Zodiacal background

The NICMOS optics emissivity is one minus the product of the efficiencies of the NICMOS optical elements except for the shield window, filter and detector which are cold. Again this is a conservative number relative to the emissivity goals of the instrument. The detector noise is for a two readout (beginning and end) read mode and is assumed to be independent of the integration time. Equation 1 describes the zodiacal background in photons per second per cm per (an per steradian. 

At the Detector Noise Moduie, the Noise Equivalent Power (NEP) associated to the detectors and the 1 // noise are calculated. In parallel, with the physical properties of the system defined, the Background Power Module calculates the background power noise due to the instmment and the Cosmic Microwave Background (CMB), Cosmic Infrared Background (CIB) and Zodiacal Light. 

The simulator also takes into account the emission from the Cosmic Microwave Background (CMB), the Cosmic Infrared Background (CIB) and Zodiacal light (Zodi), at the respective temperatures Tcmb, Tcib and Tzoi,-. From internal communication, the emlssivites for these three components are... 

Cooling the telescope will leave emission from zodiacal dust particles as the dominant source of background radiation. Near 90 elongation in the ecliptic plane, the measured surface brightness of the zodiacal emission around 10 y is about 6 x 10" w cm" sterad" with a color temperature of about 300 K [1]. 

Although vastly superior to ground-based observations, infrared observations from space are not completely background free. The primary source of infrared background radiation in space is refiection and emission by zodiacal dust. This is dust in our solar system that is distributed mainly in plane of the planets or zodiac. As shown in Fig. 9, there are two minima in the background radiation, one at 3 pm and another at 400 pm. 

As a result of this extreme cold, we espect the sky over the South Pole to be exquisitely dark in this restricted range of wavelengths. Compared to a good mid-latitude site, we ve calculated that the thermal background from the telescope and atmosphere will be lower by a factor of over 700 - at or near the limit set by the zodiacal light. If this is true, then a telescope at the South Pole can do qtdte well compared with a telescope situated at any of the usual sites a 60 cm telescope can compete effectively with a 4 m telescope ... 

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