Spectral line shifts were determined for a number of telescope pointings
relative to the line position at zenith. The set-up comprised Grism #8
with 0.7" slit and unbinned pixels. Three exposures of helium arc lamp
spectra were taken at each position except at zenith where 4 sets of 3
exposures each were taken. The results are plotted below.
Each consecutive circle represents 15 degrees of zenith distance.
The location of a box represents the corresponding telescope pointing.
The numbers within a box represent the shift, mean value (upper line) and
rms (lower line), in the spectrum between that position and zenith
NOTE : It has recently been noticed that strong
fringes in the red part of the spectrum are not eliminated competely using
dome flats. They did disappear when internal flats taken at the same position
were used. We are carrying out tests to see if this can be reproduced
and if it is related to flexure.