[mmaimcal]comments on Y+ vs. ring configuration notes from stephane

[Bryan Butler]

all,

as followup on our discussion at the imcal meeting, here are
some comments on stephane's notes regarding the Y+ vs. ring
configurations, which can be found at:
http://www.cv.nrao.edu/~awootten/mmaimcal/asac/yplus.txt
i hope mark and john will comment as well,
as they are certainly more knowledgeable about this than i.

stephane lists as arguments in favor of the Y+ config:

1) more continous reconfiguration, which gives more flexibility and 
   less time lost in non-optimal hybrid configurations.
2) less roads, so cheaper
3) less roads, so shorter reconfiguration time
4) shorter fibers, so probably better phase stability
5) better expandability towards even longer baselines
6) more flexibility in re-weighting (tapering and/or uniform) data to
   adjust angular resolution
7) possibly less pads, because no complex hybrid between the 5 km  
   Gaussian and the 14 km array is needed.

let me make comments on each of these.

1) this may or may not be an advantage.  while there are more
   hybrid configurations, it is not clear that there will be
   significant demand for them.  will there be significant demand
   for an, e.g., 10 km hybrid?  i suspect not, as the demand will
   be for either the highest resolution or for the intermediates
   (5 km and less).  in that case, we have a bunch of pads that
   may end up not being needed, in these hybrids.  now, the extra
   flexibility *is* there, so if it turned out that there was
   demand for the hybrids, it could be accomodated much easier
   than for the ring, admittedly.
2) & 3).  there is actually not a large difference in the road
   length, at least as i understand the latest ring and Y+ 
   designs.  the road cost is relatively minor, as well, so these
   are pretty minor advantages, i think.
4) i don't think that the phase stability in the fiber is an issue.
   mark commented yesterday that bill shillue echoed this sentiment
   (essentially, the residual fiber phase is << the residual
   atmospheric phase, so is unimportant in the residual phase budget).
5) this is true, but isn't this beyond the current project?  while
   we shouldn't cast a blind eye to the future, we also need to
   consider what is in the best interests of the present.
6) i don't have a good handle on how big an issue this is.  mark
   h. says he can quantify it, and i await such quantification.
7) i thought we had decided at the configuration CDR to do away
   with the hybrid configuration?  this also relates to my comment
   on 1) above.

now, i think there is one big cost issue that stephane missed that
is an advantage for the Y+.  namely that the fact that the fiber
lengths are significantly shorter for the Y+ config means that the
total fiber conduit (and power conduit) length should be significantly
shorter for the Y+.  this might be a real cost saver, depending on
what the final cost of these conduits ends up being.  mark and john,
can you quantify the difference in conduit length for the 2 configs?
i know that we don't have a good final estimate for conduit cost, but
at least if we know the relative difference between the 2 configs, this
is a good starting point.

stephane then lists as the advantages of the ring:

A) Slightly better resolution
B) Better astrometric performance 

let me again comment on each.

A) is this true?  i thought that the effective resolution of the
   2 was essentially the same now?  mark and john, can you comment?
B) this is a real key, in my opinion.  one of the primary uses of
   the most spread out configuration will be astrometry (a good example
   is the astrometry of stars to see if they have detectable wobble
   from orbiting planets).

i think another advantage that is not listed is the ability to
discriminate between two closely spaced sources, which is *not*
the same as resolution (there is an extensive literature in the
electrical engineering and signal processing fields on the difference
between these two features of any signal processing system).
it comes about for the same reason as the improved astrometry - the
distribution of baselines is different, and hence allows for better
discrimination between closely spaced sources, even though the
'resolution' is similar.  it would be nice to quantify both B)
above, and this other feature.  can the simulation tools that john
and mark and stephane's group (and maybe koh-ichiro or dave woody or
mel wright) have do this with any reliability?

one final possible advantage of the ring configuration is that it
may be true that it is easier to negotiate for the land access.
the ring fits on the current science preserve, and hence no 'extra'
land is needed.  this is not the case for the Y+, where we know that
we will need extra land to the west.  i don't know if this is a big
issue or not, but as long as we're setting these things down, i think
it should be listed as a possible advantage of the ring, at least.

i hope these comments aren't taken as an endorsement of either the ring
or the Y+ on my part.  i don't have any axe to grind here, nor any
particular current preference for either of them.  i would, however,
before dismissing one or the other, like to have a more detailed and
thought out comparison of them, and have it in writing.  i suppose
this should fall to john, as leader of the configuration group.  it
is an issue that needs immediate attention, i guess, since the land
acquisition discussion is going on even as we speak (virtually).


	-bryan