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