3.4. Dependence with Focus and Temperature

Early in the IUE mission it was discovered that the camera image format shifts frequently in both the spectral and spatial directions with time and camera temperature. Camera temperature was measured roughly by thermistors located in any of several areas of the camera head amplifier (not an ideal location to measure the temperature of the spectrograph bench itself), and one of these was selected to provide a consistent refererence. This provided a ``THDA" index which could be used to calibrate wavelength zero-point shifts and apply them to the processing of a particular observation in IUESIPS and NEWSIPS. Motion of the spectral format on the detector was generally caused by telescope flexures and, under routine operating conditions, secondarily by changes in electro-optical properties of the camera. Thus, fluctuations in satellite temperature (which was primarily a function of the telescope's orientation angle with respect to the Sun) caused the telescope to expand or contract and the focus to change. The focus was usually controlled indirectly by turning on or off a heater located behind the primary mirror. An alternate control procedure consisted of toggling a heater mounted on the spectrograph deck. Although the camera temperature and detector image shifts were correlated, a hysteresis arising from the instrument's thermal history tended to weaken the efficacy of controlling the focus by commands to the spacecraft. This control was weakened further by other environmental factors, such as the Earth's eccentric orbit around the Sun and earthglow. Consequently, the IUE Operations team developed a conservative strategy of maintaining the THDA temperature within certain limits. During the course of the IUE mission statistical correlations were derived from shifts of WAVECAL spectra as a function of the THDA and focus parameters. These shifts were parameterized by means of a polynomial fit, so NEWSIPS could remove them to first order in its assignment of wavelengths.

As part of our cross-correlation analysis, we searched for a correlation between zero-point shifts and THDA and focus values at the time of an observation. Generally, we were unable to find a convincing correlation. Except for an accident of an unrelated contemporaneous research program, this search might have stopped with this null result. However, while investigating spectral variations in B stars for another research program, we used the cross-correlation tools from the present study to shift spectra to the same wavelengths before manipulating the data further. In doing so, we noticed a time-dependent cycle of about a day (see Figure 6) in the results for a time series of 22 observations conducted in 1995 on the B3Ve star $\alpha$ Eri. In searching through the IUE archival database further, we found an extraordinary set of 181 continuous observations on the B0.5V star $\epsilon$ Per in 1996 obtained by Dr. D. Gies. Figure 7 shows that the same one-day cycle is present in the data for this star. A string of observations of HD 93521 at 1994.2 (not shown) exhibits a similar 1-day period and few km s^-1 semiamplitude over three days.

Figure 6
Cross-correlation shifts of velocity, telescope focus, and THDA temperature for an intensive time series of SWP high-dispersion observations of $\alpha$ Eridani. Dotted and dashed lines (shown for reference only) represent the x- and y-components of the satellite's orbital motion (``x" is directed toward the Sun; ``y" its perpendicular in the Earth's equatorial plane). THDA is in degrees Centigrade; focus is in instrumental units.

Figure 7
Cross-correlation shifts of velocity, telescope focus, and THDA temperature for an intensive time series of SWP high-dispersion observations of $\epsilon$ Persei. For reference only: a cross-correlation between the apparent velocities and the x-component of the (diurnal) orbital velocity shows that the velocities show a mean lag of 0.15 cycle behind the x-component.

Because the cycle lengths in these shifts are all close to one day, we investigated first whether the satellite's orbital motion might have somehow been neglected in determining wavelengths for an individual observation. However, we were able to discount that possibility.

A more successful attempt to explain the apparent 1-day velocity cycles of $\alpha$ Eri, $\epsilon$ Per, and HD 93521 was to investigate the effects of varying the instrument temperature by adjusting the telescope focus. Cycling the camera-deck heater produced correlated responses of the THDA and telescope-focus values, especially when the ambient spacecraft temperature was lower than nominal, as for the 1996 observations of $\epsilon$ Per. During this particular monitoring series, the good correlation between temperature and focus indicates that the focus was controlled by the deck heater. In the time series on $\alpha$ Eri the temperature was not quite so low. Then an adequate means of controlling the focus was to cycle the more distant telescope mirror heater. Since the telescope heater was isolated from the spectrograph, the locally measured THDA value did not correlate well with the focus changes of the telescope (see lower dashed line in figure). However, in either case the principle was the same, that heating could be applied to prevent the focus from drifting to large negative values. The important point is that this application caused an overcorrection of the focus. The focus values would then swing (relatively) positive until a low ambient temperature again reversed the change of the focus, causing a new thermal-focus cycle to ensue. All told, the changes in focus from either thermal-control technique caused the velocity to become first too negative and then too positive by a few km s^-1. Although we have found the velocity-focus correlation only in SWP camera datasets so far, it is probably present in long-wavelength data as well.

Figure 8 exhibits the correlation explicitly between telescope focus value and zero-point for the observing campaign on $\epsilon$ Per. A similar plot can be constructed for THDA instead of focus, but the $\alpha$ Eri data imply that the correlation of velocity with THDA for the $\epsilon$ Per data is a result of temperature excursions changing the focus, and not a shift caused by the temperature variation within the camera. Note especially that Fig. 8 shows that the correlation arises only in the limited focus range of -2.0 to -3.7 (instrumental units). As noted above, we also searched our results for a dependence on focus and THDA values in our study of time-dependent correlations, but we found none. We suspect the reason is that IUE observations of most objects were obtained at different epochs and with different target-centering practices. This would tend to conceal any correlation over a small critical subrange of focus values in our searches for trends in the stars of Table 1. However, we expect the pattern shown in Fig. 8 is present in all high-dispersion data to some extent. If so, it is a a secondary source of radial velocity error.

Figure 8
Velocity differences vs. IUE telescope focus for echellograms of the data depicted in Fig. 7