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The values for spectroscopy and imaging below are based on calculations with the Michelle Integration Time Calculator (ITC). They are consistent to within a factor of 1.5 of measured values. Sensitivity estimates are available for:
All imaging and spectroscopy sensitivities are 5 sigma in one hour of exposure time, in 70%-ile seeing, 50 %-ile transparency and water vapor, and at 1.5 airmasses (thus sensitivities can be somewhat better when conditions are very good and/or at low airmasses). Imaging and low resolution spectroscopy use the chop/nod mode with guiding in one beam only and the sensitivity values correspond to 30 minutes of on-source guided integration time, hence 30 minutes on sky, and to about 2.25 hrs of elapsed time, including a set-up time of ~15 minutes and observing overheads. The medium resolution gratings and echelle are used in stare/nod mode, and observations in both beams are guided. The sensitivity values in these cases therefore correspond to 60 minutes of on-source guided integration time. The total stare/nod elapsed time including overheads is about 1.5 hrs for every 1.0 hrs of exposure time. (See the Michelle observing overheads page for more discussion of overheads and efficiency)
Magnitudes are N or Q for a stellar (Rayleigh-Jeans) continuum. Flux densities are at the central wavelength of the filter.
|Filter||R-J Point Source
N or Q (mag)
|Ne II (12.8um)||8.0||14||13||1.6e-13|
In imaging mode the point source sensitivities were measured in apertures giving optimal S/N (i.e. optimal detectivity). For accurate photometry, larger apertures are usually required, so one would expect the S/N to be lower than the ITC values by a factor of ~2. The Ne II values are estimated from the Si-6 filter measurements.
Spectroscopy (sensitivity per pixel in the spectral direction)
|Grating - R||Wavelength||Point Source
|Unresolved Line Flux (W/m2)|
|LowN - 170||8.5um||8.9||14||2.9E-17|
|LowN - 210||10.5um||8.5||14||1.9E-17|
|LowN - 230||12.5um||7.7||20||2.4E-17|
|LowQ - 100||17.9um||5.1||110||1.5E-16|
|LowQ - 110||20.1um||4.9||110||1.4E-16|
|LowQ - 120||21.5um||4.2||150||2.2E-16|
|LowQ - 130||24.3um||3.5||250||3E-16|
|MedN1 - 900||9.0um||8.4||21||1.2E-17|
|MedN1 - 1200||12.0||7.3||35||7E-18|
|MedN2 - 2700||9.0um||7.7||40||5E-18|
|MedN2 - 3600||12.0um||6.5||70||5E-18|
|Ech - ~30000||9.0um||5.7||270||3E-18|
|Ech - ~30000||12.0um||4.6||400||3.4E-18|
|Ech - ~20000||18.0um||2.9||700||6E-18|
In spectroscopic mode estimates were made at wavelengths free of strong telluric absorption lines. In the 10um window a 2 pixel-wide slit was employed; in the 20um window a 3 pixel-wide slit was used.
The need for high S/N if accurate polarisation measurements are to be obtained, together with the need for stable conditions, means that only relatively bright mid-infrared targets can be observed in the polarimetry mode. Roughly speaking a target with a flux density of 2.5 Jy in the Si-5 11.6um filter if observed for 2 hours should yield an accuracy of 1 percent in the polarization (per pixel). With binning of the pixels the accuracy of the polarization measurement can be increased by a factor of a few, at the cost of a loss of spatial information. Sources that are significantly fainter than 1 Jy most likely cannot be observed in polarimetry mode, and indeed it would be better if any sources to be observed are somewhat brighter than this if a low level of polarization is expected. The ITC gives S/N values for an aperture, not per pixel; the ratio of the overall S/N to the per pixel S/N is about equal to the square-root of the number of pixels in the aperture.
The situation is somewhat better for the N' filter, and somewhat worse for other of the N-band filters. Note that the Qa filter sensitivity is rather worse than those of the N-band filters and the on-source efficiency is lower. As a result, targets for which polarimetry in the Qa filter are requested should be significantly brighter--at least 10 times brighter than the fluxes given in the preceding paragraph.
The above estimates are for targets which are either point-like or small as seen on the sky by Michelle. It is difficult to give sensitivity estimates for extended targets at this time.
The Gemini ITC can be used to estimate the time required to reach a good enough S/N to detect polarization. These estimates are based on a limited set of commissioning observations and should be viewed with some caution. It is recommended that PIs allow some extra leeway in their time estimates until the Michelle ITC has been tested against the results of a larger set of observations.
The commissioning observations were taken in local winter, and under average conditions. Conditions are likely to be better on the average in the summer, and whatever the season the best nights will produce better results than we had in the commissioning.