[alma-config] Forwarded mail bounced by email reflector

[Bryan Butler]

> 
> Bryan Butler writes:
>  > > I agree with Simon that the 25 km spec should be made more prominent
>  > > in the PB, and that the various other references to the array size
>  > > should be made consistent and precise.  At this stage, any proposal to
>  > > make the maximum baseline longer than 10 km should be treated as just
>  > > that -- a proposal -- until its benefits can be carefully weighed
>  > > against its costs.
>  > 
>  > i would state it as: any proposal to have fiber runs longer than 25 km
>  > should be treated as just that - a proposal.  the spec isn't on the
>  > max baseline length (is it?), but rather on the max allowable fiber run.
> 
> No.  The requirement for 10 km maximum baseline length was translated
> into an engineering spec, namely that we will support a maximum of 25
> km cable runs.  If a proposal is made to increase the max baseline,
> then we will have to have a look at what engineering changes may be
> needed to support it, and the costs thereof.  We must not get into a
> situation where the scientists are trying to second-guess the
> engineers; most of the scientists are just not competent to do this.
> 
> --Larry
> 


it is my understanding (from discussion with dick this morning) that the
25 km length wasn't derived from the desire to have a 10 km diameter
configuration, but rather from looking at the maximum length that could
be supported before optical repeaters were necessary.  if optical repeaters
are already necessary even for lengths < 25 km (as i understand the jodrell
bank folks are saying now), then this spec becomes an unneccesary restriction
on the possible range of configurations we could consider.  if the OSF is
built halfway from san pedro to the site, and the correlator is put there
(i'm not espousing this, just repeating ideas that i've heard and that seem
to be gaining some support), then we will almost certainly need optical
repeaters anyway.  if there are other engineering considerations, please
help me out by telling me what they are.  in the absence of other 
considerations, if we can design a 14 km diameter configuration that does
not need fiber runs of > 25 km, then i see no problem in the first place.

if the engineers want to place a spec on max baseline length, then they
should explicitly make such a spec, but only after extensive consultation
with the scientists.  we must not get into a situation where the engineers
are defining the science that ALMA can do; most of the engineers are just
not competent to do this.

	-bryan