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Spectroscopy with TEXES

Overview

TEXES is used for echelle slit spectroscopy in the 5 to 25 micron wavelength region. In the cross-dispersed echelle mode TEXES has a resolving power (for unresolved spectral lines) of up to 100,000 at wavelengths shorter than 10 microns, and a fixed wavenumber resolution of 0.01 inverse cm at longer wavelengths. Typical observing parameters are summarized in the table below.


TEXES STANDARD OBSERVING CONFIGUATIONS
RESOLVING POWERS, WAVELENGTH COVERAGES, SLIT LENGTHS
Gemini OT Name 5-14µm
0.52 arcsec wide slit
17-20µm, 22-25µm
0.75 arcsec wide slit
Echelon + 32 l/mm echelle R~85,000 (high)
Δλ ~ 0.006 λ
slit length: 4 arcsec
R~60,000 (high)
Δλ ~ 0.006 λ
slit length: 8 arcsec
Echelon + 75 l/mm grating R~85,000 )high)
Δλ ~ 0.25µm
slit length: 1.7 arcsec
-
32 l/mm echelle R~15,000 (medium)
Δλ ~ 0.006 λ
slit length: 20 arcsec
R~11,000 (medium)
Δλ ~ 0.006 λ
slit length: 20 arcsec
75 l/mm grating R~4,000 (low)
Δλ ~ 0.25µm
slit length: 20 arcsec
(8-14µm only)
-

Wavelength range and coverage

TEXES accessible wavelengths are 5 - 20 microns and 22 - 25 microns. Wavelengths from 20 to 22 microns are inaccessible. Wavelengths from 5.5 to 8 microns and 14 to 16.9 microns are either mostly or completely blocked by the atmosphere. Even within the 10-micron and 20-micron windows many specific wavelengths are partially or totally blocked by atmospheric absorption lines. PIs should generate detailed plots of the atmospheric transmission expected on Mauna Kea (use altitude of 13,796 feet, latitude 30 or 9, and typical "water vapor overburden"~1000-3000 microns) to check if the wavelengths they wish to observe can actually be observed, given the atmospheric absorption lines that are present. Note that the molecular hydrogen 0-0 S(1) line at 17.05 microns can be observed with TEXES if the sky is not too wet. 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.

Note than individual TEXES spectra cover very narrow wavelength intervals. For example, at 10 microns the wavelength interval is 0.06µm or 0.25µm.

Slits and gratings

In the echelle mode a 0.13 line/mm echelon 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 32 groove per mm echelle that gives 0.6% spectral coverage (i.e. 0.06 microns at 10 microns, or a velocity range of +/-900 km/s around line center), and the other is a 75 groove per mm first-order grating that gives about 0.25µm spectral coverage at 10 microns. If the wavelength coverage obtained with the 32 groove per mm echelle as the cross-disperser is not large enough for a program then the 75 groove per mm grating will have to be used and the shorter slit will then have to be used. This likely means nodding the telescope to blank sky rather than nodding along the slit. With the 75 l/mm 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 at 10 microns, and the velocity coverage is larger at shorter wavelengths and smaller at longer wavelengths.

In addition to the slits in the table, two other slit widths, with widths of 0.38 and 1.05 arc-seconds 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 little reason for proposals to use different slit widths than those given in the table.

The lower resolution modes use either the echelle with blocking filters, giving R~104 in either the 10 or 20µm regions, or the first order grating with blocking filters, giving R~4,000 in the 10µm region.

Detector

The detector is a Raytheon 256 by 256 pixel Si:As array, the same type of detector that is used in Spitzer. On Gemini TEXES' pixel scale is 0.14 arcseconds/pixel.

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 there must be 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.

Rough sensitivity estimates for TEXES on Gemini are given in the TEXES Sensitivity page.