Introduction to the Cassini Imaging Science Subsystem:
                              Wide Angle Camera
 
Instrument Overview
===================
 
The Cassini ISS consists of two fixed focal length telescopes, a
narrow angle camera (NAC) and a wide angle camera (WAC). The WAC is
55 cm long and 35 cm x 33 cm wide, and has a focal length of 200.77
+/- 0.02 mm in the clear filter. The two cameras together have a mass
of 56.9 kg, and sit on the Remote Sensing Palette (RSP), fixed to
the body of the Cassini Orbiter, between the Visual and Infrared
Mapping Spectrometer (VIMS) and the Composite Infrared Spectrometer
(CIRS), and above the Ultraviolet Imaging Spectrometer (UVIS). The
apertures and radiators of both telescopes are parallel to each other.
 
The WAC has its own set of optics, mechanical mountings, CCD, shutter,
filter wheel assembly, temperature sensors, heaters, and electronics,
the latter of which consists of two parts: the sensor head subassembly
and the main electronics subassembly. The Sensor Head electronics
supports the operation of the CCD detector and the preprocessing of
the pixel data. The Main Electronics provide the power and perform
all other ISS control functions, including generating and maintaining
internal timing which is synchronized to the Command Data System
(CDS) timing of 8 Hz, control of heaters, and the two hardware data
compressors. The Cassini Engineering Flight Computer (EFC) is a
radiation-hardened processor that controls the timing, internal
sequencing, mechanism control, engineering and status data
acquisition, and data packetization.
 
The optical train of the WAC, a Voyager flight spare, is an f/3.5
refractor with a ~60 microrad/pixel image scale, a 3.5 deg x 3.5 deg
field of view (FOV), and a spectral range from 380 nm - 1050 nm. Its
filter wheel subassembly carries 18 spectral filters: 9 filters on
each of two wheels. This allows for in-line combinations of filters
for greater flexibility. Each wheel is designed to move
independently, in either the forward or reverse direction, at a rate
of 3 positions per second. A homing sensor on each wheel defines a
home wheel position, and wheel positioning can be commanded
absolutely or relatively.
 
Unlike the NAC, the WAC is not thermally isolated from the RSP. It has
less stringent image quality requirements, so its bulk temperature
control is provided by the pallet.
 
The temperature of the CCD is controlled by a passive radiator,
directly connected to the focal plane, along with an active
'performance' heater on the CCD to adjust the temperature. The
temperature of the optical elements is controlled by active heaters
positioned along the optical path. These optical elements are kept to
within 1 degree Celsius to maintain camera focus without an active
focusing mechanism. Low expansion invar spacers are also used. The
radiator subassembly also includes two sets of spacecraft-controlled
decontamination heaters which are used to minimize deposition of
volatile contaminants on either the detector or radiator and to
minimize radiation damage to the CCD. All heaters are commandable (ON
or OFF) during flight.
 
Optics
------
 
Like the NAC, the focal plane field of view of the WAC is limited by
the size of the CCD. However, due to the Voyager optics, the WAC point
spread function (PSF) is somewhat larger than a pixel, with a clear
filter full width at half maximum (FWHM) of 1.8 pixels. The nominal
pixel scale is 59.749 microradians/pixel.
 
All the optical elements within the WAC are made of either
radiation-hardened optical glass (BK7 or lithium fluoride) or fused
silica. Antireflection coatings consisting of single layer MgFl2 were
deposited on the CCD window and primary optics. A fused silica quartz
plug is placed immediately in front of the CCD package to protect the
detector against radiation damage and to minimize radiation-induced
noise in the images.
 
