A demonstration of several instrumental features is presented in this
figure. It shows the spectrum of the bright rapidly rotating
B-star alpha Eri at a central wavelength of 5395 Å. In this
wavelength region this star has no spectral features of its own and thus
serves as a continuum source.
The lower curve is the extracted spectrum without flatfielding. It
shows a ghost feature centered on pixel 2200.
Also seen is an
interference pattern as a ripple with a
period of about 40 pixels. In addition a number of sharp spikes are seen
separated by 512 pixels which are detector imperfections.
We refer to the CES Users Manual for a detailed discussion of these effects
and suggestions on how to deal with them.
The other two curves (shifted upwards for display purposes) show
versions of this spectrum after flatfielding with two different methods.
The middle spectrum corresponds to the often used method for fibre-fed
spectrographs to first extract the spectrum of star and flatfield and then
divide. This efficicently removes the spikes, but a residual of the
ghost feature is seen. The upper spectrum was produced by dividing the
star frame with a flatfield normalized by its mean profile and subsequent
extraction of the result. The correction of the ghost feature is much
better with this method.
None of the shown flatfield methods can remove the interference
pattern, which has a different amplitude and shape in the flatfield and
the star exposure. As detailed in the
CES User Manual, the removal of this pattern
must be done in star and flatfield independently and preceed the