Instrument Information
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| IDENTIFIER |
urn:nasa:pds:context:instrument:issna.co::1.1
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| NAME |
IMAGING SCIENCE SUBSYSTEM - NARROW ANGLE
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| TYPE |
IMAGER
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| DESCRIPTION |
Introduction to the Cassini Imaging Science Subsystem: Narrow Angle Camera Instrument Overview=================== The Cassini ISS consists of two fixed focal length telescopes, anarrow angle camera (NAC) and a wide angle camera (WAC). The NAC is95 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 massof 57.83 kg, and sit on the Remote Sensing Palette (RSP), fixed tothe body of the Cassini Orbiter, between the Visual and InfraredMapping Spectrometer (VIMS) and the Composite Infrared Spectrometer(CIRS), and above the Ultraviolet Imaging Spectrometer (UVIS). Theapertures 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 subassemblyand the main electronics subassembly. The Sensor Head electronicssupports the operation of the CCD detector and the preprocessing ofthe pixel data. The Main Electronics provide the power and performall other ISS control functions, including generating and maintaininginternal timing which is synchronized to the Command Data System(CDS) timing of 8 Hz, control of heaters, and the two hardware datacompressors. The Cassini Engineering Flight Computer (EFC) is aradiation-hardened processor that controls the timing, internalsequencing, mechanism control, engineering and status dataacquisition, and data packetization. The NAC is an f/10.5 reflecting telescope with an image scale of ~6microrad/pixel, a 0.35 deg x 0.35 deg field of view (FOV), and aspectral range from 200 nm - 1100 nm. Its filter wheel subassemblycarries 24 spectral filters: 12 filters on each of two wheels. Thisallows for in-line combinations of filters for greater flexibility.Each wheel is designed to move independently, in either the forwardor reverse direction, at a rate of 3 positions per second. A homingsensor on each wheel defines a home wheel position, and wheelpositioning can be commanded absolutely or relatively. Unlike the WAC, the NAC is thermally isolated from the RSP in order tominimize the effects of RSP thermal transients on the NAC imagequality. 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. Thetemperature of the optical elements is controlled by active heaterspositioned along the optical path. These optical elements are kept towithin 1 degree Celsius to maintain camera focus without an activefocusing mechanism. Low expansion invar spacers are also used. Theradiator subassembly also includes two sets of spacecraft-controlleddecontamination heaters which are used to minimize deposition ofvolatile contaminants on either the detector or radiator and tominimize radiation damage to the CCD. All heaters are commandable (ONor OFF) during flight. Optics------ The narrow angle camera optics were specially designed to improve onthe quality and resolution of images of the bodies in the Saturnsystem returned by Voyager. It is based on a Ritchey-Chretienreflector design. The focal plane field of view is limited by thesize of the CCD. The NAC point spread function (PSF) was designed tobe 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 throughthe clear filters is 1.3 pixels. The nominal pixel scale is 5.9907microradians/pixel. All the reflective optical elements within the NAC (the primary andsecondary mirrors) are manufactured of fused silica; all refractiveNAC elements (such as the field correctors and the window on thesealed CCD package) are made of either fused silica or single-crystalvacuum-UV-grade calcium fluoride. Antireflection coatings consistingof single layer MgFl2 were deposited on the field correctors and CCDwindow; a multi-layer MgFl2 coating was applied to the primary andsecondary aluminum-coated mirrors to enhance reflectivity. A fusedsilica quartz plug is placed immediately in front of the CCD packageto protect the detector against radiation damage and to minimizeradiation- induced noise in the images. Geometric fidelity in the NAC is very good: pre-flight analyticalcalculations indicate distortions of less than a pixel at the