TEXES Introduction

TEXES Demonstration Science Programs Selected

The TEXES DS call can be viewed here.

TEXES, the Texas Echelon Cross Echelle Spectrograph is a high resolution (~100,000) mid-infrared [5-25 micron] spectrometer, which was offered at Gemini North as a visitor instrument in 2006B and is offered again in 2007B. Commissioning took place in the latter part of February, 2006, with a demonstration science run following in early July of 2006 and queue observations in November of 2006. The first paper based on data obtained with TEXES at Gemini has been submitted: astro-ph/0702327. TEXES is used in a similar manner to the way it has been used at the Infrared Telescope Facility in the past.

One or more of the TEXES team must be co-Is on any proposals that are granted time (John Lacy [lacy@astro.as.utexas.edu], Matt Richter [richter@physics.ucdavis.edu], Tommy Greathouse [greathouse@lpi.usra.edu], Dan Jaffe [dtj@astro.as.utexas.edu]). They will take the observations during the blocks of time when TEXES is at Gemini North, currently planned to be in October for semester 2007B. In addition to taking the observations, they will carry out the data reduction for the various programs, delivering fully reduced spectra from their reduction pipeline program to all PIs.

Prospective PIs are encouraged to contact John Lacy or other members of the TEXES team for help with the technical details of proposals. A description of the instrument and its use is found in the paper Lacy, J., et al., PASP, vol. 114, 153, (2002).

As a visitor instrument, the way TEXES proposals is defined is slightly different than what is done for the facility instruments. All phase I proposals must be submitted to the various Gemini TACs in the usual manner using the PIT software. For those programs that are approved by the ITAC, the details of the observations will be determined in consultation with the TEXES team. Unlike the case with facility instruments, the observations will not be defined in detail in the Observing Tool, although the OT will be used to pass target and guide star coordinates to the telescope systems. Once a source has been acquired, the observations will be taken by members of the TEXES team using their own software to define telescope offsets and to take the data.

The observations approved by the ITAC will be taken during October, 2007 (as conditions allow). All targets for TEXES programs must thus be visible at that time. All the observations will be carried out on a shared-risk basis.

Overview of TEXES Capabilities

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.

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.

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. 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 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.

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

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