vector and which must be identified by the user. (Many of these are
flagged in the 
vector, however.) Help in identifying
these problems is available from the IUEDAC and operations staffs.
Telemetry Dropouts: Telemetry dropouts may occur when the signal from the spacecraft is interrupted during transmission of the image. Dropouts may be identified from the scripts or image labels. MISDAT may also be used to locate the missing or questionable quality data with respect to the order(s) of interest.
Microphonics: Microphonics or periodic noise superimposed on the spectral image can occur in some IUE images. In SWP images, the interference is often observed at a low level (i.e., 1-3 DN) over most of the image, although higher amplitude noise is sometimes observed in a narrow band. In LWR images, the microphonic noise is generally restricted to narrow bands which can severely contaminate portions of the spectrum. IUESIPS flags only LWR images processed since the fall of 1981. Affected data may otherwise be identified either from comments in the label or from visual inspection of the raw data.
Cosmic Ray Hits and Hot Pixels: The IUE detectors are efficient detectors of particle radiation. In addition, radioactive decays in the camera phosphor material can cause sharp bright spots in the image. These ``hits'' are not always identified in the data quality vector but usually can be recognized by a quick inspection of the raw image. Normal incidence cosmic ray hits resemble sharp bright spots while oblique incidence hits look more like comets. Bright spots with intensity >90 DN above the local background are flagged by IUESIPS for images processed after November 19, 1982. (Note that real emission lines are sometimes flagged this way as well.) Hits may be removed from low dispersion data using the blemish removal software discussed in the chapter on data correction.
Residual Images: Residual images occur when the IUE cameras are overexposed or optimally exposed many times in a several-hour period. Residual images may be identified by inspection of the raw image, checking the scripts, and noting the presence of over-exposures in the camera of interest prior to the acquisition of the program image.
Scattered Light: Scattered light is seen in SWP images of late-type stars
and may be identified as a relatively flat ``continuum'' seen shortward of
the flux drop-off of the star.
The classic signature of scattered light (though not always seen) is a
signal found
shortward of Ly
and continuing to the edge of the detector
faceplate. Scattered light may be most easily identified from visual
inspection of the digital data.
Camera Artifacts: Images with long exposure times may display ``features'' which are actually camera artifacts. Low dispersion spectra showing the artifacts are available for each camera for both point and extended source modes (see Chapter 11 ).