Capabilities

Capabilities






  • Instrument Modes
  • Observing Modes
  • Sensitivity
  • Effects of Atmosphere

  • Instrument Modes

    TEXES is used for echelle slit spectroscopy in the 5 to 25 micron wavelength region. TEXES has a resolution (λ/δλ where δλ is the FWHM of spectral lines, about 3 pixels) of 100,000 at wavelengths shorter than 10 microns, and a fixed wavenumber resolution of 0.01 inverse cm at longer wavelengths. Thus the resolution is between 100,000 and 40,000, the value being inversely proportional to the wavelength, for wavelengths between 10 and 25 microns.

    The wavelength coverage 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 comletely blocked by the atmosphere. Pls 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. The TEXES team members have detailed maps available. While observations can be carried 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.


    TEXES Instrument Mode 5-14μm
    0.52 (1.4) arcsec wide slit on Gemini (IRTF)
    17-20μ, 22-25μm
    0.75 (2.0) arcsec wide slit on Gemini (IRTF)
    High-med Mode: R~85,000
    Δλ ~ 0.006 λ
    slit length: 3.5 (9.3) arcsec
    R~60,000
    Δλ ~ 0.006 λ
    slit length: 7 (18.7) arcsec
    High-low Mode: R~85,000
    Δλ ~ 0.25μm
    slit length: 1.2 (3.2) arcsec
    -
    Medium Mode: R~15,000
    Δλ ~ 0.006 λ
    slit length: 20 (50) arcsec
    R~11,000
    Δλ ~ 0.006 λ
    slit length: 20 (50) arcsec
    Low Mode: R~4,000 (low)
    Δλ ~ 0.25μm
    slit length: 20 (50) arcsec
    (8-14μm only)
    -



    Observing Modes

    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. Pls should consult with the TEXES team concerning the suitability of these nodes for whatever observations they wish to carry out.


    Figure1
    A diagram of the Nod on-slit
    The object alternates between the two points
    Figure2
    A map of NeII emission in the galactic center
    [Source: Irons et al. 2012 ApJ 755 90]


    Sensitivity

    Values are given in terms of the source brightness for which S/N = 1 in 1 second on-source integration time per resolution element assuming the Earth's atmosphere is essentially transparent (see below).

    Overheads with TEXES depend on the observing mode. To be safe, we advise assuming a factor of two for nodding on the slit and a factor of 4 when nodding off the slit. For maps, the effective overhead depends on the number of steps required to complete the map and any spatial summing to be applied after data collection. Please work with a team member for the most accurate estimate.

    TEXES peaks up on the target during acquisition, and this would be difficult on a target that is a factor of 3 below the 1sigma:1second threshhold. PIs should bear in mind the difficultly of acquiring very faint targets with TEXES. Acquisition of faint targets is easier if the target is optically bright, in which case the telescope camera can be used to place the target on the TEXES slit, or if there is a bright, nearby IR object with sufficiently good coordinates such that we can offset the telescope.

    10 μm 20 μm 10 μm 20 μm
    low 0.003μm 0.25 0.5 1.5 3.0
    medium 24 km/s 0.6 1.2 3.6 7.2
    high 3.6 km/s 2.1 4.2 12.6 25.2

    1 Low-res wavelength resolution and high-res velocity resolution are approximately constant with wavelength. Medium-res velocity resolution is roughly constant, but varies with wavelength within the grating orders.



    Effects of Atmosphere

    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.

    A pdf with model atmospheric transmission for Mauna Kea can be found here (6.3 MB) . The figures are organized by wavenumber and each panel shows 5 cm-1. The two continuous traces are for 1 mm PWV (upper) and 4 mm PWV (lower). These correspond to a fairly dry and a fairly wet night at Mauna Kea. The assumed elevation is 45 degrees. Transitions of several features are included on the plots as vertical tick marks. For molecular features, the tick marks are placed in line with the responsible molecule identified at the left hand side of the plot. The diagonal bars near the bottom of each window indicate the size of the TEXES detector with the center of the bar indicating the peak of the echelon order. At wavelengths beyond around 11 microns (900 cm-1), gaps appear in the spectral coverage. The size of these gaps increase with wavelength. It is possible to tune the optics so all the diagonal "detector" bars slide left or right, but the echelon blaze efficiency does not change.

    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 λ/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.