[ ESO ] La Silla

La Silla - Science Operation Department
Scattered Light Automatic Analysis Project for CES

Scattered Light :: Overview

Scattered light (SL) is present in all spectrographs due to unwanted reflections, non-optimal optics, unwanted higher orders etc. Not all instruments suffer the same amounts due to differences in design, but typical values are around a few percent.

In order to fully characterize and monitor the performance of a given instrument it is necessary to measure the SL as often as possible and preferrably for the full wavelength range. To accomplish this, an automated procedure must be implemented for all instruments within HiRes.

Since the CES is the instument most severely affected by SL, it was decided to implement the monitoring for CES first.

(see the User Requirements)

The LiSca tool :: Overview

LiSca is a task meant to estimate the fraction of internal scattered light in CES (and in future HARPS and FEROS) spectra. This is accomplished by comparing the equivalent width of absorption lines in twilight solar spectra with the ones in a canonical solar spectrum, ideally free from the problem. LiSca was written and implemented by Luca Sbordone in October-November 2004, and is currently being tested.

Project status

  • November 2004: LiSca installed. Testing started.
  • December 2004: User requirements fulfilled (see report)
  • January 2005: Comparing Solar Spectra from various sources (day sky, Jupiter moon). Conclusion: day-sky spectra can not be used - the SL is x2 the result obtained for Jupiters moons. Results from Jupiters moons and Na D stars are consistent, check the SL monitoring page for details.
  • March 2005: LiSca has performed well at a number of wavelengths and is now accepted as the future tool for measureing scattered light in the CES at all wavelength settings.

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