| DESCRIPTION |
INSTRUMENT: INFRARED THERMAL MAPPER
SPACECRAFT: VIKING ORBITER 1
Instrument Information
======================
Instrument Id : IRTM
Instrument Host Id : VO1
Pi Pds User Id : HKIEFFER
Instrument Name : UNK
Instrument Type : RADIOMETER
Build Date : 1974
Instrument Mass : 8.400000
Instrument Length : 0.330000
Instrument Width : 0.267000
Instrument Height : 0.178000
Instrument Serial Number : 003
Instrument Manufacturer Name : SANTA BARBARA RESEARCH CENTER
Instrument Description
======================
The IRTM contains four small Cassegrainian telescopes which
each image the same, seven circular areas. There is a total of
twenty-eight channels in four surface and one atmospheric
thermal band from 6 micrometer to 30 micrometer and a broad
solar reflectance band. All channels are sampled
simultaneously, using the spacecraft scanning capability to map
the radiance over small and large areas of the planet. All
channels use thermopile detectors; spectral passbands are
determined by a combination of interference filters, detector
lens materials, antireflection coatings, and reststrahlen
optics. The scan modes are described in the PDS INSTRUMENT
MODE DESCRIPTION.
Science Objectives
==================
The objective of the Viking Orbiter infrared thermal mapper
(IRTM) is to measure the thermal emission of the Martian
surface and atmosphere and total reflected sunlight with high
spatial and flux resolution.
Operational Considerations
==========================
Most low and moderate resolution IRTM data were acquired
through using 'box scans'. These were commonly acquired
between 1-6 hours from periapsis, and utilized the scan
platform to slew back and forth in cone angle (in the direction
the IRTM chevron points) with small offsets in the same
direction between these oscillating slews. Ignoring spacecraft
motion, this pattern would generate bi-directional evenly
spaced scans with the seven IRTM detectors. Spacecraft motion
during the scan sequence, typically of 10-40 minutes duration,
created some distortion in this otherwise uniform pattern.
Typical resulting scans across the planetary surface are shown
in Figure 3 of Kieffer et al., 1976. These scans were usually
designed to extend slightly off the limb of the planet on at
least one side. These 'planet port' off-planet data provided
the best estimates of the zero radiance response of the
instrument. When the spacecraft was near periapsis, the
apparent motion of the planetary surface relative to the
spacecraft was too rapid to allow oscillating slews. At these
times, the instrument would simply 'stare' in one direction and
use the spacecraft motion to sweep the detector pattern across
the surface. These observations were usually acquired in
Normal Mode, but occasionally Fixed Planet was used. At
irregular times through the mission, 'phase function'
observations were made. These involved using the two axis scan
platform to follow one point on the ground as the spacecraft
went from horizon to horizon relative to this surface point.
In actuality, this sequence was acquired using a small number
of discrete scan platform moves, allowing the instrument to
'stare' across a short stripe centered on the target point
between slews. Such 'phase function' observations typically
yielded about 10 different viewing geometries within a single
sequence. These observations were particularly useful in
determining the influence of the atmosphere. In preparation of
the IRTM data set, all observations which were more than 1 1/2
degrees apparent angle above the nearest limb of the planet
were deleted.
Geometry Errors Due to Uncertain Timing: Early and late during
the Viking mission, orbital solutions based on the tracking
telemetry were determined every few days. During VO-1
revolution 175-603 and VO-2 revolutions 118-521, orbital
solutions were often separated by a week or more. Because
there is significant irregularity in the Martian gravitional
field, these irregularities could slowly influence the orbit of
the Viking spacecraft in unpredictable ways. The primary
influence was in the period of the orbit, resulting in
uncertainty as to exactly where the spacecraft was along its
orbit at any specific time. These uncertainties were as large
as 75 seconds in the worst case. Far from periapsis, these
timing uncertainties were not of major significance because the
spacecraft velocities were low and the projected fields of
views on the planet were large. However, near periapsis, the
IRTM field of view could move across the surface equivalent to
its full width in as little as one second. Thus, when there
was a large timing error, the computed ground intercept
locations could be in error by many fields of view. In the
worst case, these positions may be in error by up to 200 km.
When the magnitude of this problem was discovered, the SEDR
(geometry calculations) for the imaging instrument was rerun
with revised orbit solutions. However, it was impractical to
regenerate the IRTM SEDR and these errors have not been
corrected. There was an attempt by the navigational team to
estimate the magnitude of the timing error for both Viking
spacecraft for those revolutions through the affected part of
the Viking mission. This is described in the 1980 April 14
memo by Frank Palluconi, which contains estimates of the
magnitude of the error for each revolution. Hugh Kieffer has a
copy (the sole surviving copy?) of this memo. A direct
determination of the timing offset can be made from the IRTM
data alone in those instances when thermal patterns can be
unambiguously identified with surface features. Since the
dominant geometric error is in time, maps of thermal patterns
(typically as contours of observed temperature minus the
calculated standard model temperature) can be slid across the
cartographic map parallel to the subspacecraft track (if the
instrument was in fixed planet mode, this is simply sliding the
IRTM trace along its own path) until the thermal and
cartographic features are aligned. Because there are small
gaps in the IRTM coverage every 64 ICKs, the amount the IRTM
pattern must be shifted to agree with the surface morphology
can be scaled directly into a timing offset in seconds. This
has been done for a variety of high resolution scans across
Arsia Mons (by Jim Zimbelman) and for many scans across Valles
Marineris (by David Paige and Hugh Kieffer). A set of known
offsets is slowly accumulating. Hugh Kieffer has a copy (the
sole surviving copy?) of the memo discussing this problem.
Calibration Description
=======================
Relative spectral response of all channels was measured end to
end using a Perkin-Elmer 16 U monochrometer with appropriate
gratings and order filters. A globar at 1400 K was used in the
2-25 micrometer range; shortward of 2.0 micrometers a tungsten
source at 2700 K was used. The reference detector was a
thermocouple for all but the 0.4-1.1 micrometer range, where a
calibrated silicon photodiode was used. Out-of-band
measurements were made by replacing the spectrometer grating
with a plane mirror and ir materials having known cutoff and
cuton wavelengths. Flux calibration of the IRTM was performed
under a simulated space environment using a vacuum chamber
operated typically at a pressure of 1.E-6 Torr. The IRTM was
operated by means of a console which simulated the interfaces
and functions of the spacecraft FDS. A minicomputer was used
to provide all operational sequences and modes. Data were
recorded on magnetic tape for subsequent computer processing.
The calibration fixture consisted of two identical blackbodies,
one located in front of the space port and maintained at liquid
nitrogen temperature and the other in front of the planet port
and adjustable in temperature from 77 K to 350K; eleven
settings from 140 K to 330 K were used. Blackbody temperatures
were measured with platinum resistance thermometers having an
absolute accuracy of +/- 0.1 degrees C. traceable to the
National Bureau of Standards. The digitizer used in the test
console provided ten times the resolution of the FDS digitizer,
thus making the digitizing uncertainty during calibration
insignificant compared to the noise. The calibration data thus
produced are IRTM output in digitization level (DN) as a
function of blackbody temperature. Radiometrically measured
internal reference surface temperatures showed close agreement
(+/- 0.5 degrees C.) with those measured independently with a
thermister. The IRTM temperature was controlled by regulating
the temperature of a mounting base plate and the thermal shield
inside the vacuum chamber. Calibration was performed at 10
degrees C. spacing across the range of operating temperatures
expected during flight. Typical IRTM channel response to scene
brightness temperatures is shown in Fig. 8 of CHASE_ETAL_1978.
The one-sample noise on the thermal channels is less than 1 DN
except for the 15 micrometer channel where it is about 2.5 DN.
The dynamic ranges of the surface thermal bands are based on
temperatures expected for the Martian surface. The 300 K
maximum chosen for the A telescope might be exceeded by midday
summer temperatures, but temperatures above the 310-K limit of
the B telescope should not be exceeded unless active volcanic
areas were found; temperatures to 320 K and 330 K could be
measured by the 9 micrometer and 7 micrometer bands. The 15
micrometer band dynamic range was set quite large as its
resolution is noise limited rather than digitization limited.
