Michelle Sensitivity

The values below are based on calculations with the Michelle Integration Time Calculator (ITC). The values are consistent to within a factor of 1.5 of measured values.

All sensitivities are 5 sigma in one hour of exposure time, in 70%-ile seeing, 50 %-ile transparency, water vapor and background conditions, 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, observations in both beams are guided. The sensitivity values 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.

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.

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.

Magnitudes are N or Q for a stellar (Rayleigh-Jeans) continuum. Flux densities are at the wavelength of the filter.

Imaging

Spectroscopy with 2 pixel wide (10um) and 3 pixel wide (20um) slits
(sensitivity per spectral pixel)

Last updated 3 March 2006; T.R. Geballe and R. S. Fisher