Target assignment priorities for JWST MSA (NIRSPEC) Part I

after discussion in Victoria, May 2016, some notes on my thoughts
on the slit assignment from NIRSPEC MSA:

Let's presume, the plate solution is perfectly known and there are no failed shutters,
and we are interested in a 3-dither (no nods) mask design.

Let's presume we have a set of targets, with foremost attributes:
alpha,delta, size,flux,"scientific value", wavelength-range of highest interest (presuming we know z).

Let's presume, we want no spectral overlap; and we can neglect the issue of spectra
falling off the chip

The question is: what is the best combination of
a) telescope pointing (field center & orient)
b) target list

For a) we have in practice we have only 2 DOF, presuming the orient is given.
presumably these are fixed by the positions of very few "high-value" targets.
So a) and b) basically decouple.

Conjecture: in the limit that spectra cover the entire detector, the matter is simple:
in each slit position, one finds the "best available" object.
Complications foremost arise in defining which object is "best":
we need to define a merit function between
-- how intrinsically valuable is the target
-- how off-center (w.r.t. the micro shutter) should its centroid be;
   let's quantify this by a single number  log(S/N)-log(S/N_best),
  where S/N_best is defined as the S/N (given t_exp) we could get for
  a perfectly centered source with best sky subtraction.

Things to pre-compute: