In space and at short wavelengths

The current high level of understanding of CCDs in terms of their manufacture, inherent characteristics, instrumental capabilities, and data analysis techniques make these devices desirable for use in spacecraft and satellite observatories and at wavelengths other than the optical. Silicon provides at least some response to photons over the large wavelength range from about 1 to 10 000 A. Figure 7.1 shows this response by presenting the absorption depth of silicon over an expanded wavelength range. Unless aided in some manner, the intrinsic properties of silicon over the UV and EUV spectral range (1000-3000 A) are such that the QE of the device at these wavelengths is typically only a few percent or less. This low QE value is due to the fact that for these very short wavelengths, the absorption depth of silicon is near 30-50 A, far less than the wavelength of the incident light itself. Thus, the majority of the light (~ 70%) is reflected with the remaining percentage passing directly through the CCD unhindered.

Observations at wavelengths shorter than about 3000 A involve additional complexities not encountered with ground-based optical observations. Access to these short wavelengths can only be obtained via space-based telescopes or high altitude rocket and balloon flights. The latter are of short duration from only a few hours up to possibly hundreds of days and use newly developing high-altitude ultra-long duration balloon flight technologies. Space-based observations in the high energy regime from UV to shorter wavelengths usually require detectors to be "solar blind." The term solar blind means that the detector must be completely insensitive to visible light photons. This is generally accomplished by using a non-optically active type of detector or through the use of various types of filters. The majority of astronomical objects emit 104-106 visible light photons for every UV or shorter wavelength photon; thus even a visible light blocking filter with a 1% visible transmission is not nearly sufficient to remove optical contamination. In addition, most common

0 2000 4000 6000 8000

Fig. 7.1. Silicon absorption depth (in A) from 1.2 to 8500 A. The vertical axis is a log scale with major tick marks starting at 100 and ending at 106. From Bonanno (1995).

0 2000 4000 6000 8000

Fig. 7.1. Silicon absorption depth (in A) from 1.2 to 8500 A. The vertical axis is a log scale with major tick marks starting at 100 and ending at 106. From Bonanno (1995).

filters used to block visible light also absorb some of the incident higher energy radiation as well. Use of such absorbing filters causes even a high QE CCD at UV wavelengths (say 20%) to be reduced to a low effective QE near 2%.

Long-term exposure to high vacuum can cause contamination of the dewar in which the CCD resides. This contamination can be through outgassing of various materials such as vacuum grease or AR coatings (Plucinsky et al., 2004), normally not a problem in well-produced ground-based systems. Exposure to high energy radiation can cause changes in the QE of the CCD or cause permanent damage to the pixels and electronic structures within the array. Studies of the effects of high energy radiation and space environment observations on CCDs are ongoing at a number of laboratories such as the Jet Propulsion Laboratory (for NASA space-based satellites and missions), the Space Telescope Science Institute, and at the European Space Agency (ESA). Good discussions of space-based CCD usage are presented in Janesick, Hynecek, & Blouke, 1981; Janesick, Elliott, & Pool, 1988; Holtzman, 1990; Janesick & Elliott, 1992; Janesick, 2001; Strueder et al., 2002; Meidinger et al., 2004a; Meidinger et al., 2004c, and a number of the websites listed in Appendix B.

Before we discuss the details of observations at wavelengths shorter than the optical, we need to make a brief detour to look into some special issues related to space-based observations with CCDs. The more notable of these issues are the calibration of the CCD throughout the instrument or mission lifetime, the fact that the point-spread function is much smaller than generally obtained with ground-based data, and continual degradation of the CCD with time as the result of radiation damage.

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