[ ESO ] La Silla

La Silla - Science Operation Department


CES Instrumental stability

Instrumental stability

The active control of the CES predisperser and grating provides a very high wavelength stability that has been repeatedly verified. This stability implies that for many scientific programmes it is not necessary to take repeated wavelength calibrations during the night. Whether or not wavelength calibrations should be repeated depends, of course, on the specific requirements of the user for calibration accuracy. To aid this decision the typical variation of the central wavelength on the CCD is illustrated here.

[Instrumental stability in direction of dispersion]

Measured at 5400 Å a variation of only 0.05 pixels rms or 0.2 pixels peak-to-peak is seen in the course of a 10 hour night. This corresponds (at the given wavelength) to a variation in radial velocity units of only 25 m/s rms or 100 m/s peak-to-peak. The upper figure shows the wavelength drift behaviour in units of CCD pixels (y-coordinate is direction of dispersion) as measured during one night.

Observers should note in any case that La Silla observatory receives weak Earth quakes quite frequently which can occasionally lead to wavelength shifts larger than demonstrated here.

In the direction perpendicular to the dispersion (x-coordinate on the CCD) larger shifts of up to 1 - 5 pixels have been measured. An example is shown in the second figure. See the monitoring pages for recent results.

[Instrumental stability perpendicular to the dispersion direction]

These shifts correspond to a non-negligible fraction of the width of individual slices with consequences for the performance of flatfielding. If possible, it is recommended to take repeated sets of flatfield exposures during the night. At any rate before using the flatfields in the data reduction process they should be normalized, e.g. divided by their average profile.

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