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Spectroscopic (Telluric) Standards

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Telluric standard star observations are required for all near-infrared spectroscopic observations to cancel telluric (atmospheric) absorption features in the data.  (Additional standard observations such as for flux calibration or radial velocity measurements are not part of the baseline calibration set, and are not discussed further here.)  The following is a guide to assist in selecting the most appropriate telluric standard stars.

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Airmass considerations - correcting for telluric absorption

When specifying standard stars, select stars whose air masses will be the same as the average airmass of the target when they are being observed. This is particularly important for observations in the L and M windows, but is also important for reducing spectra in the shorter wavelength bands. In general one should not choose standards which are at the same RA as the science target. Normally the standard will be observed just prior or just following the target, and this time interval should be taken into account, particularly if the integration on the target is a long one. As an example, if the science target spectrum will require 2 hours of time (including overheads), acceptable "before" and "after" standards would have approximately the same declination as the target but RAs approximately 1 hour less than and greater than the target. Other choices of RA and Dec also will satisfy the airmass matching criterion, but are more sensitive to the hour angle of observation.

Exposure Times for Standards

Because spectra are often obtained in better conditions than those specified in Phase2, exposure times on standard stars be set conservatively. For GNIRS one can refer to this table for guidance.

1-5um Spectral Features in Standard Stars: Which Spectral Type is Best?

So-called "telluric standards" are not standards in the classical sense at all; they are simply bright stars of known spectral type for which the intrinsic stellar features are either negligible, removable using model spectra, or easily separated from features introduced by the earth's atmosphere.  Spectral features in main sequence stars (i.e., dwarfs, luminosity class V) are present throughout the 1-5um spectral region.  The choice of the best spectral type depends on the resolution of your data and the wavelength regime of interest.  Early- to mid-A and late B dwarfs possess primarily hydrogen recombination lines, which are quite strong and highly pressure broadened.  However, these lines can be fit and removed in a fairly straightforward way, and techniques and software exist to do this, so these are the spectral types most commonly used. . Spectral type 0 and early B have very few spectral features, but are relatively rare and so may be hard to find at a good airmass match to your target.  If you are interested specifically in measuring hydrogen line profiles in your science targets, they may not be the best choice.  At G0V the hydrogen lines are considerably weaker (and many of the weaker lines seen in A dwarfs are essentially negligible); however, lines of Mg, Si, Fe and other elements have become strong.  Intrinsic lines in G2V spectra can be fit using high-resolution, high signal-to-noise solar spectra.  At early-mid F many of the hydrogen lines still are intermediate in strength and width and almost all of the lines of other atomic species are not yet very noticeable. Hence these spectral types may offer a reasonable compromise, particularly at low to moderate resolution and when the S/N of the target spectrum is low (<30, say). Later spectral types (late G, K and M) have too many intrinsic features to be generally useful for telluric corrections.  At high resolution O, B and early A dwarfs are best if the target spectrum is high S/N. In general one should look specifically at the wavelength range of interest to see where the telluric features fall, and where the stellar lines are for stars of various spectral type.  This is only a brief summary-- listed below are a few references from the literature on this subject:

A Method of Correcting Near-Infrared Spectra for Telluric Absorption, Vacca W., Cushing M. & Rayner J., 2003, PASP, 115, 389 [ADS]

Penetrating the Fog - Correcting Ground based CCD Spectroscopy, Stevenson, C.C., 1994, MNRAS, 267, 904 [ADS]

A Spectral Atlas of Hot, Luminous Stars at 2 Microns, Hanson M., Conti P. & Rieke M., 1996, ApJS, 107, 281 [ADS]

A list of hydrogen recombination line wavelengths is available.

See the NOAO Digital Library for a collection of medium and high-resolution stellar spectra in the infrared (J, H, K and L) and a high-resolution telluric spectrum from 1-5um

Magnitude limits

Magnitude limits vary with instrument and mode; higher resolution observations naturally require brighter standards.  In order not to degrade the science data during telluric division, signal-to-noise in the standard observations should be several times that of the science data whenever possible.  One should endeavour to find the brightest standards that can be used for a given instrument and mode such that e.g. S/N >100 is achievable in no more than a few minutes on-source. See the individual instrument webpages for guidelines.

Obtaining 2MASS JHK magnitudes of spectroscopic standards from the OT using the Catalog Navigator

To obtain 2MASS JHK magnitudes of your selected standard, do the following in the OT:

  • a. In the "Target Environment," enter the name of the std (e.g. HIP 23456) and hit "return," and then click on "Image.":

  • b. On the image screen (i.e., "Position Editor") click on the "Catalog" pulldown menu. First click on Image Servers and select the one you want; then click on "Catalogs" (at very top of pulldown menu) and select 2MASS.

  • c. Again on the image screen, open the "Catalogs" (Catalog Navigator) window which gives a table of all 2MASS objects. Then in the image itself, click on the std; this will highlight the std and its JHK magnitudes in the "Catalog Navigator" table.

  • d. Check to see that the JHK magnitudes and colors do not suggest a bright unresolved companion.

Hipparcos Standards

This table lists all 19,015 stars included in the Gemini telluric search utility grouped by spectral type and sorted by right ascension.

Hipparcos Telluric Standard Stars

You can use the CADC Hipparcos catalogue search engine to find your own Hipparcos calibration stars. The Canadian Astronomy Data Center is operated by the Dominion Astrophysical Observatory for the National Resarch Council of Canada's Herzberg Institute of Astrophysics.

Verify your standard stars

Many stars in the Hipparcos catalog are not suitable to be used as telluric standard stars, either because of multiplicity or peculiarities in their spectra. It is the PI's responsibility to verify that the standard stars they have selected in their Phase II will meet their scientific requirements. A good way to check your star is to look it up in Simbad.

Very bright star lists

For very high resolution (Phoenix and GNIRS R=18000), this table of Telluric Reference Stars for Cerro Pachon (from NOAO), provides lists of very bright stars (limiting magnitude 3) in the southern hemisphere, grouped by local sidereal time.

Updated 2006 June 8; Andrew Stephens & Bernadette Rodgers

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