A derivation of nano-diamond optical constants: Here be nano-diamonds
  % context {Nano-diamonds are an enticing and enigmatic dust component yet
their origin is still unclear. They have been unequivocally detected in only a
few astronomical objects, yet they are the most abundant of the pre-solar
grains, both in terms of mass and number.} %aims {Our goal is to derive a
viable set of nano-diamond optical constants and optical properties to enable
their modelling in any type of astrophysical object where, primarily, the local
(inter)stellar radiation field is well-determined.} % methods {The complex
indices of refraction, $m(n,k)$, of nano-diamonds, constrained by available
laboratory measurements, were calculated as a function of size, surface
hydrogenation, and internal (dis)order, using the THEMIS a-C(:H) methodology
optEC$_{\rm (s)}$(a).} % results {To demonstrate the utility of the optical
properties (the efficiency factors $Q_{\rm ext}$, $Q_{\rm sca}$, and $Q_{\rm
abs}$), calculated using the derived $m(n,k)$ data, we show that nano-diamonds
could be abundant in the interstellar medium (ISM) and yet remain undetectable
there.} % conclusions {The derived optical constants provide a means to explore
the existence and viability of nano-diamonds in a wide range of astronomical
sources. Here we show that up to a few percent of the available carbon budget
could be hidden in the form of nano-diamonds in the diffuse ISM, in abundances
comparable to the pre-solar nano-diamond abundances in primitive meteorites.}