Telescope D channels were calibrated using a different method.
The radiance source was a mercury-xenon lamp and narrowband
filter centered at 0.896 micrometer with a bandwidth of 425 nm.
The in-band radiance of the lamp was known by direct comparison
with a standard lamp acquired from the National Bureau of
Standards, using a silicon photodiode as a transfer standard.
The relative spectral response measurements then allowed
extension of the one-point absolute calibration to the entire
passband. Gains for the D channels were set to give full scale
for 75% of the diffuse reflection of solar irradiance at Mars
average distance from the sun. Using integrals of the Planck
function and the measured spectral response, the flux response
of the IRTM is found to be close to linear in the thermal
channels. The best fit quadratic functions, normalized to full
scale, typically had constant and quadratic coefficients of
0.002 and 0.02, respectively. The solar band channels, which
had much higher absolute flux levels at full scale, showed a
decrease in response at high signal levels corresponding to a
quadratic coefficient of 0.07. With the IRTM in the vacuum
chamber, the instrument response was measured at four lamp
currents. An additional series of wide band measurements
utilizing a NBS standard lamp and a barium sulfate diffusing
screen, in which only the lamp-screen distance was changed, was
used to determine in detail the solar band nonlinearity.
During spacecraft thermal-vacuum testing and in flight, a small
drift of about 1-min duration was found to be induced when the
scan mirror moved to the reference position in normal mode.
This appears to be caused by the decrease in radiative heat
loss from the instrument when the telescopes do not view space.
The shape of this postreference drift was accurately determined
during normal mode sequences when the spacecraft was well away
from Mars, and this effect is removed in the data reduction.
The change of the thermal state of the IRTM caused by large
scan platform slew or planetary radiation near periapsis can
introduce significant drifts of the zero-flux level. These
shifts have a time constant of 1-2 min or longer, and their
magnitude increases with inband wavelength and preamplifier
gain. It is probably due primarily to very small temperature
gradients induced in the detector packages as the general
instrument temperature changes. A significant design feature
of the IRTM is that the space DN level of each channel is
measured immediately prior to and after the restore which
occurs each minute in normal mode. A linear interpolation
between these zero-flux DN levels is used in data
decalibration. The remaining quadratic and higher order drift
is generally negligible.
Section 'IRTM'
==============
Total Fovs : 7
Data Rate : 250.000000
Sample Bits : 10
'IRTM' Detectors
----------------
A
'IRTM' Electronics
------------------
IRTM
'IRTM' Filters
--------------
SOLAR UV-22
T11
T15
T20
T7
T9
'IRTM' Section Optic IDs
------------------------
A
B
C
D
In modes
--------
FIXED PLANET
FIXED REFERENCE
FIXED SPACE
MODIFIED NORMAL
NORMAL
'IRTM' Section FOV Shape 'CIRCULAR'
-----------------------------------
Section Id : IRTM
Fovs : 7
Horizontal Pixel Fov : 0.292000
Vertical Pixel Fov : 0.292000
Horizontal Fov : 2.402000
Vertical Fov : 1.347000
'IRTM' Section Parameter 'RADIANCE A'
-------------------------------------
The A telescope measured wavelengths between 16 and 30
microns. This parameter is the radiance in that band.
Instrument Parameter Name : RADIANCE A
Sampling Parameter Name : TIME
Instrument Parameter Unit : WATT_METER**-2_MICROMETER**-1
Minimum Instrument Parameter : 0.000000
Maximum Instrument Parameter : 0.002030
Noise Level : 0.000001
Sampling Parameter Interval : 1.120000
Sampling Parameter Resolution : 1.120000
Sampling Parameter Unit : SECOND
'IRTM' Section Parameter 'RADIANCE B'
-------------------------------------
The B telescope measured wavelengths between 10 and 13
microns. This parameter is the radiance in that band.
Instrument Parameter Name : RADIANCE B
Sampling Parameter Name : TIME
Instrument Parameter Unit : WATT_METER**-2_MICROMETER**-1
Minimum Instrument Parameter : 0.000000
Maximum Instrument Parameter : 0.003120
Noise Level : 0.000001
Sampling Parameter Interval : 1.120000
Sampling Parameter Resolution : 1.120000
Sampling Parameter Unit : SECOND
'IRTM' Section Parameter 'RADIANCE C1'
--------------------------------------
The C1 set of 3 detectors (out of 7 in the C telescope) were
limited by filtering to wavelengths between 6 and 8 microns.
This parameter is the radiance in that band.
Instrument Parameter Name : RADIANCE C1
Sampling Parameter Name : TIME
Instrument Parameter Unit : WATT_METER**-2_MICROMETER**-1
Minimum Instrument Parameter : 0.000000
Maximum Instrument Parameter : 0.003190
Noise Level : 0.000001
Sampling Parameter Interval : 1.120000
Sampling Parameter Resolution : 1.120000
Sampling Parameter Unit : SECOND
'IRTM' Section Parameter 'RADIANCE C2'
--------------------------------------
The C2 set of 3 detectors (out of 7 in the C telescope) were
limited by filtering to wavelengths between 8 and 10 microns.
This parameter is the radiance in that band.
Instrument Parameter Name : RADIANCE C2
Sampling Parameter Name : TIME
Instrument Parameter Unit : WATT_METER**-2_MICROMETER**-1
Minimum Instrument Parameter : 0.000000
Maximum Instrument Parameter : 0.001980
Noise Level : 0.000001
Sampling Parameter Interval : 1.120000
Sampling Parameter Resolution : 1.120000
Sampling Parameter Unit : SECOND
'IRTM' Section Parameter 'RADIANCE C3'
--------------------------------------
The C3 detector/filter combination within the C telescope
responded to wavelengths between 14.5 and 15.5 microns. This
parameter is the radiance in that band.
Instrument Parameter Name : RADIANCE C3
Sampling Parameter Name : TIME
Instrument Parameter Unit : WATT_METER**-2_MICROMETER**-1
Minimum Instrument Parameter : 0.000000
Maximum Instrument Parameter : 0.006340
Noise Level : 0.000003
Sampling Parameter Interval : 1.120000
Sampling Parameter Resolution : 1.120000
Sampling Parameter Unit : SECOND
'IRTM' Section Parameter 'RADIANCE D'
-------------------------------------
The D telescope measured wavelengths between 0.3 and 3.0
microns. This parameter is the radiance in that band.
Instrument Parameter Name : RADIANCE D
Sampling Parameter Name : TIME
Instrument Parameter Unit : WATT_METER**-2_MICROMETER**-1
Minimum Instrument Parameter : 0.000000
Maximum Instrument Parameter : 135.840000
Noise Level : 0.030000
Sampling Parameter Interval : 1.120000
Sampling Parameter Resolution : 1.120000
Sampling Parameter Unit : SECOND
Instrument Detector 'A'
=======================
Detector Type : THERMOPILE ARRAY
Detector Aspect Ratio : 1.000000
Minimum Wavelength : 0.300000
Maximum Wavelength : 100.000000
Nominal Operating Temperature : 275.000000
Description
-----------
The seven-element thin-film antimony-bismuth thermopile array
used in the IRTM is shown in Fig. 7 of CHASE_ETAL_1978. The
chevron arrangement was based on the need for uniform
coverage irrespective of scan platform orientation; it also
allowed the detectors to all be approximately the same
distance from the telescope optic axis. In this application
thermopiles were found to be better than other thermal
detectors because they operate to dc and exhibit no 1/f
noise. Thus, no optical chopper is needed. Also, no bias
supply, another potential source of 1/f noise, is needed.
Cooled quantum detectors were not practical, considering the
duration and weight constraints of the Viking Mission. The
array was made by evaporating the various components onto a
sapphire film using photoetched masks for dimensional
control. The film, about 200 nm thick, is supported by a
sapphire disk. The film was made by anodizing aluminum foil
and etching away the aluminum. The black circular dots in
the figure are the sensitive areas overlaid with bismuth
mass. Characteristics
Active area 7.E-4 cm**2
Number of junctions 6
Resistance 13.E3 ohm
Time constant 80-100 msec
Responsivity 130 V/Watt
Detectivity (D*) 2.E8 cm_Hz**0.5_W**-1
To obtain full sensitivity the detectors must be evacuated.
