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4.1.4 Performance and Sensitivities
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EXES delivered performance appears consistent with expectations over the flight series so far. There are some variations from observation-to-observation, but we believe the values quoted here are fair estimates of what is typical. The angular resolution of EXES will match that achieved by the telescope. For the latest sensitivities, observers are recommended to consult the online Exposure Time Calculator (ETC) at http://irastro.physics.ucdavis.edu/exes/etc/. The ETC also provides the slit length as a function of wavelength and instrument configuration (and therefore whether on-slit nodding is possible or not), as well as the wavelength coverage in a single setting and echelon orders that can be targeted.
The wavelength coverage ranges from 4.5-28.3 μm. There are three resolution regimes -- high, medium and low -- with the exact resolving power depending on wavelength, grating angle and slit width. Generally, the resolution will be higher at shorter wavelengths in each regime. The high-resolution configurations will use the echelon grating and will achieve R = 50,000-100,000. If the cross disperser echelle angle is 35-65°, the configuration is called High_Medium and if 10-25° it is called High_Low. For these high-resolution configurations, there is non-continuous spectral coverage in high-resolution configuration for λ > 19 μm, but the central wavelength can be tuned so that lines of interest do not fall in the gaps. The Medium configuration will use high angles on the echelle grating to achieve R = 5,000-20,000, and the Low configuration will use low angles to achieve R = 1,000-3,000.
The High_Medium configuration slits are 4.5″-45″ long, and the High_Low slits are 1-12″ long. The shorter slits in High_Low allow for more orders to be packed onto the array, thus increasing the instantaneous wavelength coverage, while maintaining the high spectral resolution (see Fig. 4-6 for an example). In the Medium and Low configurations the slit lengths vary from 25″ to 180″ depending on the number of rows to be read out.
The sensitivity of the instrument is shown in Figures 4-1 through 4-4 for the High_Medium, Medium, and Low configurations for both point sources and extended sources. The Noise Equivalent Flux Density for S/N of 10σ in a clock-time (Note that the other instruments in this Handbook report sensitivities based on the total time on-source, not the clock-time. The latter includes the total time on-source + applicable overheads, excluding target acquisition and instrument set-up time.) of 900 seconds is plotted as a function of wavelength. These values have been calculated for a point source assuming image quality between 2" and 4" (FWHM) and the narrowest of the available 1.4" to 3.2" slits, both of which vary with wavelength, and take into account estimated instrument efficiency. They assume an altitude of 41,000 feet, 40° elevation, and 7 μm precipitable water vapor.