cornersof the field of view, and subsequent observations of the Pleiades andthe open cluster M35 set the value to 0.45 pixels. Filters------- The ISS filter assembly design -- consisting of two filter wheels anda filter changing mechanism -- is inherited from the Hubble SpaceTelescope WF/PC camera. Each wheel is designed to move independently,in either the forward or reverse direction, at a rate of 3 positionsper second in the NAC. A homing sensor on each wheel defines a homewheel position: wheel positioning can be commanded absolutely orrelatively. The Cassini Imaging Science Team has deliberately duplicated 63% ofthe filters in both the NAC and WAC. These include sevenmedium/broadband filters from the blue to the near-IR forspectrophotometry, 2 methane and 2 continuum band filters foratmospheric vertical sounding, 2 clear filters, and a narrow band Halpha filter for lightning observations. The clear filter is in the 'home' slot of each filter wheel, since itwas deemed that sticking of a filter wheel, should it occur, was mostlikely to occur in the home position. Typically a clear filter in onewheel is combined with a color filter in the other wheel, thoughtwo-filter combinations can also be used. Because of its reflecting optics and its unique ability to see in theUV, only the NAC carries filters for UV observations. The lumigencoating provides a unique spectral capability, unavailable on eitherthe Voyager or Galileo imaging systems, which Cassini carries to theouter solar system for the first time. It enables spectral responsedown to 200 nm. To take advantage of this capability, we have spannedthe range from 230 nm to 390 nm with three UV filters: UV1, UV2, andUV3. The NAC filter wheel also contains narrow-band filters for atmosphericstudies. Methane absorption bands and continuum wavelengths areavailable using the MT1/CB1, MT2/CB2 and MT3/CB3 filters. (CB1 is a2-lobed continuum filter, with lobes on each side of the methaneabsorption band.) A HAL filter is also included for observing H-alphaemissions from lightning. Finally, the NAC carries three polarization filters covering thevisible spectrum: P0, P60 and P120. As their names indicate, thesepolarizers have principle transmission axes separated by 60 degrees,in order to measure intensity and the degree and direction of linearpolarization regardless of camera orientation. The NAC also has asingle infrared polarizer, IRP0. The polarizers are, of course, to be used in combination with otherspectral filters, so filter placement was important. In the NAC, the 3visible polarizers and the one IR polarizer can all be used inconjunction with a suite of spectral filters on the opposite wheelcovering the UV to the near-IR. Table 1: ISS NAC Filter Characteristics Filter Lambda_cen Lambda_eff Science Justification----------------------------------------------------------------------UV1 258W 264 aerosolsUV2 298W 306 aerosols, broadband colorUV3 338W 343 aerosols, broadband color,polarizationBL2 440M 441 medium-band color, polarizationBL1 451W 455 broadband colorGRN 568W 569 broadband colorMT1 619N 619 methane band, vertical soundingCB1 619N 619 2-lobed continuum for MT1CB1a 635 635CB1b 603 603RED 650W 649 broadband colorHAL 656N 656 H-alpha/lightningMT2 727N 727 methane band, vertical soundingCB2 750N 750 continuum for MT2IR1 752W 750 broadband colorIR2 862W 861 broadband color; ring absorption bandMT3 889N 889 methane band, vertical soundingCB3 938N 938 continuum for MT3, see thru Titan hazeIR3 930W 928 broadband colorIR4 1002LP 1001 broadband colorCL1 611 651 wide open, combine w/wheel 2 filtersCL2 611 651 wide open, combine w/wheel 1 filtersP0 617 633 visible polarization, 0 degreesP60 617 633 visible polarization, 60 degreesP120 617 633 visible polarization, 120 degreesIRP0 746 738 IR polarization, see thru Titan haze Table 2: NAC Two-Filter Bandpasses Filters lambd_cen lambda_eff--------------------------------UV2-UV3 316 318RED-GRN 601 601RED-IR1 702 702IR2-IR1 827 827IR2-IR3 902 902IR4-IR3 996 996 (All wavelengths in nm. Central wavelengths (lambda_cen) are computedusing the full system transmission function. Effective wavelengths(lambda_eff) are computed using the full system transmission functionconvolved with a solar spectrum. Bandpass types: SP = short wavelengthcutoff; W = wide; N = narrow; LP = long wavelength cutoff.) With the exception of the clear filters and the polarizers, thefilters are all interference filters manufactured using an ion-aideddeposition (IAD) process which has the effect of making the filterstemperature and moisture tolerant, and resistant to delamination.Conventional interference filters have passbands which shift withtemperature. The shift can be significant for narrowband filterstargeted to methane absorption bands or the H_alpha line. Temperatureshifts for IAD filters is typically an order of magnitude or moresmaller than for conventional filters and is insignificant over thetemperature range (room temperature to 0 degrees C) relevant tocalibration and operation of the Cassini cameras. The NAC visible polarizers consist of a thin film (less than 1 micronsthick) of a polarizing polymer deposited between two fused silicaplates. 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 isorthogonal to the principal axis of the polarizer. The visiblepolarizers fall short of this ideal behavior in two ways. Theytransmit too little of either polarization in the ultraviolet, andtoo much of the light polarized orthogonal to the principal axis inthe near-infrared. Their performance is best between 450 nm and 650nm where the principal axis transmission is between 0.45 and 0.65,and the orthogonal transmission is less than 1%. The useable range ofthe visible polarizers extends from the UV3 filter near 350 nm to theCB2 filter at 750 nm. The infrared polarizer has much betterperformance over its range (700 nm - 1100 nm) where the principaltransmission is greater than 0.9 and the orthogonal transmission is0.001 or less. Shutter------- Between the filter wheel assembly and the CCD detector is the shutterassembly, a two blade, focal plane electromechanical system derivedfrom that used on Voyager, Galileo and WFPC. To reduce scatteredlight, the shutter assembly was put in the optical train `backwards ,with the unreflective side towards the focal plane. Each blade movesindependently, actuated by its own permanent magnet rotary solenoid,in the sample direction: i.e., keeping the blade edge parallel to thecolumns of the CCD. The shutter assembly is operated in 3-phases: open(one blade sweeps across the CCD), close (the other blade sweepsacross the CCD to join the first), and reset (both bladessimultaneously sweep across the CCD in the reverse direction to thestart position). There are 64 commandable exposure settings which can be updated duringflight if so desired. These correspond to 63 different exposure times,ranging from 5 milliseconds to 20 minutes, and one `No Operationsetting. The shortest nonzero exposure is 5 msec. In the ISS flightsoftware, the time tag on the image is the time of the close of theshutter. Because of mechanical imperfections in the shutter mechanism,there is a difference between the commanded exposure time and theactual exposure time, and a gradient in exposure time across the CCDcolumns. At an operating temperature of 0 degrees C, the meandifferences in the NAC for commanded exposure times of 5, 25 and 100ms were measured to be 0.98, 1.52 and 0.97 ms, respectively. In allcases 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 firstcolumn. This value is independent of exposure time and reasonablyindependent of temperature. The expected precision or repeatabilityof an exposure (equal to the standard deviation of actual exposuredurations measured at any one location on the CCD in ground tests) is
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| MODEL IDENTIFIER |
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| NAIF INSTRUMENT IDENTIFIER |
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| SERIAL NUMBER |
not applicable
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| REFERENCES |
Porco, C.C., R.A. West, S. Squyres, A. McEwen, P. Thomas, C.D. Murray, A. DelGenio, A.P. Ingersoll, T.V. Johnson, G. Neukum, J. Veverka, L. Dones, A. Brahic, J.A. Burns, V. Haemmerle, B. Knowles, D. Dawson, T. Roatsch, K. Beurle, and W. Owen, Cassini Imaging Science: Instrument Characteristics and Capabilities and Anticipated Scientific Investigations at Saturn, Space Science Reviews 115,363-497, 2004.
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