3.9 Choice of Long-Wavelength Camera

The LWP is currently the prime camera for use by Guest Observers. Its characteristics have been described in several articles (Settle et al. 1981, Holm 1981, Barylak 1983, Cassatella and Harris 1983, Imhoff 1983, Thompson 1983, Harris 1984, Imhoff 1984a, Oliversen 1984, Harris 1985, Cassatella et al. 1988, Teays and Garhart 1990, and Gonzalez-Riestra 1990).

The LWR camera was the default long-wavelength camera until October 1983. The changeover was due to the appearance of a "flare" or discharge in the camera's ultraviolet converter (UVC). The flare had grown in strength, forcing the reconfiguration to a lower UVC voltage at which the flare did not appear. This reconfiguration has reduced the overall sensitivity of the camera by a factor of 1.37, but its performance is otherwise essentially unchanged (see Harris 1985 and Imhoff 1985).

Guest Observers may request use of the LWR camera in its new, lower sensitivity configuration. This may be arranged with the RA prior to the observing shift. However, the GO is expected to absorb the overhead required to switch between long-wavelength cameras. For thermal stability, only one of the two long-wavelength cameras may be used at any given time. A camera may be turned on and off only once during a given 8-hour shift. Finally, the LWP camera must be on at the time the spacecraft is handed over to the next observer unless he or she also wishes to use the LWR camera and has made advance arrangements to do so.

The following is a brief list of comparisons between the LWP and LWR cameras:

1. The overhead required to switch to the LWR camera will be 15 to 45 minutes, depending on how much of the camera's operations can be "hidden" in other activities such as slewing the spacecraft. The overhead to turn the LWP camera back on and verify its operational status will normally be about 15 minutes.
2. The sensitivity of the LWP camera is significantly greater than that of the LWR except at the shortest wavelengths (see Figure 3.5).
3. The LWP camera (low dispersion) has somewhat better signal to noise (S/N) at wavelengths longward of 2500 Å than the LWR.
4. The repeatabilities of the two cameras are about equal (i.e. about ±3.5 percent for binned, single, point-source spectra; Sonneborn and Garhart 1986).
5. The LWR camera is affected by microphonic noise; however, this can generally be avoided by requesting the use of a special avoidance technique (see Section 4.2) which requires an extra 4 minutes of overhead prior to reading the image.
6. The camera preparation sequence takes longer for the LWP camera because longer tungsten lamp exposures are required. The LWP SPREP cycle is about 4.5 minutes longer, the XSPREP cycle 10.4 minutes longer. The total SPREP and XSPREP cycle times are thus 18 and 35 minutes respectively.
7. Due to increased use, the LWP camera now experiences somewhat greater fogging due to phosphorescence during low radiation shifts than the LWR camera. The LWP is also about 80 percent more sensitive than the LWR to particle radiation during the US2 shift for a given exposure time (see Section 4.9). However, since the camera is also more sensitive to flux from the target, the fogging due to either radiation or phosphorescence is nearly constant when integrating to a given exposure level, such as 200 DN.
8. The sensitivity of the LWR camera has changed with time, differentially with wavelength. The LWP camera has shown a small linear degradation with time (see Teays and Garhart 1990).
9. The LWP camera has been known to experience failures in its scan control logic, but since the camera went into routine daily use in 1983, such anomalies have not occurred.