Project DR-06

Telluric absorption correction

This project will provide a CPL-based tool for correcting telluric absorption features in reduced spectra. The tool described here is able to take into account the actual atmospheric profile (i.e., the variation of pressure and temperature with altitude), which allow corrections to reach an accuracy of better than a few percent. The development of the CPL-based recipes and Reflex workflows will be based on an IDL prototype code provided by ESO.

Telluric absorption features are commonly removed by the observations of telluric standard stars observed in the same direction as the science target, as soon as possible before or after the science target, and with a difference of airmass as small as possible. This method is the usual practice in IR and mid-IR observations, where telluric absorption lines are very numerous. In addition, it is also sometimes used in the optical.

This method works well, but has a number of drawbacks:

• Airmass difference should be as small as possible. At ESO, a rule of thumb is that the absolute difference of the airmass of the target and the airmass of the standard star should be smaller than 0.2. This means that the residual error on the correction of unsaturated features can be as large as 20%.
• If, for any reason, the telluric star cannot be observed, the science observations must usually be repeated.
• In case of bright science targets, telescope time spent on observing telluric stars may be larger than the one spent on the science target itself.
• For some instruments, the high resolution and small wavelength range (e.g. of CRIRES) makes the execution time spent on observing telluric stars larger, as it is unlikely that one telluric observation can be used for several science targets.

 An advance made in the modeling of the Earth’s atmosphere, and in the knowledge of the molecular parameters, makes it possible to use a model-based method to correct for telluric features.

References

• The idea of using models of the atmospheric transmission to correct for telluric features dates back (at least) from the early '70s. Improvements have occurred regularly; an example is the paper published by Bailey, Simpson & Crisp (PASP 2007, 119,228; http://www.journals.uchicago.edu/doi/abs/10.1086/512824, who discuss the problems associated with the 'telluric standard star' method.
• See also the paper Measuring the amount of precipitable water vapour with VISIR by Smette, A., Horst, H., Navarrete, J. 2007, Proceedings of the 2007 ESO Instrument Calibration Workshop, Springer-Verlag series "ESO Astrophysics Symposia", eds. F. Kerber & A. Kaufer (http://www.eso.org/sci/facilities/paranal/sciops/CALISTA/pwv/pdf/pwv_smette.pdf)

Applicable instruments and modes

Most algorithms required for this task have been implemented in the IDL prototype that will be provided by ESO. The prototype is currently evaluated on CRIRES data, but the method should work for other instruments (in particular, NACO, UVES, and X-SHOOTER red-ward of 700 nm should be investigated).

The applicability of the prototype code to ESO data is still under investigation: This tool has already helped to analyze a UVES gamma ray-burst spectrum. Applications to low-resolution spectra is, in principle, possible, but has not been examined so far. The utility of the tool in the framework of medium-resolution FLAMES-GIRAFFE observations is currently under investigation. Sky emission line spectra have already been used to improve the ISAAC wavelength calibration in the L and M bands (Papadaki & Schmidtobreick, 2007 ESO calibration workshop), and are at the base of the calibration of VISIR and CRIRES spectroscopy. The tool could also potentially be used for the sky subtraction of SINFONI or GIRAFFE data. Such applications have not been tested so far, but will be done in the next few months; OH emission lines are, however, likely to cause problems as their emission mechanism is very different from the lines considered here.

Required archive, calibration or observation data

Data from the above instruments are available from the VLT archive. Reduced UVES spectra are available, but other data sets will need to be reduced. Pipeline reduced spectra will be provided by the ESO Data Products Department.
 

Required algorithm developments

The IDL prototype to create atmospheric transmission spectrum relies on existing software packages and databases. Agreements may need to be obtained by ESO before the tools can be distributed or made available to the user community. The IDL prototype is making use of the following packages:

1. An atmospheric radiative transfer code: the Reference Forward Model, a FORTRAN-77 base code made freely available for use within ESO: http://www.atm.ox.ac.uk/RFM/index.html. Current agreement limits its use to within ESO. ESO will contact the authors of RFM to define the conditions of usage by the ESO community.
2. A database of molecular parameters: typically HITRAN (http://cfa-www.harvard.edu/hitran/). No distribution of this database can be done without prior agreement;
3. An atmospheric profile valid for the time of the observations: the tool is using GDAS, a product of the NCEP Global Data Assimilation System model, created by the Air Resources Laboratory of the National Oceanic and Atmospheric Administration (USA). Such an atmospheric profile provides reports the pressure, temperature and humidity for a number of layers of the atmosphere, at any point on the Earth for all recent dates (going back to December 2004), on a 3-hour grid basis (http://www.arl.noaa.gov/ready/cmet.html).

 The tool is currently written in IDL for development purposes. A first routine allows one to select the molecules that can appear in a given spectral range, as for example, in the science spectrum of interest. A second routine prepares the atmospheric profile; it can interpolate between two atmospheric profiles corresponding to times before and after the observation to provide an estimate of the atmosphere at the time of the observation. It has been mainly developed for telluric corrections of CRIRES data, but could certainly be adapted to spectra obtained by other instruments.

The specific developments for this project will include:

1. Definition of a product interface, conditions that must be fulfilled by a pipeline data product to be suitable for this task.
2. Definition of a database and model interface (access to HITRAN, database of atmospheric profiles, etc…).
3. Identification of the specific IDL fitting routines used in the prototype and design of a CPL-based equivalent.
4. The main interface to the model shall be provided as a CPL function for interfacing with the ESO operational environment, together with the associated CPL recipes and Reflex workflows.
5. In addition the recipe will be invoked from a Web interface like the ESO Exposure Time Calculators. ESO will provide support for the invocation of the CPL recipes from the Web.

Required software developments

All software will be developed following the guidelines of the VLT-SPE-ESO-19000-1618 document. The software delivery will include in particular the CPL-based recipes, test reports, user documentation and Reflex workflows. The low-level interface to the model shall be provided as a CPL function for interfacing with the ESO operational environment. A Web-based interface to the recipe will be developed with the support of ESO.

Validation

The validation of the CPL recipes will be performed against the IDL prototype for a selected set of observation data (e.g. CRIRES, NACO, UVES, X-Shooter).