The larger field of view of the WAC makes it more susceptible than the
NAC to geometric distortions. Measurements of distortion and its
dependence on temperature and spectral bandpass in the WAC were made
on the ground and in flight. Ground based measurements suggested
distortions up to about 3.6 +/- 0.2 pixels in the corners of the CCD,
independent of spectral bandpass, in the optics temperature range of
-10 degrees C to +25 degrees C. Subsequent observations of the
Pleiades and the open cluster M35 showed a consistent distortion
parameter of k = -6.27 +/- 0.25, and slight changes in focal length
as a function of filter combination. The WAC focal length in the
clear filter is 200.77 +/- 0.02 mm. Focal lengths in other filter
combinations range from 200.71 mm to 201.22 mm, yielding a range in
image radius of 1.27 pixels for a nominal 500 pixel radius object.
Thus, individual filter combinations need to be fully calibrated to
determine specific focal length. The distortion parameter remains
essentially constant in the different filters. In-flight distortion
measurements for the WAC are consistent with those taken from the
ground: 3.36 pixels in the corners.
 
Filters
-------
 
The ISS filter assembly design -- consisting of two filter wheels and
a filter changing mechanism -- is inherited from the Hubble Space
Telescope WF/PC camera. Each wheel is designed to move independently,
in either the forward or reverse direction, at a rate of 2 positions
per second in the WAC. A homing sensor on each wheel defines a home
wheel position: wheel positioning can be commanded absolutely or
relatively.
 
The WAC filter wheel contains both medium and broad-band filters that
cover the spectral range of the CCD, as well as narrow-band filters
for atmospheric studies. The former include the BL1, GRN, RED, IR1,
IR2, IR3, and IR4 filters (available on both cameras) as well as VIO
and IR5 (WAC only). The latter include the MT2, MT3, CB2 and CB3
filters, used to investigate methane absorption bands and continuum
wavelengths. A HAL filter is also included for observing H-alpha
emissions from lightning.
 
The Cassini Imaging Science Team has deliberately duplicated 63% of
the filters in both the NAC and WAC. These include seven
medium/broadband filters from the blue to the near-IR for
spectrophotometry, 2 methane and 2 continuum band filters for
atmospheric vertical sounding, 2 clear filters, and the HAL filter.
 
The clear filter is in the 'home' slot of each filter wheel, since it
was deemed that sticking of a filter wheel, should it occur, was most
likely to occur in the home position. Typically a clear filter in one
wheel is combined with a color filter in the other wheel, though
two-filter combinations can also be used. However, with the spare
Voyager optics on the WAC, we encountered difficulty in achieving a
sharp focus in the near-IR (at which the Voyager vidicon detector was
not sensitive). The solution was to place all near-IR filters on one
wheel and a special, thin clear filter on the other wheel. As a
result of this decision, and because the WAC lacks the UV filters,
the only useful 2-filter bandpass in the WAC is IR1-IR2.
 
Though both cameras are capable of seeing into the near-IR at ~1.0
micron, the wide angle camera is 9 times faster for a given exposure
than the NAC and is consequently better equipped to sense this
spectral region for either broadband color imaging or atmospheric
sounding where the CCD quantum efficiency and solar flux are
declining and a large camera throughput is desired, though this
benefit is reduced somewhat by the Voyager optical coatings.
 
Finally, the WAC carries two orthogonal infrared polarizers, IRP0 and
IRP90, which can provide intensity and the Stokes parameter, Q,
referenced to the principal axes of the polarizers. If the polarizers
are oriented parallel or perpendicular to the scattering plane, the
information provided by Q is in most cases as informative as that
provided by three polarizers because the polarized electric vector is
usually aligned parallel or perpendicular to the scattering plane.
Estimates of Q referenced to the scattering plane can be made for
other orientations but with diminishing precision as the angle
between the scattering plane and the polarizer axis approaches 45
degrees at which point the measurement of Q is not useful.
 
The polarizers are, of course, to be used in combination with other
spectral filters and so filter placement was important. In the WAC the
3 broad-band filters at 867 nm, 950 nm, and 977 nm (cutoff filter),
and the four narrow-band filters at 727 and 889 nm (methane) and 750
and 940 nm (continuum), are all placed in the same wheel opposite the
wheel containing the 2 IR polarizers.
 
