Introduction to the Cassini Imaging Science Subsystem:
                            Narrow 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 NAC is
95 cm long and 40 cm x 33 cm wide, and has a focal length of 2002.70
+/- 0.07 mm in the clear filter. The two cameras together have a mass
of 57.83 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 NAC 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 NAC is an f/10.5 reflecting telescope with an image scale of ~6
microrad/pixel, a 0.35 deg x 0.35 deg field of view (FOV), and a
spectral range from 200 nm - 1100 nm. Its filter wheel subassembly
carries 24 spectral filters: 12 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 WAC, the NAC is thermally isolated from the RSP in order to
minimize the effects of RSP thermal transients on the NAC image
quality.
 
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
------
 
The narrow angle camera optics were specially designed to improve on
the quality and resolution of images of the bodies in the Saturn
system returned by Voyager. It is based on a Ritchey-Chretien
reflector design. The focal plane field of view is limited by the
size of the CCD. The NAC point spread function (PSF) was designed to
be approximately the same physical size as a pixel in the near-IR.
The full width at half maximum (FWHM) of the PSFs of the NAC through
the clear filters is 1.3 pixels. The nominal pixel scale is 5.9907
microradians/pixel.
 
All the reflective optical elements within the NAC (the primary and
secondary mirrors) are manufactured of fused silica; all refractive
NAC elements (such as the field correctors and the window on the
sealed CCD package) are made of either fused silica or single-crystal
vacuum-UV-grade calcium fluoride. Antireflection coatings consisting
of single layer MgFl2 were deposited on the field correctors and CCD
window; a multi-layer MgFl2 coating was applied to the primary and
secondary aluminum-coated mirrors to enhance reflectivity. 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.
 
Geometric fidelity in the NAC is very good: pre-flight analytical
calculations indicate distortions of less than a pixel at the corners
of the field of view, and subsequent observations of the Pleiades and
the open cluster M35 set the value to 0.45 pixels.
 
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 3 positions
per second in the NAC. A homing sensor on each wheel defines a home
wheel position: wheel positioning can be commanded absolutely or
relatively.
 
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 a narrow band H
alpha filter for lightning observations.
 
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.
 
Because of its reflecting optics and its unique ability to see in the
UV, only the NAC carries filters for UV observations. The lumigen
coating provides a unique spectral capability, unavailable on either
the Voyager or Galileo imaging systems, which Cassini carries to the
outer solar system for the first time. It enables spectral response
down to 200 nm. To take advantage of this capability, we have spanned
the range from 230 nm to 390 nm with three UV filters: UV1, UV2, and
UV3.
 
The NAC filter wheel also contains narrow-band filters for atmospheric
studies. Methane absorption bands and continuum wavelengths are
available using the MT1/CB1, MT2/CB2 and MT3/CB3 filters. (CB1 is a
2-lobed continuum filter, with lobes on each side of the methane
absorption band.) A HAL filter is also included for observing H-alpha
emissions from lightning.
 
Finally, the NAC carries three polarization filters covering the
visible spectrum: P0, P60 and P120. As their names indicate, these
polarizers have principle transmission axes separated by 60 degrees,
in order to measure intensity and the degree and direction of linear
polarization regardless of camera orientation. The NAC also has a
single infrared polarizer, IRP0.
 
The polarizers are, of course, to be used in combination with other
spectral filters, so filter placement was important. In the NAC, the 3
visible polarizers and the one IR polarizer can all be used in
conjunction with a suite of spectral filters on the opposite wheel
covering the UV to the near-IR.
 
Table 1: ISS NAC Filter Characteristics
 
Filter  Lambda_cen  Lambda_eff  Science Justification
----------------------------------------------------------------------
UV1     258W        264         aerosols
UV2     298W        306         aerosols, broadband color
UV3     338W        343         aerosols, broadband color,polarization
BL2     440M        441         medium-band color, polarization
BL1     451W        455         broadband color
GRN     568W        569         broadband color
MT1     619N        619         methane band, vertical sounding
CB1     619N        619         2-lobed continuum for MT1
CB1a    635         635
CB1b    603         603
RED     650W        649         broadband color
HAL     656N        656         H-alpha/lightning
MT2     727N        727         methane band, vertical sounding
CB2     750N        750         continuum for MT2
IR1     752W        750         broadband color
IR2     862W        861         broadband color; ring absorption band
MT3     889N        889         methane band, vertical sounding
CB3     938N        938         continuum for MT3, see thru Titan haze
IR3     930W        928         broadband color
IR4     1002LP      1001        broadband color
CL1     611         651         wide open, combine w/wheel 2 filters
CL2     611         651         wide open, combine w/wheel 1 filters
P0      617         633         visible polarization, 0 degrees
P60     617         633         visible polarization, 60 degrees
P120    617         633         visible polarization, 120 degrees
IRP0    746         738         IR polarization, see thru Titan haze
 
Table 2: NAC Two-Filter Bandpasses
 
Filters   lambd_cen   lambda_eff
--------------------------------
UV2-UV3   316         318
RED-GRN   601         601
RED-IR1   702         702
IR2-IR1   827         827
IR2-IR3   902         902
IR4-IR3   996         996
 
(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 NAC visible polarizers consist of a thin film (less than 1 microns
thick) of a polarizing polymer deposited between two fused silica
plates. The infrared polarizer has 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.
Ideal polarizers block only photons whose electric vector is
orthogonal to the principal axis of the polarizer. The visible
polarizers fall short of this ideal behavior in two ways. They
transmit too little of either polarization in the ultraviolet, and
too much of the light polarized orthogonal to the principal axis in
the near-infrared. Their performance is best between 450 nm and 650
nm where the principal axis transmission is between 0.45 and 0.65,
and the orthogonal transmission is less than 1%. The useable range of
the visible polarizers extends from the UV3 filter near 350 nm to the
CB2 filter at 750 nm. The infrared polarizer has much better
performance over its 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 NAC for commanded exposure times of 5, 25 and 100
ms were measured to be 0.98, 1.52 and 0.97 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 NAC,
this column is illuminated for ~ 0.3 msecs 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