TEXES Components

Specific elements of  TEXES include:

TEXES Slits

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.

However the slit length varies in different configurations. Using the 31 groove per mm echelle as the cross disperser the slit length projected on the sky is expected to be about 3 arc-seconds at a wavelength of 10 microns. Using the 71 groove per mm grating instead the slit will be about half as long. The projected slit length is proportional to the wavelength.

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.

If the wavelength coverage of close to 0.5% of the central wavelength 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. This alternate mode gives about 1.5% relative wavelength coverage at 10 microns. 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.

The TEXES 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.

Last update 21 February 2006; Kevin Volk