Table 1: ISS WAC Filter Characteristics
 
Filter  Lambda_cen  Lambda_eff  Science Justification
----------------------------------------------------------------------
VIO     420SP       420         broadband color
BL1     460W        463         broadband color
GRN     567W        568         broadband color
RED     648W        647         broadband color
HAL     656N        656         H-alpha/lightning
MT2     728N        728         methane band, vertical sounding
CB2     752N        752         continuum for MT2
IR1     742W        740         broadband color
IR2     853W        852         broadband color; ring absorption band
MT3     890N        890         methane band, vertical sounding
CB3     939N        939         continuum for MT3,see thru Titan haze
IR3     918W        917         broadband color
IR4     1001LP      1000        broadband color
IR5     1028LP      1027        broadband color
CL1     635         634         wide open, combine w/wheel 2 filters
CL2     635         634         wide open, combine w/wheel 1 filters
IRP0    705         705         IR polarization,see through Titan haze
IRP90   705         705         IR polarization,see through Titan haze
 
Table 2: WAC Two-Filter Bandpasses
 
Filters   lambd_cen   lambda_eff
--------------------------------
IR1-IR2   826         826
 
(All wavelengths in nm. Central wavelengths (lambda_cen) are computed
using the full system transmission function. Effective wavelengths
(lambda_eff) are computed using the full system transmission function
convolved with a solar spectrum. Bandpass types: SP = short wavelength
cutoff; W = wide; N = narrow; LP = long wavelength cutoff.)
 
With the exception of the clear filters and the polarizers, the
filters are all interference filters manufactured using an ion-aided
deposition (IAD) process which has the effect of making the filters
temperature and moisture tolerant, and resistant to delamination.
Conventional interference filters have passbands which shift with
temperature. The shift can be significant for narrowband filters
targeted to methane absorption bands or the H_alpha line. Temperature
shifts for IAD filters is typically an order of magnitude or more
smaller than for conventional filters and is insignificant over the
temperature range (room temperature to 0 degrees C) relevant to
calibration and operation of the Cassini cameras.
 
The WAC infrared polarizers have a 1 mm-thick layer of Polarcor
(trademark Corning) cemented between two slabs of BK7-G18 glass.
Polarcor is a borosilicate glass impregnated with fine metallic wires.
The infrared polarizers have much better performance than the NAC
visible polarizers over their range (700 nm - 1100 nm), where the
principal transmission is greater than 0.9 and the orthogonal
transmission is 0.001 or less.
 
Shutter
-------
 
Between the filter wheel assembly and the CCD detector is the shutter
assembly, a two blade, focal plane electromechanical system derived
from that used on Voyager, Galileo and WFPC. To reduce scattered
light, the shutter assembly was put in the optical train `backwards ,
with the unreflective side towards the focal plane. Each blade moves
independently, actuated by its own permanent magnet rotary solenoid,
in the sample direction: i.e., keeping the blade edge parallel to the
columns of the CCD. The shutter assembly is operated in 3-phases: open
(one blade sweeps across the CCD), close (the other blade sweeps
across the CCD to join the first), and reset (both blades
simultaneously sweep across the CCD in the reverse direction to the
start position).
 
There are 64 commandable exposure settings which can be updated during
flight if so desired. These correspond to 63 different exposure times,
ranging from 5 milliseconds to 20 minutes, and one `No Operation
setting. The shortest nonzero exposure is 5 msec. In the ISS flight
software, the time tag on the image is the time of the close of the
shutter. Because of mechanical imperfections in the shutter mechanism,
there is a difference between the commanded exposure time and the
actual exposure time, and a gradient in exposure time across the CCD
columns. At an operating temperature of 0 degrees C, the mean
differences in the WAC for commanded exposure times of 5, 25 and 100
ms were measured to be 0.15, 0.39 and 0.07 ms, respectively. In all
cases the actual exposure times are less than the commanded times.
There is also a small temperature dependence to these shutter offsets.
 
The 1024th column is illuminated first in both cameras. In the WAC,
this column is illuminated for ~ 0.1 msec longer than the first
column. This value is independent of exposure time and reasonably
independent of temperature. The expected precision or repeatability
of an exposure (equal to the standard deviation of actual exposure
durations measured at any one location on the CCD in ground tests) is