[ ESO ] La Silla

La Silla - Science Operation Department
CES image slicer diffuse light monitoring

Monitoring of image slicer diffuse light

The CES suffers from scattered diffuse light, that contaminates the background. This light has another wavelength dependence than the normal spectrum.

All measurements were done using the saturated (interstellar) NaI D lines (5890 Å and 5896 Å) seen in certain stars. The uncertainties are internal errors estimated from the S/N of the spectrum, and may not reflect the true accuracy of the measurements. Only the high-resolution slicer is monitored. This information is also available in graphical form.

Scattered light measurements using saturated interstellar Na D lines.
Date Diffuse light [%]
2000-02-10 6.70 ± 0.01
2000-06-18 7.75 ± 0.30
2001-08-26 7.00 ± 0.10
2001-12-17 6.86 ± 0.17
2002-03-30 7.25 ± 0.33
2002-12-01 6.40 ± 0.30
2003-04-17 6.97 ± 0.25
2003-08-16 7.03 ± 0.18
2004-02-05 6.04 ± 0.12
2004-09-01 5.57 ± 0.25
2007-07-30 4.30 ± 0.30

At other wavelengths one must use a different approach, based on measuring equivalent widths (EW) in a solar spectrum obtained with the CES and comparing with a "canonical" solar spectrum. Such monitoring has recently (Jan. 2005) been started, and will eventually replace the use of Na D stars. Below we give the measured scattered light using this method:

Scattered light measured by EW method. The values are averages over the spectral coverage (40-60 Å).
DateWavelength [Å]Scattered light [%]Source
2005-03-2061546.1 ± 2.2Ganymede
2005-03-1758885.3 ± 2.8Ganymede

Users aiming at measuring accurate equivalent widths should correct their spectra for scattered light, using either the values quoted here, or (better) by estimating the scattered light in Solar Spectra. We are building a database of calibrations from where the relevant data can be downloaded.

[Observing] [ESO La Silla]  [ESO]  [Index]  [Search]  [Help]  [News]