Therefore, during ground testing the detector packages were
pumped down through a permanently attached manifold. At
other times the detector packages were backfilled with xenon
to protect the detectors while still allowing gross
sensitivity checks. To avoid exposure to moisture during the
long period prior to launch when the IRTM was mounted on the
spacecraft and could not be sealed, the manifold was kept at
a slight positive pressure by a continuous flow of high
purity nitrogen. The manifold was opened to space by launch
vehicle separation.
Sensitivity
-----------
The detectivity is 2.E8 CM_HZ**0.5_W**-1
Instrument Electronics 'IRTM'
=============================
Description
-----------
The signal channels use a synchronous demodulation scheme to
provide good stability and to avoid 1/f noise in the preamp.
The input FET chopper is a full-wave type operating at 471
Hz. This and the center-tapped thermopile allow voltage
doubling of the detector signal and noise and thus reduce the
preamp noise contribution which otherwise would be
significant. The differential input connection, while
suffering a square root (2) noise disadvantage compared to
single-ended input, provides excellent common mode rejection
of chopper spikes and other input noise. Temperature
dependence of the thermopile, about -0.5%/degree C., is
compensated by a thermister network external to the hybrid
package. Preamp gain is adjustable with an external
resistor. Following the half-wave synchronous demodulator is
an integrate, hold, and reset circuit with an integrate time
of 981 msec. The integrator serves as a low pass filter
while the hold feature ensures spatial simultaneity of
corresponding detectors in each telescope. After completion
of sampling by the multiplexer, all channel hold circuits are
reset to ensure independence of data samples. The IRTM
analog signals, which have a range of +/- 6V, are digitized
by the analog-to-pulse width converter and flight data
subsystem (FDS) counter into +/- 2**9 levels, yielding 1023
data numbers (DN) which are nearly linear with radiance in
each channel. The IRTM multiplexer consists of sixty-eight
FET switches and a buffer signal amplifier. In addition to
thirty-two data channels (twenty-eight active and four
spare), thirty-two channels of engineering data are also
sample. These include eight temperature measurements from
thermisters located at four locations on the reference plate,
the electronics module, and each of the three ir detector
packages (telescopes A, B, C). Three power supply voltages
and the pre-dc restore voltage of twenty-one channels
(telescopes A, B, and C) are monitored. The pre-dc restore
monitors are diagnostic to determine the presence of large
thermal or detector offsets. The scan mirror is driven by a
four-position stepper motor through a 50/1 gear reduction. A
motor drive pulse duration of 40 msec allows a 90 degree
mirror rotation in 2 sec. The mirror position is sensed by a
two-bit encoder on the motor shaft; the contacts at the three
desired positions are about half of the width of 1.8 degree
mirror step. The motor stepping is controlled by the FDS
using a comparison of the encoder readout with the desired
position originating either from the FDS normal mode clock or
direct ground command; the motor cannot be directly
commanded. In addition to the restore which occurs
automatically in the normal model when the mirror reaches the
space position, restores can be ground commanded when the
IRTM is in the fixed planet or fixed space mode; in either
case housekeeping data are multiplexed into the data stream
during the 1-sec restore period. Whenever the mirror reaches
the reference position, the calibration lamp is turned on for
the next two integration periods. The lamp is at full
radiance throughout the second integration period, which is
used for gain determination of the D telescope channels. In
the fixed reference mode, science and housekeeping data are
sampled alternatively.
Instrument Filter 'A - T20'
===========================
Filter Name : T20
Filter Type : RESTSTRAHLEN
Minimum Wavelength : 17.700000
Maximum Wavelength : 30.000000
Center Filter Wavelength : 21.000000
Description
-----------
Relative spectral response of all channels was measured end
to end using a Perkin-Elmer 16 U monochrometer with
appropriate gratings and order filters. A globar at 1400 K
was used in the 2-25 micrometer range; shortward of 2.0
micrometers a tungsten source at 2700 K was used. The
reference detector was a thermocouple for all but the 0.4-1.1
micrometer range, where a calibrated silicon photodiode was
used. Out-of-band measurements were made by replacing the
spectrometer grating with a plane mirror and ir materials
having known cutoff and cuton wavelengths. Flux calibration
of the IRTM was performed under a simulated space environment
using a vacuum chamber operated typically at a pressure of
1.E-6 Torr. The IRTM was operated by means of a console
which simulated the interfaces and functions of the
spacecraft FDS. A minicomputer was used to provide all
operational sequences and modes. Data were recorded on
magnetic tape for subsequent computer processing. The
calibration fixture consisted of two identical blackbodies,
one located in front of the space port and maintained at
liquid nitrogen temperature and the other in front of the
plant port and adjustable in temperature from 77 K to 350K;
eleven settings from 140 K to 330 K were used. Blackbody
temperatures were measured with platinum resistance
thermometers having an absolute accuracy of +/- 0.1 degrees
C. traceable to the National Bureau of Standards. The
digitizer used in the test console provided ten times the
resolution of the FDS digitizer, thus making the digitizing
uncertainty during calibration insignificant compared to the
noise. The calibration data thus produced are IRTM output in
digitization level (DN) as a function of blackbody
temperature. Radiometrically measured internal reference
surface temperatures showed close agreement (+/- 0.5 degrees
C.) with those measured independently with a thermister. The
IRTM temperature was controlled by regulating the temperature
of a mounting base plate and the thermal shield inside the
vacuum chamber. Calibration was performed at 10 degrees C.
spacing across the range of operating temperatures expected
during flight. Typical IRTM channel response to scene
brightness temperatures is shown in Fig. 8 of
CHASE_ETAL_1978. The one-sample noise on the thermal
channels is less than 1 DN except for the 15 micrometer
channel where it is about 2.5 DN. The dynamic ranges of the
surface thermal bands are based on temperatures expected for
the Martian surface. The 300 K maximum chosen for the A
telescope might be exceeded by midday summer temperatures,
but temperatures above the 310-K limit of the B telescope
should not be exceeded unless active volcanic areas were
found; temperatures to 320 K and 330 K could be measured by
the 9 micrometer and 7 micrometer bands. The 15 micrometer
band dynamic range was set quite large as its resolution is
noise limited rather than digitization limited.
Instrument Filter 'B - T11'
===========================
Filter Name : T11
Filter Type : MULTILAYER INTERFERENCE
Minimum Wavelength : 9.800000
Maximum Wavelength : 12.500000
Center Filter Wavelength : 11.200000
Description
-----------
Relative spectral response of all channels was measured end
to end using a Perkin-Elmer 16 U monochrometer with
appropriate gratings and order filters. A globar at 1400 K
was used in the 2-25 micrometer range; shortward of 2.0
micrometers a tungsten source at 2700 K was used. The
reference detector was a thermocouple for all but the 0.4-1.1
micrometer range, where a calibrated silicon photodiode was
used. Out-of-band measurements were made by replacing the
spectrometer grating with a plane mirror and ir materials
having known cutoff and cuton wavelengths. Flux calibration
of the IRTM was performed under a simulated space environment
using a vacuum chamber operated typically at a pressure of
1.E-6 Torr. The IRTM was operated by means of a console
which simulated the interfaces and functions of the
spacecraft FDS. A minicomputer was used to provide all
operational sequences and modes. Data were recorded on
magnetic tape for subsequent computer processing. The
calibration fixture consisted of two identical blackbodies,
one located in front of the space port and maintained at
liquid nitrogen temperature and the other in front of the
plant port and adjustable in temperature from 77 K to 350K;
eleven settings from 140 K to 330 K were used. Blackbody
temperatures were measured with platinum resistance
thermometers having an absolute accuracy of +/- 0.1 degrees
C. traceable to the National Bureau of Standards. The
digitizer used in the test console provided ten times the
resolution of the FDS digitizer, thus making the digitizing
uncertainty during calibration insignificant compared to the
noise. The calibration data thus produced are IRTM output in
digitization level (DN) as a function of blackbody
temperature. Radiometrically measured internal reference
surface temperatures showed close agreement (+/- 0.5 degrees
C.) with those measured independently with a thermister. The
IRTM temperature was controlled by regulating the temperature
of a mounting base plate and the thermal shield inside the
vacuum chamber. Calibration was performed at 10 degrees C.
