Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/hst/wfc3/documents/ISRs/2002/WFC3-2002-13.pdf Дата изменения: Fri Sep 26 03:24:55 2008 Дата индексирования: Thu Jan 15 04:56:27 2009 Кодировка: Поисковые слова: zodiacal light

Instrument Science Report WFC3 2002-13

An Estimate of the Thermal Background of WFC3 for the HST Exposure Time Calculator
Mauro Giavalisco October 11, 2002

ABSTRACT We present an estimate of the expected thermal background that will be recorded by the Infra-Red (IR) channel of Wide Field Camera 3 (WFC3) in the spectral range 0.8<<1.7 µm. This component of the total background dominates the zodiacal light and Earthshine at wavelengths >1.6 µm, and is calculated with a detailed model of the thermal emission of the HST Optical Telescope Assembly (OTA) and the instrument assembly. The thermal background has been incorporate in the WFC3/IR module of the HST Exposure Time Calculator.

Introduction
The near-IR channel of WFC3 (WFC3-IR) will dramatically expand the imaging capabilities of HST at IR wavelengths by providing a camera capable of high-resolution imaging in the spectral range 0.8<<1.7 µm with a scale of 0.13 arcsec/pix and a field of view of ~2.2 x 2.2 arcmin. These features combine with the high quantum efficiency of the detectors of the IR channel and the low sky background at the HST orbit to provide a sensitivity to point sources that improves upon that of the most powerful ground-based telescopes by ~1.5-2 magnitude. WFC3 will greatly advance the survey and discovery capability of HST; the instrument will be particularly effective in the identification and study of galaxies up to z~10, the physics of star formation in distant and nearby galaxies, accurate determination of the baryonic mass by detecting stars at the limit of the hydrogen-burning


Instrument Science Report WFC3 2002-13 sequence, extra-solar Jupiter mass planets, and the study of planetary objects in the solar system. For more information on WFC3, see the Wide Field Camera 3 Instrument MiniHandbook (Giavalisco et al. 2002) at http://www.stsci.edu/instruments/wfc3/wfc3docs.html. An Exposure Time Calculator (ETC) is the primary software tool for predicting the performances of HST instruments and for verifying the feasibility of proposed observations. The availability of a realistic ETC ETC for WFC3-IR seems particularly important, since this instrument will undoubtly be often used at the limit of its capabilities. Since the signal-tonoise ratio (S/N) of faint observations will be primarily limited by the intensity of the background, an accurate estimate of the spectral energy distribution of background light is critical to the robustness of the simulations carried out by the WFC3 ETC. The three major contributors to the total sky background visible to WFC3-IR include the zodiacal light, the Earthshine, and the thermal emission, which results from the contribution of the emission from the OTA and from the instrument assembly itself. The thermal background becomes an increasingly important component at the longer wavelengths of the spectral range of the instrument, ~1.5 µm and eventually dominate the background. This Instrument Science Report discusses the expected WFC3 thermal background. The spectrum of the WFC3 thermal background has been derived by Giavalisco, Stiavelli and Robberto (2001) from detailed modeling of thermal characteristics of WFC3 and HST Optical Telescope Assembly (OTA), including the emission by all the components along the optical path, from the telescope entrance through the instrument to the detector. This ISR documents this thermal model and presents the corresponding background emission. Estimates of the contribution to the background by the zodiacal light and by the Earthshine at UV through near-IR wavelengths are the subject of a separate ISR (Giavalisco, Sahu & Bohlin, 2002). The spectrum of the thermal background presented in this paper has been incorporated in the WFC3 ETC module of the recently released Astronomy Proposal Tool (APT), as well as in the previous CGI Web-based version, which is still being used.

The Thermal Background of WFC3
We have modeled the thermal background visible to WFC3 as the sum of the emission of the OTA of HST and of the mechanical assembly of WFC3, including the optical elements. The instrument includes a cold stop mask that blocks the thermal emission by internal parts of the spacecraft that would otherwise enter the optical path from the primary mirror hole, after being reflected into the optical path by the secondary mirror (see Giavalisco, Stiavelli, & Robberto 2001 for a discussion on the design and specifications of the cold stop mask).

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Instrument Science Report WFC3 2002-13 The contribution of the OTA consists of the emission of the primary and secondary mirrors, the supports (or spiders) of the secondary mirror, and the hold-down pads of the primary mirror. The contribution of the instrument assembly includes the emission of the pick off mirror, all the optical elements, namely four mirrors, the Refractive Corrector Plate (RCP), one spectral element (filter or grism), the window of the cold enclosure, and the dewar. The contribution of each components is modeled as a black body emission with known temperature and emissivity; Table 1 lists the temperature; the emissivity, and the etandue of the components that we have considered in the model. The dewar is not isothermal, the top being at T~191.2 K, the side at T~190.8 K, and the bottom at T~190.4 K. However, its overall temperature is very low compared to the other elements that contribute to the background; as we shall see, its relative contribution is minor. Since the temperature gradient is comparable to the uncertainty within which the true temperatures are currently known, we have neglected it and assumed that the dewar is isothermal with temperature T=191.2 K. This considerably simplifies the calculations and provides a conservative approach, since the corresponding background value is a slight overestimate of the true one.
ELEMENT Temperature K window filter dewar internal mirrors RCP pick off mirror cold stop mask secondary mirror pads primary mirror spiders 243 243 191 288 243 288 253 290 288 288 256 0.02 1-Tr() 1 0.006 0.02 0.03 1 0.03 1 0.03 1 Emissivity A sr cm2 1.94в10-8 1.94в10-8 1.02в10-5 1.94в10-8 1.94в10-8 1.94в10-8 2.11в10-9 1.67в10-8 2.12в10-10 1.67в10-8 3.53в10-10

Table 1. The Temperature, emissivity, and etandue of the individual components of the HST OTA and WFC3 IR channel's mechanical assembly used to model the thermal background.

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Instrument Science Report WFC3 2002-13 The contribution to the background by each component at the detector is the product of the black body emissivity of that component and its etandue (listed as A in Table 1). For components at the location of the primary we have computed their etandue as

Figure 1. Top. The total thermal background expected at the detector of the WFC3 IR channel as a function wavelength in the spectral range 0.8<<1.8 µm. The F160W spectral element has been used in the calculation, a conservative assumption, since it maximizes the background. In practice, no appreciable background is present at <1.5 µm. Bottom. The total thermal background in the range 1.4<<1.7 µm, expressed in units of photons per second per pixel, using the pixel size of the WFC3 IR channel (0.13 arcsec per pixel). Also shown is the transmittance of the F160W passband.

the product of the total etandue of the optical system (which is 1.94в10-8 sr cm2) times the area of the component expressed in unit of the area of the primary mirror. When computing the contribution to the total thermal emission, we have taken into account the position of each component in the optical path and included in the calculation the transmittance and reflectivity of the optical elements (lenses, mirrors and the filter) placed between that

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Instrument Science Report WFC3 2002-13 component and the detector. Since the spectral element controls the background produced before it in the optical path, we have conservatively used the transmittance of the F160W passband, because this is the wide filter with the longest central wavelength in the intrument's collection and, therefore, results in the strongest thermal background being transmitted to the detector. The contribution of the dewar and its window are not controlled by the filter passband. The result of the calculation is shown in Figure 1, upper panel, where the predicted thermal background expressed in units of erg s-1 cm-2 е-1 arcsec-2 is plotted as a function wavelength in the spectral range 0.8<<1.8 µm. The lower panel shows the backround expressed in units of photons s-1 pixel-1 (assuming the pixel scale of the WFC3 IR channel, i.e. 0.13 arcsec per pixel) together with the transmittance of the F160W filter. The total number of photons per pixel per second in this passband is 0.04, which is consistent with the expected number of electrons per second per pixel at the detector calculated by Giavalisco, Stiavelli & Robberto (2001), given the quantum efficiency of the detector and the difference between the passbands used in the calculation (they used the "long" version of the F160W filter, while here we used the "short" version of this passband, which is the one selected for flight --see the WFC3 Instrument Minihandbook). Figure 2 illustrates the comparison of the thermal background with the zodiacal light and Earthshine backgrounds for the case of Low Zodi, namely the intensity of the zodiacal light at the North Ecliptic Pole, and the case of High Zodi and High Earthshine (see Giavalisco, Sahu, & Bohlin 2002 and Stiavelli 2001 for a discussion of the intensity of the zodiacal light and Earthshine). The detector of WFC3 IR has a long-wavelength cutoff at ~1.65 µm; since for sufficiently large limb angle (>50, see Giavalisco, Sahu, & Bohlin 2002) the Earthshine is, for all practical purpose, zero. Figure 2 shows that the thermal component never dominates the total background. In most cases, IR observations with WFC3 will be sky-background dominated. Figure 2 also shows that even when observing in proximity of the NEP and at high limb angle, a situation where the zodiacal light is at its lowest intensity and the Earthshine is zero, the thermal background always remains smaller than the sky background over the useful range of sensitivity of the detector, namely 0.85<< 1.65 µm, implying that even images in the F160W will be nearly skybackground limited.

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Instrument Science Report WFC3 2002-13

Figure 2. A comparison between the sky background (Zodiacal light and Earthshine) and the thermal background. Top. The thermal background (continuous line) and the Zodiacal light at the North Ecliptic Plane (short-dashed line). The thermal background becomes the dominant component at >1.7 µm. Bottom. The thermal background (continuous line) compared to the Zodiacal light (short-dashed line) and Earthshine (long-dashed line) at high intensity (see Giavalisco, Sahu, & Bohlin 2002 for a discussion on the intensity of the sky background). In this case the thermal background becomes the dominant component at >1.8 µm.

Conclusions
A model of the thermal background of the IR channel of WFC3 and compared is discussed and compared the zodiacal and Earthshine background. Because of the design of WFC3 IR and its detector, the thermal component never dominates the total background over the entire wavelength range of the instrument; in most observing conditions the observations are sky-background limited. Our spectrum of the WFC3 thermal background is now used in the HST ETCs, including both the current CGI, Web based version, as well as the module of the Astronomer Planning Tool (APT) that is about to be released.

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Instrument Science Report WFC3 2002-13

References
Giavalisco, M. et al. 2002, "Wide Field Camera 3 Instrument Mini-Handbook", Version 1.0, (Baltimore: STScI). Giavalisco, M., Sahu, K., & Bohlin, R. 2002, STScI Instrument Science Report, WFC3ISR 2002 - 02. Giavalisco, M., Stiavelli, M., & Robberto, M. 2001, STScI Instrument Science Report, WFC3-ISR 2001 - 016. Stiavelli, M. 2001, STScI Instrument Science Report, WFC3-ISR 2002 - 02.

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