• TEXES Overview
• Wavelength coverage
• Slits and gratings
• Detector
• Nodding and scan mapping
• Faint target acquisition

Overview

TEXES will be used for echelle slit spectroscopy in the 5 to 25 micron wavelength region. At this time it is not anticipated that the lower resolution spectroscopy modes of TEXES will be offered, since Michelle has overlapping capability in those areas. In the cross-dispersed echelle mode TEXES has a resolution (lambda over delta lambda, where delta lambda is the FWHM of spectral lines, about 3 pixels) of 100000 at wavelengths shorter than 10 microns, and a fixed wavenumber resolution of 0.01 inverse cm at longer wavelengths. Thus the resolution is between 100000 and 40000, the value being inversely proportional to the wavelength, for wavelengths between 10 and 25 microns.

Wavelength coverage

The wavelength coverge is from 5 to 20 microns and from 22 to 25 microns. The wavelengths from 20 to 22 microns are inaccessible for the echelle. Wavelengths from 5.5 to 8 microns and 14 to 16.9 microns are either mostly or completely blocked by the atmosphere. However the molecular Hydrogen line at 17.05 microns can be observed with TEXES. PIs should consult detailed plots of the atmospheric transmission expected at Mauna Kea to see that the wavelength they wish to observe can actually be observed, given whatever atmospheric absorption lines are going to be present. While observations can be carried out down to 5 microns, the TEXES detector has poor sensitivity at wavelengths near 5 microns compared to the InSb detectors used in most modern near-infrared instruments.

Slits and gratings

In the echelle mode a 0.3 line/mm grating is used as the main dispersive element in the spectrometer. It has an order separation of 0.662 inverse cm, or 0.0066 microns at 10 microns. Users have a choice of two different cross-disperser gratings for separating the orders: one is a 31 groove per mm echelle that gives 0.5% spectral coverage (i.e. 0.05 microns at 10 microns, or a velocity range of +/-750 km/s), and the other is a 75 groove per mm first-order grating that gives about 1.5% spectral coverage at 10 microns. If the wavelength coverage obtained with the 31 groove per mm echelle as the cross-disperser, which is equivalent to about about 1500 km/s velocity range, is not large enough for a program then the 71 groove per mm grating will have to be used and the shorter slit will then have to be used. With the grating as the cross-disperser the wavelength range is more or less constant with the selected wavelength, so the velocity range covered is about 4500 km/s around 10 microns, and the velocity coverage is larger at shorter wavelengths and smaller at longer wavelengths.

In both cases the slit width is chosen to be close to 2 lambda/D ~ 0.5 arc-seconds at 10 microns for Gemini, but the slit length as projected on the sky is larger for the 31 groove per mm echelle (about 4 arc-seconds at 10 microns) than it is for the first-order grating (about 1.7 arc-seconds at 10 microns). The projected slit length is proportional to the wavelength.

A variety of slits are available in TEXES. However experience has shown that there is no particular gain by changing the slit from a width given by 2 lambda/D with lambda being the wavelength and D the mirror diameter. This gives a value of about 0.52 arc-seconds at 10 microns. Larger slits do not give a better sensitivity since the sky noise increases proportional to the slit width, but degrade the spectral resolution. Narrower slits do not increase the spectral resolution. Thus there is no reason for proposals to use different slit widths.

It is possible to reduce the length of the slit from the maximum length, if there is a reason to do so. But if the slit is too small then it will not be possible to nod along the slit and then it would be necessary to nod off the slit thereby reducing the on-target efficiency by a factor of two. We do not anticipate that PIs will want the slit to be shorter than the default values, so they should set whether they want the longer or shorter slit in the OT by choosing which cross-disperser is to be used.

Detector

The detector is a Raytheon 256 by 256 pixel Si:As array, the same type of detector that is used in Spitzer. The detector has a measured quantum efficiency of about 50% at 7.7 microns, which is expected to increase to about 60% for the region between 10 and 20 microns. At short wavelengths the quantum efficiency falls to about 30%. The pixel to pixel variations in quantum efficiency are about 3% (one sigma). The well size is 190000 electrons for the normal operating bias. The read noise is low, about 16 electrons. The detector is background noise limited.

Nodding and scan mapping

The usual operating procedure for TEXES is to nod the target along the slit during the observation, so that taking the difference of the observations at the two positions removes the sky emission. It is also possible to scan the slit across an extended target, such as a planet, to produce a spatial map of the spectrum. PIs should consult with the TEXES team concerning the suitability of these modes for whatever observations they wish to carry out.

Faint target acquisition

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 ensure 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.

Rather preliminary sensitivity estimates for TEXES on Gemini are given in the TEXES Sensitivity page. These values are uncertain by a factor of 1.5 to 2 due to poor weather during the commissioning period.

Last update March 09, 2007; K. Volk & R. Mason