spacing across the range of operating temperatures expected
during flight. Typical IRTM channel response to scene
brightness temperatures is shown in Fig. 8 of
CHASE_ETAL_1978. The one-sample noise on the thermal
channels is less than 1 DN except for the 15 micrometer
channel where it is about 2.5 DN. The dynamic ranges of the
surface thermal bands are based on temperatures expected for
the Martian surface. The 300 K maximum chosen for the A
telescope might be exceeded by midday summer temperatures,
but temperatures above the 310-K limit of the B telescope
should not be exceeded unless active volcanic areas were
found; temperatures to 320 K and 330 K could be measured by
the 9 micrometer and 7 micrometer bands. The 15 micrometer
band dynamic range was set quite large as its resolution is
noise limited rather than digitization limited.
Instrument Filter 'C1 - T7'
===========================
Filter Name : T7
Filter Type : MULTILAYER INTERFERENCE
Minimum Wavelength : 6.100000
Maximum Wavelength : 8.300000
Center Filter Wavelength : 7.200000
Description
-----------
Relative spectral response of all channels was measured end
to end using a Perkin-Elmer 16 U monochrometer with
appropriate gratings and order filters. A globar at 1400 K
was used in the 2-25 micrometer range; shortward of 2.0
micrometers a tungsten source at 2700 K was used. The
reference detector was a thermocouple for all but the 0.4-1.1
micrometer range, where a calibrated silicon photodiode was
used. Out-of-band measurements were made by replacing the
spectrometer grating with a plane mirror and ir materials
having known cutoff and cuton wavelengths. Flux calibration
of the IRTM was performed under a simulated space environment
using a vacuum chamber operated typically at a pressure of
1.E-6 Torr. The IRTM was operated by means of a console
which simulated the interfaces and functions of the
spacecraft FDS. A minicomputer was used to provide all
operational sequences and modes. Data were recorded on
magnetic tape for subsequent computer processing. The
calibration fixture consisted of two identical blackbodies,
one located in front of the space port and maintained at
liquid nitrogen temperature and the other in front of the
plant port and adjustable in temperature from 77 K to 350K;
eleven settings from 140 K to 330 K were used. Blackbody
temperatures were measured with platinum resistance
thermometers having an absolute accuracy of +/- 0.1 degrees
C. traceable to the National Bureau of Standards. The
digitizer used in the test console provided ten times the
resolution of the FDS digitizer, thus making the digitizing
uncertainty during calibration insignificant compared to the
noise. The calibration data thus produced are IRTM output in
digitization level (DN) as a function of blackbody
temperature. Radiometrically measured internal reference
surface temperatures showed close agreement (+/- 0.5 degrees
C.) with those measured independently with a thermister. The
IRTM temperature was controlled by regulating the temperature
of a mounting base plate and the thermal shield inside the
vacuum chamber. Calibration was performed at 10 degrees C.
spacing across the range of operating temperatures expected
during flight. Typical IRTM channel response to scene
brightness temperatures is shown in Fig. 8 of
CHASE_ETAL_1978. The one-sample noise on the thermal
channels is less than 1 DN except for the 15 micrometer
channel where it is about 2.5 DN. The dynamic ranges of the
surface thermal bands are based on temperatures expected for
the Martian surface. The 300 K maximum chosen for the A
telescope might be exceeded by midday summer temperatures,
but temperatures above the 310-K limit of the B telescope
should not be exceeded unless active volcanic areas were
found; temperatures to 320 K and 330 K could be measured by
the 9 micrometer and 7 micrometer bands. The 15 micrometer
band dynamic range was set quite large as its resolution is
noise limited rather than digitization limited.
Instrument Filter 'C2 - T9'
===========================
Filter Name : T9
Filter Type : MULTILAYER INTERFERENCE
Minimum Wavelength : 8.300000
Maximum Wavelength : 9.800000
Center Filter Wavelength : 9.000000
Description
-----------
Relative spectral response of all channels was measured end
to end using a Perkin-Elmer 16 U monochrometer with
appropriate gratings and order filters. A globar at 1400 K
was used in the 2-25 micrometer range; shortward of 2.0
micrometers a tungsten source at 2700 K was used. The
reference detector was a thermocouple for all but the 0.4-1.1
micrometer range, where a calibrated silicon photodiode was
used. Out-of-band measurements were made by replacing the
spectrometer grating with a plane mirror and ir materials
having known cutoff and cuton wavelengths. Flux calibration
of the IRTM was performed under a simulated space environment
using a vacuum chamber operated typically at a pressure of
1.E-6 Torr. The IRTM was operated by means of a console
which simulated the interfaces and functions of the
spacecraft FDS. A minicomputer was used to provide all
operational sequences and modes. Data were recorded on
magnetic tape for subsequent computer processing. The
calibration fixture consisted of two identical blackbodies,
one located in front of the space port and maintained at
liquid nitrogen temperature and the other in front of the
plant port and adjustable in temperature from 77 K to 350K;
eleven settings from 140 K to 330 K were used. Blackbody
temperatures were measured with platinum resistance
thermometers having an absolute accuracy of +/- 0.1 degrees
C. traceable to the National Bureau of Standards. The
digitizer used in the test console provided ten times the
resolution of the FDS digitizer, thus making the digitizing
uncertainty during calibration insignificant compared to the
noise. The calibration data thus produced are IRTM output in
digitization level (DN) as a function of blackbody
temperature. Radiometrically measured internal reference
surface temperatures showed close agreement (+/- 0.5 degrees
C.) with those measured independently with a thermister. The
IRTM temperature was controlled by regulating the temperature
of a mounting base plate and the thermal shield inside the
vacuum chamber. Calibration was performed at 10 degrees C.
spacing across the range of operating temperatures expected
during flight. Typical IRTM channel response to scene
brightness temperatures is shown in Fig. 8 of
CHASE_ETAL_1978. The one-sample noise on the thermal
channels is less than 1 DN except for the 15 micrometer
channel where it is about 2.5 DN. The dynamic ranges of the
surface thermal bands are based on temperatures expected for
the Martian surface. The 300 K maximum chosen for the A
telescope might be exceeded by midday summer temperatures,
but temperatures above the 310-K limit of the B telescope
should not be exceeded unless active volcanic areas were
found; temperatures to 320 K and 330 K could be measured by
the 9 micrometer and 7 micrometer bands. The 15 micrometer
band dynamic range was set quite large as its resolution is
noise limited rather than digitization limited.
