Telluric and spectro-photometric calibration
Telluric correction: The most prominent feature in IR spectra are the telluric features of the Earth's Atmosphere. Unfortunately, some of the telluric lines do not scale linearly with airmass, so it is necessary to observe a star, which we will call a telluric standard, at the same airmass and with the same instrument setup as those of the science target. This standard is divided directly into that of the science target. Flux calibration: In the optical domain, one uses spectrophotometric standards that have tabulated fluxes and wavelengths. In the IR, there are no such standards. There are some pseudo-standards, in the sense that tables of flux versus wavelength do exist for some stars, but their fluxes are derived from models (for DA white dwarfs) and scaled with the observed photometry in the near-IR bands.
In KMOS, both standards (Telluric and flux specto-photometric) are taken with the same template, the
- Telluric standard: Only the Spectral Type parameter is filled.
- Spectro-photometric standard: Only the magnitude in the observed band is given.
- Both Telluric and spectro-photometric standard: Both magnitude and spectral Type parameter are completed.
To save time, the observer is encouraged to select standard star that can be use for both Telluric and spectro-photometric calibration.
Choosing the standard star for both telluric and spectro-photometric calibration
The stars should:
- be a hot star between B0 and B9, with negative V-I colors, a solar analogue G0V to G4V, or a Vega analogue;
- be without compagnion and not variable;
- have avaible magnitudes in the requiered near-IR bands;
- To maintain the exposure time to reasonable values the magnitude of the star should be between 6.6 to 7.5 in the observed band.
As a reference, suitable stars selected from the XHIP expended Hiparccos compilation (Anderson et al. 2012) are provided below:
- Solar Analogue G0V-G4V: text- VO table - Skycat catalogue
- Hot stars B0-B4V: text- VO table - Skycat catalogue
- Hot stars B5V-B9V: text- VO table - Skycat catalogue
- Vega Analogue A0V: text- VO table - Skycat catalogue
The field of view of KMOS IFU is small, 2.8"x2.8", it is thus important to have the coordinates of the standard and those of associated guide star with a good astrometry. As such:
- Do not forget add the proper motions of the standard star in the target information.
- Avoid guide stars with high proper motion. The I/322 catalogue, available from Vizier, is a good source of guiding stars containing information on the proper motion.
Exposure times can be estimated using the KMOS ETC. The spectrum should have signal to noise over 100 per pixel in order to properly correct from the telluric lines. Indicative exposure time are tabulated hereafter: