  Spectroscopic diagnostics of solar coronal plasmas critically depends on the
uncertainty in the measured line intensities. One of the main sources of
uncertainty is instrumental scattered light, which is potentially most
important in low-brightness areas. In the solar corona, such areas include
polar and equatorial coronal holes, which are the source regions of the solar
wind; instrument-scattered light must thus pose a significant obstacle to
studies of the source regions of the solar wind. In this paper we investigate
the importance of instrument-scattered light on observations of equatorial
coronal holes made by the Hinode/EIS spectrometer in two different phases of
the solar cycle. We find that the instrument-scattered light is significant at
all temperatures, and in both regions it amounts to approximately 10% of the
average intensity of the neighboring quiet Sun regions. Such contribution
dominates the measured intensity for spectral lines formed at temperatures
larger than Log T = 6.15 K, and has deep implications for spectroscopic
diagnostics of equatorial coronal hole plasmas and studies of the source
regions of a large portion of the solar wind which reaches Earth. Our results
suggest that the high temperature tail of in the coronal hole plasma
distribution with temperature, however small, is an artifact due to the
presence of scattered light.
