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Space Topics: Cassini-Huygens
Imaging Science Subsystem (ISS)
Optical Remote Sensing Instrument for the Cassini Orbiter
Scientific Objectives -
How It Works -
Saturn Exploration Context -
Camera Facts
Scientific Objectives
The Imaging Science Subsystem (ISS) is used for multispectral imaging of
Saturn, Titan, rings, and the icy satellites to observe their properties.
Specifically:
To study the Saturn and Titan atmospheres:
- To map 3-dimensional structure and motions. (Some wavelengths of light
penetrate farther into the atmosphere than others, so images at different
wavelengths can produce vertical maps of the atmospheres.)
- To study the composition, distribution, and physical properties of clouds
and aerosols. (Materials with different compositions preferentially absorb
different wavelengths of light, so color images reveal compositional
differences among different locations.)
- To investigate scattering, absorption, and solar heating.
- To search for evidence of lightning, aurorae, airglow, and planetary
oscillations.
To study Saturn's rings:
- To study the gravitational interactions between the rings and Saturn's
satellites by examining ring structures and how they change over time.
- To determine the rate and nature of energy and momentum transfer within
the rings.
- To determine ring thickness and sizes, composition, and physical nature
of ring particles.
To study Saturn's moons:
- To map the surfaces of the satellites (including Titan) to study their
geological histories.
- To determine the nature and composition of the icy satellite surface
materials.
- To determine the rotation states of the icy satellites.
Also, the cameras
will also be used for optical navigation: images of moons against the
background of the stars will help planners keep the spacecraft on course.
How It Works
Cassini's Optical Remote Sensing Pallet
All of Cassini's Optical Remote Sensing instruments are located on a pallet
on one side of the orbiter. Credit: NASA / JPL
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Fields of view of the Cassini Optical Remote Sensing Instruments
All of Cassini's Optical Remote Sensing instruments (ISS, UVIS, VIMS, and
CIRS) share the same boresight. This diagram compares the fields of view
of the four instruments. The wide- and narrow-angle camera fields of view
are shown in green.
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The Imaging Science Subsystem consists of two cameras, a Wide Angle
Camera (ISS-WAC) and a Narrow Angle Camera (ISS-NAC). Both cameras produce
images 1024 by 1024 pixels in size. Each camera is equipped with a pair of
filter wheels that give the cameras excellent color vision: 23 different filters
for the Narrow Angle and 17 for the Wide Angle Camera, spanning wavelengths
of light from the ultraviolet to the near-infrared. Because the filters
are on two overlapping filter wheels, filters can be combined to yield a great
many more effective filter combinations (about 100 for the NAC and about
50 for the WAC).
The Wide Angle Camera will provide context images for all of the other optical
remote sensing instruments. The Narrow Angle Camera will be used for high-resolution
studies.
The ISS cameras are affixed to Cassini's Remote Sensing Pallet. The Remote
Sensing Pallet also contains the CIRS, VIMS, and UVIS instruments. In order
for ISS to capture images, the Cassini spacecraft must rotate to point the
Remote Sensing Pallet to a target. Consequently, whenever ISS captures an
image of a target, the other three Optical Remote Sensing instruments are
also pointed in the same direction and can study the same target. This is
both a strength and a weakness: a strength, because there is no question that
the whole instrument suite is focused on exactly the same spot, and a weakness,
because the four instruments cannot operate independently. The four instrument
teams cooperate in planning their science observations.
The resolution of the cameras depends upon their distance from the target.
When Cassini is 1000 km from her target, the Narrow Angle Camera will achieve
a resolution of 6 meters per pixel.
How Does the Imaging Science Subsystem Fit in the Context of Saturn Exploration?
The Imaging Science Subsystem is the main camera system aboard Cassini. The
images it produces will create enduring public impressions of the Saturn system,
much like the Voyager Imaging Science Subsystem did, 25 years ago. Despite
the similarity in name, though, Cassini's ISS is much more powerful than Voyager's.
Cassini ISS uses a CCD (the same sort of detector as in commercial digital
cameras), so her vision is much sharper than Voyager, which used a vidicon.
More importantly, Cassini's ISS has four times as many filters as Voyager
had, spanning twice as wide a range of the electromagnetic spectrum. Images
of the same target captured through different filters can be combined into "true
color" (if red, green, and blue filters are used) or "false color" images
to highlight different information depending on the composition or brightness
of the target.
Cassini's filters were carefully selected for their usefulness for studying
the Saturn system. In particular, there are three sets of filters designed
to exploit the spectral characteristics of methane. Over the ISS wavelength
range, methane strongly absorbs light at three wavelengths: 619, 727, and
890 nanometers. The ISS cameras have "methane band" filters at
these wavelengths, as well as "continuum band" filters at wavelengths
outside the methane absorption bands. These filters, in combination with several
polarizing light filters, will allow Cassini to see to great depths in Saturn's
and Titan's atmospheres, and even all the way to Titan's surface.
Camera Facts
The Imaging Science Subsystem consists of two framing cameras, a Wide Angle
Camera (ISS-WAC) and a Narrow Angle Camera (ISS-NAC).
| Location |
Remote Sensing Pallet (side of Cassini orbiter) |
| Image size |
1024 x 1024 pixels |
| Field of View |
ISS-WAC: 3.5° x 3.5° (61 by 61 milliradians)
ISS-NAC: 0.35° x 0.35° (6.1 by 6.1 milliradians) |
| Angular Resolution |
ISS-WAC: 60 microradians
ISS-NAC: 6 microradians |
| Spectral Range |
ISS-WAC: 380-1100 nanometers, 18 filters
ISS-NAC: 200-1100 nanometers, 24 filters |
Filters: Narrow-Angle Camera
The Narrow-Angle Camera has two filter wheels, each with twelve filters.