Instrument Filter 'C3 - T15'
============================
Filter Name : T15
Filter Type : MULTILAYER INTERFERENCE
Minimum Wavelength : 14.560000
Maximum Wavelength : 15.410000
Center Filter Wavelength : 15.000000
Description
-----------
Relative spectral response of all channels was measured end
to end using a Perkin-Elmer 16 U monochrometer with
appropriate gratings and order filters. A globar at 1400 K
was used in the 2-25 micrometer range; shortward of 2.0
micrometers a tungsten source at 2700 K was used. The
reference detector was a thermocouple for all but the 0.4-1.1
micrometer range, where a calibrated silicon photodiode was
used. Out-of-band measurements were made by replacing the
spectrometer grating with a plane mirror and ir materials
having known cutoff and cuton wavelengths. Flux calibration
of the IRTM was performed under a simulated space environment
using a vacuum chamber operated typically at a pressure of
1.E-6 Torr. The IRTM was operated by means of a console
which simulated the interfaces and functions of the
spacecraft FDS. A minicomputer was used to provide all
operational sequences and modes. Data were recorded on
magnetic tape for subsequent computer processing. The
calibration fixture consisted of two identical blackbodies,
one located in front of the space port and maintained at
liquid nitrogen temperature and the other in front of the
plant port and adjustable in temperature from 77 K to 350K;
eleven settings from 140 K to 330 K were used. Blackbody
temperatures were measured with platinum resistance
thermometers having an absolute accuracy of +/- 0.1 degrees
C. traceable to the National Bureau of Standards. The
digitizer used in the test console provided ten times the
resolution of the FDS digitizer, thus making the digitizing
uncertainty during calibration insignificant compared to the
noise. The calibration data thus produced are IRTM output in
digitization level (DN) as a function of blackbody
temperature. Radiometrically measured internal reference
surface temperatures showed close agreement (+/- 0.5 degrees
C.) with those measured independently with a thermister. The
IRTM temperature was controlled by regulating the temperature
of a mounting base plate and the thermal shield inside the
vacuum chamber. Calibration was performed at 10 degrees C.
spacing across the range of operating temperatures expected
during flight. Typical IRTM channel response to scene
brightness temperatures is shown in Fig. 8 of
CHASE_ETAL_1978. The one-sample noise on the thermal
channels is less than 1 DN except for the 15 micrometer
channel where it is about 2.5 DN. The dynamic ranges of the
surface thermal bands are based on temperatures expected for
the Martian surface. The 300 K maximum chosen for the A
telescope might be exceeded by midday summer temperatures,
but temperatures above the 310-K limit of the B telescope
should not be exceeded unless active volcanic areas were
found; temperatures to 320 K and 330 K could be measured by
the 9 micrometer and 7 micrometer bands. The 15 micrometer
band dynamic range was set quite large as its resolution is
noise limited rather than digitization limited.
Instrument Filter 'D - SOLAR UV-22'
===================================
Filter Name : SOLAR UV-22
Filter Type : MULTILAYER INTERFERENCE
Minimum Wavelength : 0.300000
Maximum Wavelength : 3.000000
Center Filter Wavelength : 1.600000
Description
-----------
Relative spectral response of all channels was measured end
to end using a Perkin-Elmer 16 U monochrometer with
appropriate gratings and order filters. A globar at 1400 K
was used in the 2-25 micrometer range; shortward of 2.0
micrometers a tungsten source at 2700 K was used. The
reference detector was a thermocouple for all but the 0.4-1.1
micrometer range, where a calibrated silicon photodiode was
used. Out-of-band measurements were made by replacing the
spectrometer grating with a plane mirror and ir materials
having known cutoff and cuton wavelengths. Flux calibration
of the IRTM was performed under a simulated space environment
using a vacuum chamber operated typically at a pressure of
1.E-6 Torr. The IRTM was operated by means of a console
which simulated the interfaces and functions of the
spacecraft FDS. A minicomputer was used to provide all
operational sequences and modes. Data were recorded on
magnetic tape for subsequent computer processing. The
calibration fixture consisted of two identical blackbodies,
one located in front of the space port and maintained at
liquid nitrogen temperature and the other in front of the
planet port and adjustable in temperature from 77 K to 350K;
eleven settings from 140 K to 330 K were used. Blackbody
temperatures were measured with platinum resistance
thermometers having an absolute accuracy of +/- 0.1 degrees
C. traceable to the National Bureau of Standards. The
digitizer used in the test console provided ten times the
resolution of the FDS digitizer, thus making the digitizing
uncertainty during calibration insignificant compared to the
noise. The calibration data thus produced are IRTM output in
digitization level (DN) as a function of blackbody
temperature. Radiometrically measured internal reference
surface temperatures showed close agreement (+/- 0.5 degrees
C.) with those measured independently with a thermister. The
IRTM temperature was controlled by regulating the temperature
of a mounting base plate and the thermal shield inside the
vacuum chamber. Calibration was performed at 10 degrees C.
spacing across the range of operating temperatures expected
during flight. Typical IRTM channel response to scene
brightness temperatures is shown in Fig. 8 of
CHASE_ETAL_1978. The one-sample noise on the thermal
channels is less than 1 DN except for the 15 micrometer
channel where it is about 2.5 DN. The dynamic ranges of the
surface thermal bands are based on temperatures expected for
the Martian surface. The 300 K maximum chosen for the A
telescope might be exceeded by midday summer temperatures,
but temperatures above the 310-K limit of the B telescope
should not be exceeded unless active volcanic areas were
found; temperatures to 320 K and 330 K could be measured by
the 9 micrometer and 7 micrometer bands. The 15 micrometer
band dynamic range was set quite large as its resolution is
noise limited rather than digitization limited.
Instrument Optics 'A'
=====================
Telescope Diameter : 0.058000
Telescope F Number : 3.500000
Telescope Focal Length : 0.203000
Telescope Resolution : 0.005100
Telescope T Number : UNK
Telescope Transmittance : UNK
Description
-----------
The A telescope (17.7-24 micrometer) is shown schematically
in Fig. 3 of CHASE_ETAL_1978. It is an f/3.5, 20.3-cm focal
length Cassegrainian design with an aperture diameter of
5.8-cm., spherical surfaces, and, except for mirror
materials, is identical to the B and C telescopes. By using
relatively slow fore optics, degradation of filter sharpness
normally caused by operating an interference filter in a low
f-number beam is negligible. The focal plane contains a
field-defining aperture plate with seven 0.107-cm diameter
holes arranged in a chevron pattern. The fields of view thus
defined are nested with those of the MAWD and imaging systems
(Fig. 4 of CHASE_ETAL_1978). Behind each hole in the field
stop plate is a lens which produces a 0.0254-cm diam image of
the telescope aperture on the detector, which itself is about
the same size. The final optical speed at the detector is
f/1. Optical materials used in the four telescopes are shown
in Table II, and the resulting spectral response is shown in
Fig. 5 of CHASE_ETAL_1978. Mirrors are made of hot-pressed
uncoated zinc oxide for both primary and secondary mirrors.
The reststrahlen reflection properties of ZnO are the major
factors in the A telescope spectral response. Minimizing
extrafield sensitivity (EFS) was an important aspect of the
optical design since on previous Mariner radiometers EFS
contribution seriously compromised observations of scenes
near large temperature contrasts (points near the planetary
limb and polar caps). During instrumentation development,
IRTM image quality was determined in two angular regions. In
the near-field region, a laboratory collimator and ir source
were used to measure the 2-D spatial response out to 16-mrad
diam (three fields of view). Point source field of view
measurements in this region are shown in Fig. 6 of
CHASE_ETAL_1978. For far field measurements, sensitivity
constraints dictated an approach in which the fraction of
energy within a given angular annulus is measured. A 30.5-cm
diam, concentric grooved, blackbody plate with a series of
restricting apertures was used at several distances (30.5-cm,
140-cm, and 610-cm) to define angular response regions from
about one field of view out to 1-rad diam. That is, with the
telescope focused at 610-cm, a disk 3.17-cm in diameter at
that distance defines one half-response field of view (5.2
mrad). The source was held at 95 degrees C. by a
heater/regulator and integral water jacket. To prevent
difficulties with atmospheric transmission, the entire
apparatus was contained in a polyethylene bag flushed with
dry N2. The EFS problem was more severe for the longer
wavelength A telescope than the others, possibly owing to the
higher reflectance at longer wavelengths of the black paint
used inside the telescope. Tests using this apparatus led to
several telescope modifications designed to reduce EFS (see
Fig. 3 of CHASE_ETAL_1978): (1) A postfocal baffle was
placed between the field lens and the detectors to confine
energy to the sensitive area of the detectors. (2) A spider
baffle, added to the outer edges of the secondary mirror
support spider, was designed to reduce reflection off the
sides of the spider legs. (3) A cone baffle coated with CTL
15 black paint was placed on the central dead spot of the
secondary mirror. This was designed to prevent focal plane
reflections from falling on the detectors. Of these three
modifications, only the cone baffle gave significant
improvement, although all three were incorporated in the
design. These results of the final EFS measurements are
shown in Fig. 6 of CHASE_ETAL_1978. The calculated response
due to diffraction and the measured values are shown. The
integrated EFS response between 12 mrad and 1-rad diam was
about 4%. Of this, about 1/2 is due to diffraction effects.
