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Instrumental Factors
Detectors
The detector properties which will affect the sensitivity are simply those familiar to ground-based optical and IR observers, namely dark current and read noise, and the detector quantum efficiency (DQE). Laboratory and preliminary on-orbit measurements have determined the read noise for the three NICMOS flight arrays to be ~35 electrons. The measured numbers are given in Table 7.1 on page 106.
Optics
NICMOS is a relatively simple instrument in layout, and thus contains a fairly small number of elements which affect the sensitivity. These are the filter transmission, the field of view (determined by the NICMOS optics external to the dewar, in combination with the HST mirrors), the reflectivities of the various external mirrors and the transmission of the dewar window. Background Radiation
At long wavelengths the dominant effect limiting the NICMOS sensitivity will be the thermal background emission from the telescope. How large this will be depends on the areas of the primary and secondary mirror and their optical configuration, temperatures, and emissivities. We discussed the issue of thermal background and its stability in Chapter 3.
At shorter NICMOS wavelengths, sensitivities will be affected by the zodiacal background which is given by the equation in Chapter 3; the overall expected background is shown in Figure 3.6.
Background radiation will be a slightly worse problem in the case of Multi-Object Spectroscopy (MOS) than in the case of imaging observations. Every pixel on the array will always see the entire background radiation integrated over the grism bandpass. The expected detected background rate per pixel is shown in Table 5.3.