Performance

The angular resolution of the three cameras is limited by the MTF of the image intensifiers. A typical plot of the MTF is shown in Figure 9. The dimensions of the image that is focused on the photocathode of the intensifier is 18.4 mm × 18.4 mm. For an array of 256 × 256 pixels the corresponding resolution is 7 line pairs/mm. The MTF is 0.45 and adequate for auroral imaging. For an array of 512 × 512 pixels, or 14 line pairs/mm, the MTF decreases to 0.15. For this reason the CCD output is summed into 2 × 2-pixel blocks. This CCD pixel block is referred throughout this paper as a pixel, although it is possible to telemeter the entire 512 × 512-pixel array. The angular resolution of the pixel blocks is suitable for auroral imaging. The pixel dimensions as a function of source distance for the three cameras are tabulated in Table 6.

The instrument sensitivity S is defined as the charge accumulation rate in a CCD pixel per unit brightness of the emission, in units of electrons/kR-pixel-s. The 2 × 2-pixel blocks are used for the calculation of sensitivity. The overall gain of the intensifier is sufficient to record individual MCP events, or avalanches, in the CCD. As an example, the sensitivity of the low-resolution camera at 557.7 nm is computed. The rate of intensifier avalanches, or counts, is determined by the area of the entrance pupil A = 3.14 cm², the pixel solid angle = 1.7 × 10^-7 sr, reflectivity of 6 mirrors = r⁶ where r = 0.90, transmission of the passband filter t_f = 0.38, transmission of the intensifier window t_w = 0.96, quantum efficiency of the photocathode = 0.10, and efficiency of the MCP to initiate an avalanche c = 0.5. Thus for a 1-kiloRayleigh (kR) aurora, the count rate R of the intensifier is

The integration time for accumulation of one count in a pixel is 2.5 s for a 1-kR aurora at 557.7 nm. The light gain of the image intensifier is set by ground command to provide 100 electrons in the 2 × 2-pixel block of the CCD for each avalanche in the MCP. Thus the sensitivity S of the low-resolution camera at 557.7 nm is 41 electrons/kR-pixel-s. The CCD can be operated in inverted and noninverted modes. For fully inverted operation the full well is about 3 × 10⁵ electrons/pixel. The dynamic range is then approximately 3000. The auroral intensities corresponding to a single count for several wavelengths and integration times are presented in Table 7 for the low-resolution camera. A similar summary of the brightnesses for filling the CCD well in the inverted mode is given in Table 8. The onset of nonlinear responses of the intensifier occurs only for the most intense aurora, 200 kR, with saturation at about 600 kR. For comparison the threshold brightness for one count in a pixel for the low-resolution camera during an integration time of 12 s is similar for that for a pixel in a DE-1 image acquired over a time interval of 720 s. The intensifier is operated at temperatures in the range -15° to 0° C. The background contributions at these temperatures that are due to photocathode thermionic emissions are 100 R. The rate of thermionic charge accumulation in the CCD at -90° C for inverted operation corresponds to an equivalent auroral brightness 1 R. The primary source of background will be due to penetrating energetic particle fluxes when the spacecraft passes through the inner radiation zone.

There are twelve filters on the filter wheel that is shared by the low-resolution and medium-resolution cameras. These twelve filters are listed in Table 9. The wavelength at peak transmission is , is the full width at half maximum for the passband, t_f is the filter transmission at and 5° C, and S is the sensitivity at in units of CCD electrons/kR-pixel-s. S is given in the table for the low-resolution camera. For the moderate-resolution camera these values for S must be multiplied by 4.

The Earth camera is equipped with a single broadband filter with full width at half-maximum transmission of 25 nm. The passband is 124 to 149 nm. The field-of-view is 20° × 20°. In the 256 × 256-pixel format the solid angle corresponding to a pixel is 1.9 × 10⁶ sr. The count rate R is 0.6 counts/kR-pixel-s. For a 12 s accumulation period the single-count threshold corresponds to 140 R. The sensitivity S for 100 CCD electrons/MCP avalanche is 60 electrons/kR-pixel-s. For operation of the CCD in the fully inverted mode the well depth is again about 3 × 10⁵ electrons. For a 12 s accumulation period the well is filled by a brightness of 420 kR. For comparison, the solid angle corresponding to a DE-1 pixel is 2.0 × 10⁵ sr, about 10 times larger than that of the Earth camera, with a single-count threshold of ~300 R/pixel during an accumulation time of 720 s.