Optical or Infrared — the Elusive Boundary
In years gone by, the “near infrared” part of the spectrum was considered to be the region just redward of 7000 Å, where the human eye was no longer sensitive. With the advent of semiconductor silicon array technology for imaging applications at very low light levels, optical astronomy extended its territorial claims to 1.1 microns — the cut-off wavelength defined by the silicon band-gap. Now, in the past few years, solid-state arrays employing lower band-gap semiconductors have become commercially available for IR imaging out to much longer wavelengths. So where is the optical-IR boundary? In this paper I suggest that it is at a wavelength of about 2.5 microns and therefore the availability of IR arrays of short-wavelength cut-off material, like Mercury-Cadmium-Telluride (HgCdTe), is an extremely important development for “optical” observatories.
KeywordsDust Attenuation Emissivity Alan
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