TEXES Performance and Use

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):

Wavelength	Point Source magnitude	Point Source (Jy)	Surface Brightness (1.e-07 W/m2/ster)	Unresolved Line Flux (1.e-18 W/m2)
5.0	7.8	0.12	1.3	0.8
8.0	6.2	0.2	1.5	0.8
9.0	5.1	0.43	2.6	1.5
10.0	4.8	0.48	2.6	1.5
11.0	4.8	0.26	1.0	0.8
12.0	4.9	0.28	1.0	0.9
13.0	4.7	0.31	1.0	1.0
17.0	3.4	0.57	1.0	1.7

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 wavelenghs 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.

In addition to science observations, it is generally necessary to observe a comparison object for each TEXES spectral setting to allow removal of atmospheric absorption features and to correct residual flat-fielding errors. In some cases the comparison object can also provide flux calibration information, although flux calibration is necessarily uncertain due to TEXES' narrow slit.

Due to molecular features in their spectra, K and M stars do not make good comparison objects. Asteroids are generally the best choice longward of about 10 um, and asteroids brighter than 100 Jy (N = -1) are usually available, although not typically near a target. Hot stars (spectral type A, typically) are often preferred at shorter wavelengths.

Unlike the case with other Gemini instruments, PIs must include time for telluric standards in their proposals. For programs involving bright objects where the on-source integrations are short the additional time needed for these telluric standards can lead to a factor of 2 increase in the total time requested, in the case where every science observation is preceeded by or followed by a telluric standard observation. A rule of thumb is to allow 30 mintues clock time for a telluric standard observation, which includes time slewing to the standard, setting up, and taking the observation. The actual time needed may turn out to be shorter than this on the average, but we do not yet have enough experience with the use of TEXES on Gemini to give a more accurate estimate. The actual time needed will depend on what asteroid is chosen and the wavelength of the observation.

Prospective PIs are encouraged to check the type of standards that are needed and how long they are likely to take with members of the TEXES team, since both of these things are very wavelength dependent.

Acquisition of optically bright objects that are faint in the mid-infrared can be done by placing the target on the TEXES "hot spot", which should put the object accurately on the slit. For objects that are bright in the mid-infrared the object will be peaked-up on so that the maximum signal is on the slit. We anticipate that for most targets these two acquisition methods will be sufficient.

If a target is optically faint as well as too faint in the mid-infrared for it to be peaked-up on in the slit, the acquisition can still be carried out provided there is a brighter target nearby which can be put into the slit. This is the same process as "blind or offset acquisition" for the near-infrared instruments such as GNIRS. It requires accurate relative astrometry of the two targets to be provided. Only small offsets should be used if at all possible because the accuracy of say a 30 arc-second telescope offset is limited by the PWFS2 probe-mapping. Positional errors of 0.2 arc-seconds or more (for a 30 to 40 arc-second offset) are possible if the PWFS2 probe happens to positioned in an area where the mapping is poor. This is the worst case, normally the accuracy is of order 0.1 arc-seconds for a 30 arc-second offset. Nonetheless, small offsets are strongly recommended in any blind offsetting acquistions.

If a PI wishes to carry out this type of acquisition they need to insure that there is a suitable PWFS2 guide star that can be used for both the bright target and the faint target. The PI will be required to provide relative offsets on the sky between the two targets if such an acquisition is to be carried out. This is different than the case for the facility instruments where the offset is defined in the Phase 2 OT file. For TEXES the offsetting will be carried out by the TEXES control software. The process of defining "User1" target coordinates for blind offsetting, which is used for other instruments, is not used with TEXES.