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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.
ATRAN TRANSMISSION AND S/N THROUGH VARIOUS MID-IR FILTERS ON MAUNA KEA AND CERRO PACHO (S/N est. includes 4% telescope + instrument emissivity) ===================================================================== MAUNA KEA Transmission S/N (norm to secz=1,1mm H2O) 1.0mm 1.6mm 3.0mm 5.0mm H2O 1.0mm 1.6mm 3.0mm 5.0mm airmass Si-1
Table 1 below shows the time taken for the background (sky + telescope) to reach 50% full-well capacity of the NIRI Aladdin array in spectroscopy mode. Table 2 shows the brightness of a stellar source (in magnitudes) that will fill 80% of the well of a NIRI array pixel in a one-second exposure (0.2 sec exposure at M).
The specification of the Target component differs somewhat depending on whether a Natural Guide Star (NGS) or the Laser Guide Star (LGS) is being employed. There also can be differences in the target component, depending on whether NIRI or NIFS is being used, since the NIFS OIWFS can be employed to provide tip/tilt information.