Telluric Standard Stars with EMIR

This page is a simple guide to correcting for the effects of the Earth's atmosphere for spectroscopic observations with EMIR. As telluric contamination is severe in the near-IR domain, every science case should attempt to correct for this effect. The most typical way one implements this correction is by observing a star that is known to exihibit a small number of spectral features in the wavelength region of interest. Using these observations the user creates a telluric model, which is then used to correct the final reduced science spectra.


Currently the user must include the additional time required to observe a telluric standard star in their total time requested budget.


How to select a good telluric standard

Selecting a good telluric standard requires several considerations:

It is important to take all of these into account when choosing a telluric standard star. In practice one observes a signle, non-variable star that is easy to model (i.e. shape dominated by blackbody, small number spectral features) in close proximity with the science observations with a S/N at least five times greater than the science object.


Lists of telluric standard stars

Potential telluric standard stars can be selected from the extended Hipparcos compilation ( Anderson et al. 2012), which have radial velocities and spectral types for all entries. Lists of the most appropriate choices of star are found in the links below:

The best choices for telluric standard stars will depend upon the science goals but typically stars where the features are well known and easilly modelled are good choices.

In addition, a useful resource for selecting a good telluric standard is to use the Gemini Telluric Standard Search facility, that allows users to search the Hipparcos catalogue for appropriate telluric standards.


Note: The lists above are taken from the ESO webpages, please check that these targets have the appropriate magnitude for your observations.


Checking your choices

Once one has selected a telluric standard star, it is of vital importance to check this choice. A simple checklist:

If the selected star selected meets these criteria, then it is likely a good choice for a telluric standard star.

Alternatives to telluric standard star observations [UNTESTED with EMIR]

A more complicated and untested way to implement a telluric correction is to use software like MOLECFIT to perform the telluric correction. This software minimises the time spent on sky by creating a synthetic atmospheric model by fitting the telluric features in the science data using an atmospheric transmission model.

The accuracy of this will be tested by the EMIR team and this page subsequently updated. Currently, we reccommend that users observe telluric standard stars as detailed above.