The two filter wheels can be moved independently, allowing filters on different
wheels to be stacked.
| Color |
Wavelength* |
Purpose |
| UV1 |
ultraviolet |
258 nm, wide |
atmospheric aerosols |
| UV2 |
ultraviolet |
298 nm, wide |
aerosols, broadband color |
| UV3 |
ultraviolet |
338 nm, wide |
aerosols, broadband color, polarization |
| BL2 |
blue |
440 nm, medium |
medium-band color, polarization |
| BL1 |
blue |
451 nm, wide |
broadband color |
| GRN |
green |
568 nm, wide |
broadband color |
| MT1 |
near infrared |
619 nm, narrow |
weak methane band, vertical sounding |
| CB1b |
near infrared |
603 nm, narrow |
continuum band for MT1 |
| CB1a |
635 nm, narrow |
| RED |
red |
650 nm, wide |
broadband color |
| HAL |
hydrogen alpha |
656 nm, narrow |
hydrogen alpha / lightning |
| MT2 |
near infrared |
727 nm, narrow |
medium methane band, vertical sounding |
| CB2 |
near infrared |
750 nm, narrow |
continuum band for MT2 |
| IR1 |
near infrared |
752 nm, wide |
broadband color |
| IR2 |
near infrared |
862 nm, wide |
broadband color; ring absorption band |
| MT3 |
near infrared |
889 nm, narrow |
strong methane band, vertical sounding |
| CB3 |
near infrared |
938 nm, narrow |
continuum band for MT3; see through Titan haze |
| IR3 |
near infrared |
930 nm, wide |
broadband color |
| IR4 |
near infrared |
1002 nm, long-pass |
broadband color |
| CL1 |
all colors, ultraviolet
to near infrared |
611 nm, wide open |
filter wheel 1 clear (combine with wheel 2 filters) |
| CL2 |
all colors, ultraviolet
to near infrared |
611 nm, wide open |
filter wheel 2 clear (combine with wheel 1 filters) |
| P0 |
visible polarization, 0° |
617 nm |
visible polarization, 0° |
| P60 |
visible polarization, 60° |
617 nm |
visible polarization, 60° |
| P120 |
visible polarization, 120° |
617 nm |
visible polarization, 120° |
| IRP0 |
IR polarization, 0° |
746 nm |
IR polarization, 0°; see through Titan haze |
* short-pass: filter allows only short wavelengths to
pass through (detector can see down to 200 nm)
long-pass: filter allows only long wavelengths to pass through (detector
can see up to 1100 nm)
wide: filter passes a broad range of wavelengths (50-200 nm or so)
medium: filter passes a medium range of wavelengths (20-50 nm)
narrow: filter passes a narrow range of wavelengths (less than 20 nm)
wide open: no filter; all light passed straight through to detector |
Filters: Wide-Angle Camera
The Wide-Angle Camera has two filter wheels, each with nine filters. The
two filter wheels can be moved independently, allowing filters on different
wheels to be stacked.
| Color |
Wavelength* |
Purpose |
| VIO |
violet |
420 nm, short-pass |
broadband color |
| BL1 |
blue |
460 nm, wide |
broadband color |
| GRN |
green |
567 nm, wide |
broadband color |
| RED |
red |
648 nm, wide |
broadband color |
| HAL |
hydrogen alpha |
656 nm, narrow |
hydrogen alpha / lightning |
| MT2 |
near infrared |
727 nm, narrow |
medium methane band, vertical sounding |
| CB2 |
near infrared |
752 nm, narrow |
continuum band for MT2 |
| IR1 |
near infrared |
742 nm, wide |
broadband color |
| IR2 |
near infrared |
853 nm, wide |
broadband color; ring absorption band |
| MT3 |
near infrared |
890 nm, narrow |
strong methane band, vertical sounding |
| CB3 |
near infrared |
939 nm, narrow |
continuum band for MT3; see through Titan haze |
| IR3 |
near infrared |
918 nm, wide |
broadband color |
| IR4 |
near infrared |
1001 nm, long-pass |
broadband color |
| IR5 |
near infrared |
1027 nm, long-pass |
broadband color |
| CL1 |
all colors, ultraviolet
to near infrared |
635 nm, wide open |
filter wheel 1 clear (combine with wheel 2 filters) |
| CL2 |
all colors, ultraviolet
to near infrared |
635 nm, wide open |
filter wheel 2 clear (combine with wheel 1 filters) |
| IRP0 |
IR polarization, 0° |
705 nm |
IR polarization, 0°; see through Titan haze |
| IRP90 |
IR polarization, 90° |
705 nm |
IR polarization, 90°; see through Titan haze |
* short-pass: filter allows only short wavelengths to
pass through (detector can see down to 380 nm)
long-pass: filter allows only long wavelengths to pass through (detector
can see up to 1100 nm)
wide: filter passes a broad range of wavelengths (50-200 nm or so)
narrow: filter passes a narrow range of wavelengths (less than 20 nm)
wide open: no filter; all light passed straight through to detector |
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