V. Optical Techniques

V.A. Zodiacal Scattered Light

A 1-zodi cloud would have a face-on surface brightness of about V = +22 mags arcsec^-2 at 1 AU from the primary star. This would be entirely scattered/reflected light from the primary. Figure 6 gives the zodiacal surface brightness at 1, 2, 3, 4, and 5 AU from the Sun (upper to lower curves, respectively) as a function of wavelength. The reflected sunlight component from about 0.4 to 4 µm is modeled from the observed visible zodiacal light, and the thermal infrared emission from 5-20 µm is based on the IRAS model. The transition from scattered light to thermal emission occurs at about 3.5 µm for our local zodiacal cloud. The near-IR brightness is about the same or slightly brighter than in the visible because the grains are grey or slightly red.

Figure 6: Spectral energy distribution of the zodiacal cloud surface brightness as a function of wavelength and of distance from the Sun. The nested curves are for dust at 1 AU (highest) to 5 AU (lowest) from the sun. Each SED has a minimum in the near-IR at the transition from scattered solar light to intrinsic thermal emission; the transition moves to longer wavelengths for cooler dust.

The surface brightness of exozodiacal light (extended emission) with a given optical depth at a chosen angular distance from a primary star will be proportional to the stellar apparent brightness.

Exozodiacal optical surface brightness will not be easily distinguished from background and instrumental sources of scattered light without detecting a radial "edge" or azimuthal asymmetry in the exozodiacal cloud. Based on experience with galaxy surface photometry, detection is probably possible to exozodiacal surface brightness as low as 6 mags arcsec^-2 below the background, which will require a dramatic reduction in the normal background for the relevant wavelengths, apertures, and angular scales. Exozodiacal cloud detection will be much more difficult than, e.g., finding galaxies around quasars (Ftaclas 1998).

V.B. HST and NGST

The superior surface accuracy of HST plus advanced-technology coronagraphic cameras expected in the near future can probably reach the 10-zodi level. An 8-m NGST with coronagraph and "slow" adaptive optics might detect the visual surface brightness of a 1-zodi cloud at 10 pc in a diffraction-limited beam if the instrument can compensate for NGST's expected modest surface accuracy compared with HST.

The envisioned NGST instrumentation includes an adaptive mirror for "static" wavefront correction over a very small field (1 arcsec diameter). The post-correction background brightness would consist of residual diffraction and scatter from actuator noise (wavefront sensor errors, position errors, etc.). A critical concern is the unknown angular-scale power spectrum of that post-correction background.

Figure 7a

Figure 7: Zodiacal light surface brightness at visual wavelengths compared with solar and planetary diffraction patterns seen from 10 pc by an NGST-class telescope. a) Solar diffraction reduced by "slow" adaptive optics in the case of system noise with a "white" spatial spectrum. b) Same, but for a plausible case of "non-white" noise.

Figure 7b 

If it is "white noise" (Figure 7a), calculations show that the adaptive mirror needs to be corrected to an accuracy of about 2 Å . This requirement could be relaxed in some cases of background spatial noise that is not "white" (Figure 7b). Note that the exozodiacal detection problem at visual wavelengths with an 8-m aperture is more difficult than directly detecting Jovian planets and comparable to detecting individual terrestrial planets (Ftaclas 1998).