The effect of response outside of the nominal field of view
can be estimated directly from data obtained on scans across
the hot (subsolar) planetary limb. Assuming that the
response is circularly symmetric, and all evidence indicates
this to be closely followed, the signature of a half space
would also be symmetric. A plot of fractional energy derived
from a Viking 1 IRTM scan across the sunlit limb of Mars is
shown in Fig. 6 of CHASE_ETAL_1978. The alignment was
determined using a 20.3-cm (8-in.) collimator to illuminate
all four telescopes with a small source of high temperature
blackbody radiation. Measurements were taken simultaneously
in twenty-eight channels over a 1.5-mrad square grid pattern.
For each channel, a parabolic ellipsoid was fit to data where
the measured intensity was more than 10% of the peak
intensity in that channel. The alignment of each telescope
was ascertained by combining the center of response so
determined for the seven channels in the telescope. This
procedure allowed for the alignment of the four telescopes to
be determined with an estimated precision of 0.1 mrad. The
back of the secondary mirror of the B telescope was
aluminized and used as the alignment reference for this
procedure and for instrument alignment on the spacecraft.
The instrument pointing direction was verified in the same
manner just prior to planetary encounter using Mars as a
5-mrad diam source and using the science platform motion to
generate a 5-mrad spaced grid. The in-flight alignment is
shown in Fig. 4 of CHASE_ETAL_1978.
Instrument Optics 'B'
=====================
Telescope Diameter : 0.058000
Telescope F Number : 3.500000
Telescope Focal Length : 0.203000
Telescope Resolution : 0.005100
Telescope T Number : UNK
Telescope Transmittance : UNK
Description
-----------
The A telescope (17.7-24 micrometer) is shown schematically
in Fig. 3 of CHASE_ETAL_1978. It is an f/3.5, 20.3-cm focal
length Cassegrainian design with an aperture diameter of
5.8-cm., spherical surfaces, and, except for mirror
materials, is identical to the B and C telescopes. By using
relatively slow fore optics, degradation of filter sharpness
normally caused by operating an interference filter in a low
f-number beam is negligible. The focal plane contains a
field-defining aperture plate with seven 0.107-cm diameter
holes arranged in a chevron pattern. The fields of view thus
defined are nested with those of the MAWD and imaging systems
(Fig. 4 of CHASE_ETAL_1978). Behind each hole in the field
stop plate is a lens which produces a 0.0254-cm diam image of
the telescope aperture on the detector, which itself is about
the same size. The final optical speed at the detector is
f/1. Optical materials used in the four telescopes are shown
in Table II, and the resulting spectral response is shown in
Fig. 5 of CHASE_ETAL_1978. The B mirror is made of
aluminized and SiO overcoated fused silica. The spectral
bandpass of the ir channel is determined by interference
bandpass filter and an AR coated detector lense. The
out-of-band response for the B telescope is less than 0.1% of
full scale for an object of 1.E-6 the radiance of a 5800-K
blackbody, the level expected for reflectance from the
subsolar region of Mars. Minimizing extrafield sensitivity
(EFS) was an important aspect of the optical design since on
previous Mariner radiometers EFS contribution seriously
compromised observations of scenes near large temperature
contrasts (points near the planetary limb and polar caps).
During instrumentation development, IRTM image quality was
determined in two angular regions. In the near-field region,
a laboratory collimator and ir source were used to measure
the 2-D spatial response out to 16-mrad diam (three fields of
view). Point source field of view measurements in this
region are shown in Fig. 6 of CHASE_ETAL_1978. For far
field measurements, sensitivity constraints dictated an
approach in which the fraction of energy within a given
angular annulus is measured. A 30.5-cm diam, concentric
grooved, blackbody plate with a series of restricting
apertures was used at several distances (30.5-cm, 140-cm, and
610-cm) to define angular response regions from about one
field of view out to 1-rad diam. That is, with the telescope
focused at 610-cm, a disk 3.17-cm in diameter at that
distance defines one half-response field of view (5.2 mrad).
The source was held at 95 degrees C. by a heater/regulator
and integral water jacket. To prevent difficulties with
atmospheric transmission, the entire apparatus was contained
in a polyethylene bag flushed with dry N2. Tests using this
apparatus led to several telescope modifications designed to
reduce EFS (see Fig. 3 of CHASE_ETAL_1978): (1) A postfocal
baffle was placed between the field lens and the detectors to
confine energy to the sensitive area of the detectors. (2) A
spider baffle, added to the outer edges of the secondary
mirror support spider, was designed to reduce reflection off
the sides of the spider legs. (3) A cone baffle coated with
CTL 15 black paint was placed on the central dead spot of the
secondary mirror. This was designed to prevent focal plane
reflections from falling on the detectors. Of these three
modifications, only the cone baffle gave significant
improvement, although all three were incorporated in the
design. These results of the final EFS measurements are
shown in Fig. 6 of CHASE_ETAL_1978. The calculated response
due to diffraction and the measured values are shown. The
integrated EFS response between 12 mrad and 1-rad diam was
about 4%. Of this, about 1/2 is due to diffraction effects.
The effect of response outside of the nominal field of view
can be estimated directly from data obtained on scans across
the hot (subsolar) planetary limb. Assuming that the
response is circularly symmetric, and all evidence indicates
this to be closely followed, the signature of a half space
would also be symmetric. A plot of fractional energy derived
from a Viking 1 IRTM scan across the sunlit limb of Mars is
shown in Fig. 6 of CHASE_ETAL_1978. The alignment was
determined using a 20.3-cm (8-in.) collimator to illuminate
all four telescopes with a small source of high temperature
blackbody radiation. Measurements were taken simultaneously
in twenty-eight channels over a 1.5-mrad square grid pattern.
For each channel, a parabolic ellipsoid was fit to data where
the measured intensity was more than 10% of the peak
intensity in that channel. The alignment of each telescope
was ascertained by combining the center of response so
determined for the seven channels in the telescope. This
procedure allowed by the alignment of the four telescopes to
be determined with an estimated precision of 0.1 mrad. The
back of the secondary mirror of the B telescope was
aluminized and used as the alignment reference for this
procedure and for instrument alignment on the spacecraft.
The instrument pointing direction was verified in the same
manner just prior to planetary encounter using Mars as a
5-mrad diam source and using the science platform motion to
generate a 5-mrad spaced grid. The in-flight alignment is
shown in Fig. 4 of CHASE_ETAL_1978.
Instrument Optics 'C'
=====================
Telescope Diameter : 0.058000
Telescope F Number : 3.500000
Telescope Focal Length : 0.203000
Telescope Resolution : 0.005100
Telescope T Number : UNK
Telescope Transmittance : UNK
Description
-----------
The A telescope (17.7-24 micrometer) is shown schematically
in Fig. 3 of CHASE_ETAL_1978. It is an f/3.5, 20.3-cm focal
length Cassegrainian design with an aperture diameter of
5.8-cm., spherical surfaces, and, except for mirror
materials, is identical to the B and C telescopes. The focal
plane contains a field-defining aperture plate with seven
0.107-cm diameter holes arranged in a chevron pattern. The
fields of view thus defined are nested with those of the MAWD
and imaging systems (Fig. 4 of CHASE_ETAL_1978). Behind
each hole in the field stop plate is a lens which produces a
0.0254-cm diam image of the telescope aperture on the
detector, which itself is about the same size. The final
optical speed at the detector is f/1. Optical materials used
in the four telescopes are shown in Table II, and the
resulting spectral response is shown in Fig. 5 of
CHASE_ETAL_1978. Mirrors are made of aluminized and SiO
overcoated fused silica except for the A telescope, which
uses hot-pressed uncoated zinc oxide for both primary and
secondary mirrors. The spectral bandpass of the other ir
channels is determined by interference bandpass filters and
AR coated detector lenses. The out-of-band response for the
B and C telescopes is less than 0.1% of full scale for an
object of 1.E-6 the radiance of a 5800-K blackbody, the level
expected for reflectance from the subsolar region of Mars.
