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Mid-IR Astrometry

Mid-IR observing programmes would often benefit from absolute astrometry with subarcsecond accuracy, for example for comparing mid-IR images of galaxy nuclei or star-forming regions with images at other wavelengths. In this page we guide the user through the steps necessary to set up accurate absolute astrometry in the OT. The procedure at mid-IR wavelengths differs from that used in the optical because of low probability that two stars close enough to use the same guide star will be both bright at 10 or 20 microns.

Astrometry

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Telluric, flux and wavelength calibration

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.

Imaging Calibrations

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.

Mid-IR Spectroscopy Reduction

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

Data Reduction

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.

MIR Baseline Calibrations

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.

Polarimetry: special conditions

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