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As at near-IR wavelengths, telluric standard star observations are required for mid-infrared spectroscopic observations to cancel telluric (atmospheric) absorption features in the data. The following is a guide to assist in selecting the most appropriate telluric standard stars. Two telluric standard 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.
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.
The midir package now includes a spectral reduction pipeline, msreduce. This can be used either with a spectrum of a single object or with two spectra, one of the science object and one of a telluric standard. In the former case a wavelength- (but not flux-) calibrated spectrum is produced, while in the latter case a flux-calibrated output spectrum can also be obtained. This section is a step-by-step description of the reduction process. The examples
IRAF tools have been developed by Gemini staff to expedite reduction of Mid-IR data. Scripts for basic imaging and spectral data processing of Michelle and T-ReCS data have been released as part of the Gemini IRAF package. These are found in the midir package under the main Gemini IRAF package. Tasks are also available for very basic imaging polarimetry reductions, but these should normally be used just to obtain images at the different waveplate positions.
For all queue observations, a set of "baseline" calibrations is required to ensure the long-term utility of data in the archive. The baseline calibration set will vary with mode (and sometimes instrument) but includes all the observations deemed necessary to produce scientifically useful data. PIs are free to request additional calibrations beyond the baseline set. However, while baseline calibrations are not charged to the program, additional calibrations are charged to the program and time to observe them must be included in the original proposal.
Michelle's imaging polarimetry capabilities are described in detail in the Michelle-specific pages; here we outline the special S/N requirements for polarimetric measurements and the weather conditions necessary for these to be obtained.
Polarisation accuracy and signal-to-noise ratio
On this page we describe the principal observing and calibration issues that users of Gemini's mid-IR instruments should consider when planning their programs and writing their proposals. We recommend that proposers weigh the importance of each issue to their program's scientific goals and formulate their observing plans accordingly. For Michelle and T-ReCS, the principal concerns are the relatively small field of view, the need to chop and nod, and the restricted chopping amplitude. A secondary concern is astrometric accuracy.
Compared to observing from space platforms such as Spitzer, observing at mid-IR wavelengths from large ground-based telescopes on good sites brings two principal benefits. First the image quality from such sites is usually diffraction limited (thus scaling with the telescope diameter) and therefore on Gemini, for example, is higher by more than an order of magnitude than on mid-IR space telescopes such as Spitzer.