The A band has less than 0.1% response for wavelengths less
than 16 micrometer, and wavelengths longer than 30 micrometer
are limited by the Irtran 6 field lens. Minimizing
extrafield sensitivity (EFS) was an important aspect of the
optical design since on previous Mariner radiometers EFS
contribution seriously compromised observations of scenes
near large temperature contrasts (points near the planetary
limb and polar caps). During instrumentation development,
IRTM image quality was determined in two angular regions. In
the near-field region, a laboratory collimator and ir source
were used to measure the 2-D spatial response out to 16-mrad
diam (three fields of view). Point source field of view
measurements in this region are shown in Fig. 6 of
CHASE_ETAL_1978. For far field measurements, sensitivity
constraints dictated an approach in which the fraction of
energy within a given angular annulus is measured. A 30.5-cm
diam, concentric grooved, blackbody plate with a series of
restricting apertures was used at several distances (30.5-cm,
140-cm, and 610-cm) to define angular response regions from
about one field of view out to 1-rad diam. That is, with the
telescope focused at 610-cm, a disk 3.17-cm in diameter at
that distance defines one half-response field of view (5.2
mrad). The source was held at 95 degrees C. by a
heater/regulator and integral water jacket. To prevent
difficulties with atmospheric transmission, the entire
apparatus was contained in a polyethylene bag flushed with
dry N2. Tests using this apparatus led to several telescope
modifications designed to reduce EFS (see Fig. 3 of
CHASE_ETAL_1978): (1) A postfocal baffle was placed between
the field lens and the detectors to confine energy to the
sensitive area of the detectors. (2) A spider baffle, added
to the outer edges of the secondary mirror support spider,
was designed to reduce reflection off the sides of the spider
legs. (3) A cone baffle coated with CTL 15 black paint was
placed on the central dead spot of the secondary mirror.
This was designed to prevent focal plane reflections from
falling on the detectors. Of these three modifications, only
the cone baffle gave significant improvement, although all
three were incorporated in the design. These results of the
final EFS measurements are shown in Fig. 6 of
CHASE_ETAL_1978. The calculated response due to diffraction
and the measured values are shown. The integrated EFS
response between 12 mrad and 1-rad diam was about 4%. Of
this, about 1/2 is due to diffraction effects. The effect of
response outside of the nominal field of view can be
estimated directly from data obtained on scans across the hot
(subsolar) planetary limb. Assuming that the response is
circularly symmetric, and all evidence indicates this to be
closely followed, the signature of a half space would also be
symmetric. A plot of fractional energy derived from a Viking
1 IRTM scan across the sunlit limb of Mars is shown in Fig.
6 of CHASE_ETAL_1978. The alignment was determined using a
20.3-cm (8-in.) collimator to illuminate all four telescopes
with a small source of high temperature blackbody radiation.
Measurements were taken simultaneously in twenty-eight
channels over a 1.5-mrad square grid pattern. For each
channel, a parabolic ellipsoid was fit to data where the
measured intensity was more than 10% of the peak intensity in
that channel. The alignment of each telescope was
ascertained by combining the center of response so determined
for the seven channels in the telescope. This procedure
allowed by the alignment of the four telescopes to be
determined with an estimated precision of 0.1 mrad. The back
of the secondary mirror of the B telescope was aluminized and
used as the alignment reference for this procedure and for
instrument alignment on the spacecraft. The instrument
pointing direction was verified in the same manner just prior
to planetary encounter using Mars as a 5-mrad diam source and
using the science platform motion to generate a 5-mrad spaced
grid. The in-flight alignment is shown in Fig. 4 of
CHASE_ETAL_1978.
Instrument Optics 'D'
=====================
Telescope Diameter : 0.037000
Telescope F Number : 5.500000
Telescope Focal Length : 0.203000
Telescope Resolution : UNK
Telescope T Number : UNK
Telescope Transmittance : UNK
Description
-----------
The D telescope has a reduced aperture of 3.7 cm and a focal
length of f/5.5. By using relatively slow fore optics,
degradation of filter sharpness normally caused by operating
an interference filter in a low f-number beam is negligible.
The focal plane contains a field-defining aperture plate with
seven 0.107-cm diam holes arranged in a chevron pattern. The
fields of view thus defined are nested with those of the MAWD
and imaging systems (Fig. 4 of CHASE_ETAL_1978). Behind
each hole in the field stop plate is a lens which produces a
0.0254-cm diam image of the telescope aperture on the
detector, which itself is about the same size. The final
optical speed at the detector is f/1. Optical materials used
in the four telescopes are shown in Table II, and the
resulting spectral response is shown in Fig. 5 of
CHASE_ETAL_1978. Mirrors are made of aluminized and SiO
overcoated fused silica except for the A telescope, which
uses hot-pressed uncoated zinc oxide for both primary and
secondary mirrors. The spectral bandpass of the other ir
channels is determined by interference bandpass filters and
AR coated detector lenses. The transmission elements of the
D telescope insure that it is thermally blind. Special
coatings on the D telescope mirrors were used to obtain a
reasonably gray response to solar radiation. Minimizing
extrafield sensitivity (EFS) was an important aspect of the
optical design since on previous Mariner radiometers EFS
contribution seriously compromised observations of scenes
near large temperature contrasts (points near the planetary
limb and polar caps). During instrumentation development,
IRTM image quality was determined in two angular regions. In
the near-field region, a laboratory collimator and ir source
were used to measure the 2-D spatial response out to 16-mrad
diam (three fields of view). Point source field of view
measurements in this region are shown in Fig. 6 of
CHASE_ETAL_1978. For far field measurements, sensitivity
constraints dictated an approach in which the fraction of
energy within a given angular annulus is measured. A 30.5-cm
diam, concentric grooved, blackbody plate with a series of
restricting apertures was used at several distances (30.5-cm,
140-cm, and 610-cm) to define angular response regions from
about one field of view out to 1-rad diam. That is, with the
telescope focused at 610-cm, a disk 3.17-cm in diameter at
that distance defines one half-response field of view (5.2
mrad). The source was held at 95 degrees C. by a
heater/regulator and integral water jacket. To prevent
difficulties with atmospheric transmission, the entire
apparatus was contained in a polyethylene bag flushed with
dry N2. The EFS problem was more severe for the longer
wavelength A telescope than the others, possibly owing to the
higher reflectance at longer wavelengths of the black paint
used inside the telescope. Tests using this apparatus led to
several telescope modifications designed to reduce EFS (see
Fig. 3 of CHASE_ETAL_1978): (1) A postfocal baffle was
placed between the field lens and the detectors to confine
energy to the sensitive area of the detectors. (2) A spider
baffle, added to the outer edges of the secondary mirror
support spider, was designed to reduce reflection off the
sides of the spider legs. (3) A cone baffle coated with CTL
15 black paint was placed on the central dead spot of the
secondary mirror. This was designed to prevent focal plane
reflections from falling on the detectors. Of these three
modifications, only the cone baffle gave significant
improvement, although all three were incorporated in the
design. These results of the final EFS measurements are
shown in Fig. 6 of CHASE_ETAL_1978. The calculated response
due to diffraction and the measured values are shown. The
integrated EFS response between 12 mrad and 1-rad diam was
about 4%. Of this, about 1/2 is due to diffraction effects.
The effect of response outside of the nominal field of view
can be estimated directly from data obtained on scans across
the hot (subsolar) planetary limb. Assuming that the
response is circularly symmetric, and all evidence indicates
this to be closely followed, the signature of a half space
would also be symmetric. A plot of fractional energy derived
from a Viking 1 IRTM scan across the sunlit limb of Mars is
shown in Fig. 6 of CHASE_ETAL_1978. The alignment was
determined using a 20.3-cm (8-in.) collimator to illuminate
all four telescopes with a small source of high temperature
blackbody radiation. Measurements were taken simultaneously
in twenty-eight channels over a 1.5-mrad square grid pattern.
