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Standard star observations are required for mid-IR imaging observations to allow the calibration of data onto a magnitude or flux scale. The following is a guide to assist in selecting appropriate flux standard stars. Two standard star observations must be included in the phase II definition for each observation lasting more than ~30 minutes, one airmass-optimized for measurement before the science target, and one for after the target. The Gemini staff member at the telescope will select one of these for baseline calibration, which will not be charged to the science program. If the PI desires both calibration stars or any additional calibration stars to be observed, these additional calibrations will be charged to the science program and should be accounted for in the initial time request and explicitly defined in the phase II observing programme. Please see the definition of baseline calibrations for more information, and the observing tool libraries for example observations. Additional standard observations for accurate PSF calibration are not part of the baseline calibration set.
Phase II science programs for imaging observations must specify standards for flux calibration, usually from the following lists:
- Complete set of "Cohen" standards; xml files for use in the observing tool are available from the OT library
- Table of primary and secondary Cohen standards
- Very bright southern standard stars (many of which will saturate Michelle and T-ReCS under good conditions unless the instruments are defocussed)
The Cohen System
Because of the high thermal background in the 8-25 micron wavelength range and the resulting relatively low sensitivity of ground-based mid-IR instruments, mid-IR observations are flux calibrated using a small number of bright standard stars. The most extensive effort to bring a set of infrared standards into a unified calibration scheme has been led by Martin Cohen. Cohen's basic product is a continuous spectrum of each star that is carefully tied to observational data. The spectra can be used to flux-calibrate mid-IR imaging and low-resolution spectroscopic data by smoothing to the appropriate resolution or integrating over the filter bandpass. (The calibration spectra are of relatively low spectral resolution, so further effort is needed to establish reliable calibration methods for R ~ 1000 spectroscopy.) Gemini recommends adopting Cohen's standards and calibration methods for mid-infrared imaging observations.
Cohen's work is described in a series of papers beginning in 1992. The paper which establishes a large set of standards scattered over the sky, "Paper X", Cohen et al. 1999, AJ 117, 1864, includes a summary of the previous papers. The numerical spectral data from the papers were tabulated in the AAS CDROM series. Cohen's primary and secondary standards (see below) along with the set of standards in Paper X are the main calibration stars which are used for T-ReCS and Michelle observations.
Cohen's calibration is based on two primary standards: Alpha Lyr (A0 V) and Alpha CMa (A1 V). Alpha Lyr is defined to be 0 mag between 1 and 20 microns but at longer wavelengths allowance must be made for emission from dust. For this reason Cohen favors Alpha CMa as the primary standard over all wavelengths. Kurucz model atmospheres fitted to observational data of these two stars define the absolute flux calibration (Cohen et al. 1992, AJ, 104, 1650, Paper I in the series).
Cohen extends his system to ten bright secondary standards which are tied to the primaries via well-documented ratio spectra obtained from several ground-based, airborne, and space-based platforms (Papers II, IV, VI, and VII). The secondary standards are K and M giants which exhibit broad CO and SiO absorption features in the M band and near 8 microns, so approximating their spectra with blackbodies would introduce calibration errors of 10-20%. Using Cohen's absolute spectra, errors should be under a few percent.
In Paper X
(Cohen et al. 1999, AJ 117, 1864), spectral templates for 422 stars are presented, potentially defining a dense all-sky network. The templates can be obtained from the electronic version of the paper. However, many of these templates have not been verified observationally, so these stars must be used with some caution.
Many other 8-20 micron calibration systems exist. For a list of references, see Chapter 7, "Infrared Astronomy," by Alan Tokunaga, in "Allen's Astrophysical Quantities, 4th ed." (ed. A. Cox, Springer-Verlag:New York, 2000). Two useful summaries are Tokunaga 1984, AJ, 89, 172, and Hanner & Tokunaga 1991, in "Comets in the Post Halley Era" (eds. R.L. Newburn, Jr. et al., Kluwer) p. 67.
Phase II Science Programs must specify two calibration stars, one that would match the airmass of the target if observed before it and one that would match it if observed after. The PI should factor into their choice of calibration stars the amount of time to be spent observing the science target. If, for example, the science target requires two hours, ideally the first standard would have an RA 1 hr earlier than and a Dec equal to that of the science target and the second would have an RA one hour later than the target and a similar Dec. The elevation plot facility in the observing tool can be used to check that the chosen standard stars are appropriately matched in airmass to the science target.
In addition to using these stars as flux calibration standards, some of them can be used to calibrate the PSF. Those Cohen standard stars of spectral type K or earlier are likely to be suitable as PSF standards, as are most M giants. M supergiants may have extended emission from dust shells and should be avoided. Extremely bright stars should also be avoided, as they can produce array artifacts that can affect the shape of the PSF (see this discussion).