The instrument sensitivity on Gemini was only measured at two wavelengths, 12.3 microns and 17.0 microns, due to poor observing conditions during the commissioning period in February 2006. The measured sensitivity to point sources was about 4 times better than recent measurements with TEXES on the IRTF. However, poorly optimized optics resulted in TEXES using only about 7 meters of the Gemini primary mirror during the engineering tests. With redesigned optics and better observing conditions TEXES is expected to gain an additional 20% to 50% in sensitivity.

For the purpose of an observing time estimator, we assume a relatively conservative 25% improvement over the February 2006 performance, i.e. a factor of 5 improvement over the IRTF. Expected sensitivities based on this assumption are given in the following table, with several important caveats following the table.

To be consistent with the sensitivity tables for Michelle values are given in terms of the source brightness for which S/N = 5 in 1 hour (total clock time). However, normally TEXES peaks up on the target during acquisition, and this would be difficult on a target that is so faint as to only get S/N of 5 in 1 hour. PIs should bear in mind the difficultly of acquiring very faint targets with TEXES. The only circumstances where acquisition would be easy for a faint target is either if the acquisition can be done on a nearby bright target, followed by an offset to put the faint target on the slit [see below], or if a target is optically bright in which case it should be possible to place it on the TEXES "hotspot" directly during the acquisition.

The following are the estimated source brightnesses to get S/N = 5 in 1 hour (total clock time) at different wavelengths in the N-band when the seeing is better than it was in the commissioning period (all values are per resolution element, about 3 pixels, and assume 50% on-source efficiency):

Thus one sees that we anticipate being able to get S/N of about 5 per pixel for objects of brightness near 0.35 Jy, or magnitudes near 5. The TEXES sensitivity is similar to that of Michelle in its echelle mode, but with 3 times better spectral resolution and better wavelength coverage. TEXES provides significant improvement over Michelle for very narrow lines or the resolution of line structure narrower than 10 km/s. On the other hand the TEXES slit is relatively short compared to that of Michelle, which may be an issue for larger targets.

Due to the significant variations in the sensitivity with wavelength, prospective PIs are encouraged to contact members of the TEXES team for help with the sensitivity estimates for their specific wavelengths of interest.

The above estimates are assuming that the target is in the slit at both nod positions. If it is necessary to nod the target off of the slit--which is likely to be the case for extended targets--then the on-target efficiency is 2 times poorer than assumed. For such cases the sensitivities will be 1.4 times worse than shown above.

The observations at 5-13 micron are assumed to be at wavelengths where the atmosphere is essentially transparent (< 3% atmospheric emissivity or 10% total with instrument and telescope) and those at 17-24 micron are assumed to be made with 20% total background emissivity. To determine the sensitivity at a specific wavelength, it is necessary to determine the atmospheric emissivity and multiply by (1+atmo_emiss/.1)^0.5/(.93-atmo_emiss) at 5-13 micron, or (1+atmo_emiss/.2)^0.5/(.93-atmo_emiss) at 17-24 micron. It is not uncommon for this factor to degrade the sensitivity by a factor of 2. In addition, the instrumental response rolls off between echelon orders, which can degrade the sensitivity by an additional factor of 1.4. The line sensitivities assume the line is narrow compared to the resolution of lambda/100,000 shortward of 10 micron and 0.01 cm^-1 longward. For broader lines, the sensitivity numbers must be multiplied by the square root of the number of resolution elements over which the line is spread, or the NEFD numbers could be used.