[ ESO ] La Silla

La Silla - Science Operation Department


CES Instrumental features

Instrumental features

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.

[Instrumental features: ghost, spikes, ripple, flatfielding]

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 flatfielding process.

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