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Project DR-05

Sky spectrum model for the VLT and E-ELT ETCs


The sky spectrum model should provide, over any requested wavelength range in the domain 300 nm to 28 μm, a sky spectrum up to a resolution of 150,000 (actual maximum accuracy depends on input database provided by ESO) including OH lines and thermal components. The available data need to be analyzed in order to establish relevant time scales for the variation of the relative line intensities during a night. On basis of these results an observation strategy for obtaining further data probably needs to be developed. With these data a proposal for the development of a reliable sky model based on observed spectral key features (e.g. specific night sky emission lines), can be developed.


A general description of the current summary of knowledge about the sky spectrum models used at ESO for the E-ELT (http://www.eso.org/sci/facilities/eelt) and ETCs (http://www.eso.org/observing/etc) is available at the URL: http://www.eso.org/sci/facilities/eelt/science/drm/tech_data/background/


In addition, the following articles may be of interest: 

  • Patat, F., 2008, The dancing sky: 6 years of night-sky observations at Cerro Paranal, Astron.Astrophys., 481, 575-591
  • Patat, F., et al., 2006, UBVRI twilight sky brightness at ESO-Paranal, arXiv:astro-ph/0604128
  • Hanuschik, R.W., 2003, A flux-calibrated, high-resolution atlas of optical sky emission from UVES, Astron.Astrophys., 407, 1157-1164
  • Rousselot, P. et. al., 2000, Night-sky spectral atlas of OH emission lines in the near-infrared, A&A, 354, 1134-1150.
  • Osterbrock et al., 1996, Night-Sky High-Resolution Atlas of OH and O2 Emission Lines for Echelle Spectrographs Wavelength Calibration, PASP, 108, 277-308, as well as PASP 109 614-627.
  • Ramsay et al., 1992, Non-thermal emission in the atmosphere above Mauna Kea, MNRAS, 259, 751-760


Applicable instruments and modes

The main domain of application is for the ESO Exposure Time Calculators (ETC), the sky model is used for all existing instruments, and also for preliminary studies, including the E-ELT ETC.

Required archive, calibration or observation data

The scope of this project will require extensive archive data retrieval and processing for different instruments in different bands and at different spectral resolutions. This project may require the acquisition of new observation data.

Required algorithm developments

A recent evaluation by ESO (October 2008) of the available models and observation data yielded the conclusion that, in the optical, we can expect significant improvement of the model currently used by the ETCs by including much improved measurements description of dark time sky brightness values, their dependence on various parameters, a much better description of the lunar and zodiacal components, and eventually by incorporating much improved measurements of the extinction. In the NIR there are essentially no new theoretical advances or new data, but a re-implementation of the existing methods in a consistent and well documented and tested ANSI C-based framework will be necessary for the ETCs, the E-ELT studies, etc The improvements in the NIR will mostly consist of the inclusion of seasonal OH variations and eventually of an empirical OH line list from CRIRES. In the MIR the present situation is essentially already satisfactory and so no improvements to the accuracy are foreseen.

A detailed technical specification of the sky spectrum model for VLT and ELT ETCs is in preparation. It describes the different components of the sky spectrum in the optical, NIR, and MIR domains.


Required software developments

The software for DR-05 will be generally developed following the guidelines of the VLT-SPE-ESO-19000-1618 document. For these modules, the implementation language will be ANSI-C (not using the CPL library), and will include test reports and user documentation.



The sky spectrum model will be validated primarily by comparison with dispersed observation data taken from different ESO instruments in the optical, NIR, MIR domains at different resolutions, and also by comparison with the present model used in the ESO Exposure Time Calculators (www.eso.org/etc).

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