelength, multi-temperature, multi-focal-plane SiC structure and
optics include 1) ability to easily assemble and align at room
temperature, 2) retention of optical alignment as the instrument then
was cooled to cryogenic operating temperatures, 3) demonstrated stable
optical alignment in sustaining large temperature differences between
focal planes, 4) demonstrated optical throughput over full wavelength
range (ultraviolet through near infrared), and 5) maintain stable focal
plane operating temperatures.

Thermal stability of the MICAS optical design was solidly demonstrated
in pre-flight thermal vacuum tests. The system was aligned at room
temperate and cycled down to cryogenic instrument operating temperatures
through multiple cycles. The optical system exhibited no measurable
change in alignment over these cycles; interferrometric measurement
showed optical stability down to a precision of 1/10 wave. In addition
the MICAS multi-wavelength performance was demonstrated in the
laboratory to provide the optical throughput over the full wavelength
range, ultraviolet through infrared.

Early during flight the MICAS ultraviolet channel detector failed. The
device continues to respond to external stimuli (e.g. stray-light) and
exhibits normal temperature-dependent characteristics, but the data are
scrambled\-\--pixels sheared, shifted, and summed. Tests by reading out
the MICAS buffer have isolated the cause to the data scrambling to the
detector itself. Currently the best candidate failure mode is a
breakdown in a gate in the serial output circuit of the UV
frame-transfer CCD.

Inflight tests of VISCCD, the APS, and the SWIR further validated the
overall optical, thermal, and multi-wavelength performance of the
integrated MICAS SiC optical/structural system. One of the best examples
is shown in Figures 2 and 3. Early in November 1999 a series of APS and
VISCCD images were acquired of Mars. Mars was at a range of roughly
5.5x10^7^ km and subtended about 7 pixels in the APS and 9 in the
VISCCD. Because they were relatively short exposures they provide
excellent data to look at the point spread function (PSF) for both
channels. Figures 2 and 3 show the results of such a analysis for the
APS camera. The blue diamonds of Figure 2 represent the APS image
profile across Mars (roughly equatorial from terminator to bright limb);
the phase angle was \~52.8. A photometric model for Mars was convolved
with a Gaussian PSF function; the resultant fit is shown as the red
dashed line in Figure 3. The model PSF derived in this process is shown
in Figure 2. The full width at half maximum (FWHM) of this PSF is \~1.5
pixels consistent with the original design for the optical system for
this channel. Similar images and analyses acquired for the VISCCD for
the same November 1999 Mars sequence shows PSF with a FWHM \~2 pixels
consistent with the IFOV ratios of the two channels (\~1.5:1). These are
quite sensitive measures of optical stability and shows the optical
system to have stayed in alignment and to deliver design performance in
flight.

In addition MICAS observations of Mars can be used for inflight
validation of the SWIR imaging spectrometer (demonstrating
spectro-optical, detector, focal plane, and thermal performance). This
analysis shows the end-to-end performance of the SWIR was also
successfully validated. During May of 1999 three observations were made
of Mars with the MICAS SWIR channel from a ranges of between 105 and 110
million kilometers at a phase angle of \~50 degrees. Two exposure times
were used (0.442s and 1.454s the second of which saturated at shorter
wavelength but provided better SNR at greater than 2m). These
observations were taken at three separate longitudes; all are dominated
by bright regions. Mars was about 1 SWIR pixel (SWIR IFOV is 54radian).
Fig. 4 shows all five of these calibrated and reduced spectra along with
a theoretical model of the transmittance of CO~2~ computed by D. Crisp
(1990). The agreement of the MICAS observations with the theoretical
CO~2~ transmittance is remarkable. The quality of the MICAS spectra in
the 1.4 to 1.9 m range far surpasses any observations to date, either
ground-based or space-based. Two groups of features at \~ 1.74 m and at
\~1.87 m in Fig. 4 are candidates for new surface features. Features due
to sulfates this spectral range were recently suggested. Calvin .et.
al., 1999 suggested a gypsum band might be visible in the Phobos ISM
spectra at about 1.75m and Bishop et. al. 1999 suggested that jarosite
might produce a feature in the 1.85m region. These are by no means
suggested to be definitive identifications for the MICAS features but
they do suggest that reflectance imaging spectroscopy as a viable
scientific tool that should be flown to Mars in the next decade. They
also constitute demonstrable validation of the overall end-to-end
performance of the MICAS SWIR channel.

\[1\] P.M. Beauchamp et. al. (1994) AIAA Conference Utah; \[2\] D. Crisp
(1990) JGR 95 pp. 14577-14588, \[3\] W. M Calvin et. al. 1999 5th Intl
Conf. on Mars pp. 6125-6126, \[4\] J. L. Bishop (1999) 5th Intl Conf. on
Mars pp. 6220-6221.

lsoderblom@usgs.gov

lsoderblom@gscamnlh01.wr.usgs.gov

phone: 520-556-7018

address

U. S. Geological Survey

Flagstaff Field Center

2255 North Gemini Drive

Flagstaff, AZ 86002

department: GEO-WRG-AST

Dr. Bill Sandel

Bill R. Sandel

name: sandel bill r

alias: bill-sandel

email: bill-sandel@ns.arizona.edu (sandel@arizona.edu)

office_phone: (520) 621-4305

fax: (520) 621-8364

office_address: gould-simpson: room 911a

department: lunar and planetary laboratory