Any star within 
of the ecliptic plane passes within 10 
of the sun for about 5 days once per year. There are
10-20 UV bright stars which pass this close.
and which have been observed
with Voyager and IUE. About 50 stars can be observed with the
WLC. They
can be used for visible radiometric
calibration and co-registration with LASCO.
The position of the star relative to the sun must be accurately
predicted in order to be centered in the 
arcsec
field of view of the WLC.
The WLC has no capability of tracking the star along its path, since
two mechanisms, rolling and mirror rotation, would be involved.
Therefore, the best position for observing a star must be predetermined
and then the instrument will sit and stare at the star transit.
A star will move across the pinhole in about 6 minutes.
The successful pointing and detection of a star will permit the spatial
co-registration with LASCO.
The brightness lower limits to a star detection are given by the
brightness of the corona and by the electrical noise of the PMT.
Nevertheless, since a background (corona + PMT dark counts)
integration can be performed right after the star transit, a
background subtraction gives the brightness of the star, which can be
used as a second way of calibrating in intensity the WLC, and also for a
radiometric cross-calibration with LASCO.
Repeated stellar observations with the EUV channels will
track the
changes of instrument sensitivity
due to detector degradation and other causes.
Annual observations of several stars will track
sensitivity changes, and comparison
with Voyager and IUE observations will check the
calibration against an independent
intensity scale,
which is claimed to be good to about 15%.
The best star for this purpose
is the 
sdO star Feige 110. Some B stars are brighter,
but some of them may vary in the UV.
The stellar spectra will have strong, broad interstellar
Ly
absorption, so grating positions away from those
used for solar observations should be used.
Intensity comparison with O VI channel stellar observations will
be complicated by the overlap of the primary instrument response and the
redundant Ly-
response, so that light at two different wavelengths
will fall on each pixel. It will be possible, though laborious, to use
strong absorption features in the spectrum and multiple grating positions
to separate the two contributions to the count rate.
Observations of interstellar absorption
lines can also compare the wavelength scale to those of stellar instruments.
Measurement of residual flux at the bottoms of
saturated interstellar absorption lines will place
limits on scattered light from the grating.
If the pointing can
be accurately enough predicted, we can let a star move
along the length of the slit to build up counts.
The numbers in the first column pertain
to 
Tau (B3 V, 
), which approaches the Sun on June 2. It
will have count rates
of 18 and 7.3 
at 1240 and
1040 
, respectively. The last column pertains to WLC observations.
Instrument Calibration with Stellar Observations
** For this example, the slit is placed at 5 
at a roll angle
of 
from ecliptic north, and the star drifts across the middle of the
slit in about 6 minutes. The start time for this observation must be specified carefully.