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