For each channel, a parabolic ellipsoid was fit to data where
the measured intensity was more than 10% of the peak
intensity in that channel. The alignment of each telescope
was ascertained by combining the center of response so
determined for the seven channels in the telescope. This
procedure allowed by the alignment of the four telescopes to
be determined with an estimated precision of 0.1 mrad. The
instrument pointing direction was verified in the same manner
just prior to planetary encounter using Mars as a 5-mrad diam
source and using the science platform motion to generate a
5-mrad spaced grid. The in-flight alignment is shown in Fig.
4 of CHASE_ETAL_1978.
Instrument Mode 'FIXED PLANET'
==============================
Instrument Power Consumption : 6.000000
In sections
-----------
IRTM
Description
-----------
Other than power on or power off, the only command options
for the IRTM controlled the motion of the scan mirror. This
mirror could be commanded to 3 positions, separated by 90
degrees; PLANET, SPACE, and REFERENCE. In the PLANET
position, data were acquired in all 28 channels. In SPACE,
the zero-radiance level was reset to near the bottom of the
dynamic range. In the REFERENCE position, gain calibration
data was obtained using both a grooved blackbody at ambient
instrument temperatureand a small lamp and hemispherical
diffuser for the solar telescope. The REFERENCE position was
also the 'safe' position, used when the instrument was turned
off.
In the FIXED PLANET mode, the first 0,2, or 4 ICKs could be
housekeeping data while the mirror was in motion to the
planet position. Thereafter the instrument reported planet
data untill the next command was received.
Instrument Mode 'FIXED REFERENCE'
=================================
Instrument Power Consumption : 6.000000
In sections
-----------
IRTM
Description
-----------
Other than power on or power off, the only command options
for the IRTM controlled the motion of the scan mirror. This
mirror could be commanded to 3 positions, separated by 90
degrees; PLANET, SPACE, and REFERENCE. In the PLANET
position, data were acquired in all 28 channels. In SPACE,
the zero-radiance level was reset to near the bottom of the
dynamic range. In the REFERENCE position, gain calibration
data was obtained using both a grooved blackbody at ambient
instrument temperatureand a small lamp and hemispherical
diffuser for the solar telescope. The REFERENCE position was
also the 'safe' position, used when the instrument was turned
off.
In the FIXED REFERENCE mode, the mirror was commanded to the
reference position; 0 to 5 ICKS of housekeeping data were
possible while the mirror was in motion. Thereafter, the
instrument reported radiance readings or housekeeping data on
alternate ICKs untill the next command was received.
Instrument Mode 'FIXED SPACE'
=============================
Instrument Power Consumption : 6.000000
In sections
-----------
IRTM
Description
-----------
Other than power on or power off, the only command options
for the IRTM controlled the motion of the scan mirror. This
mirror could be commanded to 3 positions, separated by 90
degrees; PLANET, SPACE, and REFERENCE. In the PLANET
position, data were acquired in all 28 channels. In SPACE,
the zero-radiance level was reset to near the bottom of the
dynamic range. In the REFERENCE position, gain calibration
data was obtained using both a grooved blackbody at ambient
instrument temperatureand a small lamp and hemispherical
diffuser for the solar telescope. The REFERENCE position was
also the 'safe' position, used when the instrument was turned
off.
In the FIXED SPACE mode, the mirror was commanded to the
space position from wherever it had been, with 2 ICKs of
housekeeping data if the mirror was not already in the space
position. Thereafter the instrument reported space data
until another command was received.
Instrument Mode 'MODIFIED NORMAL'
=================================
Instrument Power Consumption : 6.000000
In sections
-----------
IRTM
Description
-----------
Other than power on or power off, the only command options
for the IRTM controlled the motion of the scan mirror. This
mirror could be commanded to 3 positions, separated by 90
degrees; PLANET, SPACE, and REFERENCE. In the PLANET
position, data were acquired in all 28 channels. In SPACE,
the zero-radiance level was reset to near the bottom of the
dynamic range. In the REFERENCE position, gain calibration
data was obtained using both a grooved blackbody at ambient
instrument temperatureand a small lamp and hemispherical
diffuser for the solar telescope. The REFERENCE position was
also the 'safe' position, used when the instrument was turned
off.
A MODIFIED normal mode was also available, in which the only
space view was the first of the cycle, followed by 243 ICKs
of planet data. In NORMAL mode, ICKs 1 and 2 were
housekeeping data while the mirror moved to space. ICK 3 was
the space level before reset. During ICK 4, the electronics
for all channels were reset so that the sensed radiance
(meant to be the cosmic background level of essentially zero
radiance) yielded a data number (DN) of a few. There was a
filter on the reset so that the voltage change was only about
2/3 of the way to the space radiance level; this smoothed out
the zero setting, but also meant that several cycles were
required to recover from a serious drift. ICK 5 was still in
the space position and yielded the DN response to space. Ick
6 and 7 were housekeeping data while the mirror moved to the
reference position. ICK 8 and 9 were the DN response to the
reference surface; only the second reading was used in
calibration of the solar channel to allow the lamp filament
to warm up completely. ICKs 10 through 13 were housekeeping
data while the mirror moved to the planet position. The 57
ICKs 14-64 were planet data. The 7 ICK cycle to space, with
reset of the zero-radiance DN level in all channels, was
repeated each 64 ICKs, beginning on ICKs 65, 129, and 193,
with motion directly back to the planet position for another
57 ICKs of planet data.
Instrument Mode 'NORMAL'
========================
Instrument Power Consumption : 6.000000
In sections
-----------
IRTM
Description
-----------
Other than power on or power off, the only command options
for the IRTM controlled the motion of the scan mirror. This
mirror could be commanded to 3 positions, separated by 90
degrees; PLANET, SPACE, and REFERENCE. In the PLANET
position, data were acquired in all 28 channels. In SPACE,
the zero-radiance level was reset to near the bottom of the
dynamic range. In the REFERENCE position, gain calibration
data was obtained using both a grooved blackbody at ambient
instrument temperature and a small lamp and hemispherical
diffuser for the solar telescope. The REFERENCE position was
also the 'safe' position, used when the instrument was turned
off.
The NORMAL mode sequenced the pointing to PLANET most of the
time, with pointing to SPACE at intervals of 64 ICKs, and to
REFERENCE at intervals of 256 ICKs. ICK is a one syllable
acronym for 'incremental counter keeper' and represents 1.12
second duration, the basic time interval of IRTM operation.
It required 2 ICKs for the mirror to move 90 degrees and
settle; whenever the mirror was in motion the downlink data
contained housekeeping information about instrument status
and detector voltage levels. In NORMAL mode, ICKs 1 and 2
were housekeeping data while the mirror moved to space. ICK
3 was the space level before reset. During ICK 4, the
electronics for all channels were reset so that the sensed
radiance (meant to be the cosmic background level of
essentially zero radiance) yielded a data number (DN) of a
few. There was a filter on the reset so that the voltage
change was only about 2/3 of the way to the space radiance
level; this smoothed out the zero setting, but also meant
that several cycles were required to recover from a serious
drift. ICK 5 was still in the space position and yielded the
DN response to space. Ick 6 and 7 were housekeeping data
while the mirror moved to the reference position. ICK 8 and
9 were the DN response to the reference surface; only the
second reading was used in calibration of the solar channel
to allow the lamp filament to warm up completely. ICKs 10
through 13 were housekeeping data while the mirror moved to
the planet position. The 57 ICKs 14-64 were planet data.
The 7 ICK cycle to space, with reset of the zero-radiance DN
level in all channels, was repeated each 64 ICKs, beginning
on ICKs 65, 129, and 193, with motion directly back to the
planet position for another 57 ICKs of planet data.
Mounted On Platform 'SCAN PLATFORM'
===================================
Cone Offset Angle : 0.070000
Cross Cone Offset Angle : 0.030000
Description
-----------
OFFSET ANGLE SCALED FROM FIGURE 4 OF CHASE_ETAL_1978; these
were determined by scans across Mars during approach, when
the angular diameter of the planet was 1.93 and 4.79
milliradians (VO-1)
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