[mmaimcal] draft memo

[Min Yun]

Simon,

I have a few comments on your proto-memo.  It is probably useful
to write down these numbers so we can use some of these as
baseline figures for future considerations.  Let me know if you
need any clarification on some of these notes.


Introduction
------------

Memo XXX must be my proto-memo on reconfiguration.  I will try to
get it submitted officially by today.  It will have a number by then.


Transport Times
---------------

You use "mean travel distance" of one quarter of the circumference.
I would add another D/2 or R -- this is an increase from about
2R to 3R.  I know this is all sort of orders-of-magnitude
calculation, but this lets you fall on the conservative side.

In the last paragraph, you stated "travel times would be
reduced if the roads were radial, rather than circumferential."
I had thought about this as part of configuration cost optimization.
Indeed, you will save in travel time if we had a network of
radial roads.  In the end, however, radial spokes are needed to
connect these "branches" to the individual "leaves", and all
the radial spokes are ADDITIONS to the circumferential 
distribution system.  I am intrigued to find that
the gain in time is probably not significant.


Reconfiguration Times
---------------------

For the 10 and 20 km configuration, your stating that "useful
rescalable configurations will not be easy to design" is extremely
conservative.  In fact, neither the European group nor us are
considering any rescalable configuration design beyond 
10 km or longer baselines.  The rescalable array designs of
both Conway and Webster only consider the intermediate array,
between the compact and up to 3 km diameter.  I think you can
just come out and say the 10+ km configuration is separate.


Configuration Cycle
-------------------

Here and in the previous section, you assume/propose an 18 month
cycle, reconfiguring every 2 months or so.  The statement in page 4,
"Reconfiguring the ALMA every two months seems a good balance between
overhead and stasis", seems too much of a declaration.  We
need to develop a better justification for such a plan.  When I mentioned
such a plan previously, objections falling pretty evenly on both sides of
the argument were offered by all sorts of people.  Basically scientists
want all configurations at all times (i.e. reconfigure more quickly) 
while engineers and managers favor a far lower frequency since we
increase the chance of accident/damage/wear-and-tear and time/money.

When I looked at your Table 3, I was initially surprized a little because
you propose to go from 3 km config. back to compact config.  I had
imagined it would save time to reconfigure in the sequence,

	150 -> 450 -> 1km -> 3km -> 1km -> 450 -> 150

or some such order.  Based on your calculation, this does not
gain you much since the reconfiguration duration is relatively
independent of array size for <3 km configs.  Was this part of your
motivation?  It would be worthwhile stating clearly somewhere since
someone is bound to raise this issue at some point.

Also, we all agree from the practical point of view that going
to the largest config. only very other time would be better. On
the other hand, would the scientific 
demands be met by this plan?  It is difficult to predict at the
moment, but would there be a disproportionally numbers of projects
requiring the highest resolution possible?  This sounds like a question 
we should pose to the science advisory committee.  The situation with
the VLA is that the demand for all configurations are pretty much
even.  At OVRO, there is a higher standard for high resolution
proposals in order to balance the demands and available suitable 
observing times.  This is a largely operational issue, but 
this will have some implications on the array design (e.g.
transporter).  Besides, this sort of decision is seldom remade
when the array goes into operation.

Lastly, you did not mention the use of a transporter for transporting
antenna between the site and San Pedro.  Not only we will do this
during the construction phase, but we may do this for periodic
overhauls of the antennas or for major upgrades.  This is not a 
reconfiguration issue, but it is relevant one.  Can you mention
the current thinking on how long this may take and what fraction of
time this may happen?  Would this consideration increase the need for
an additional transporter?  If you think this is better suited for
my reconfiguration memo, I can add this into my memo instead.  
At any rate, I was hoping you would make a more concrete recommendation 
for the number of transporters required from the reasonable 
reconfiguration duration considerations.




					-- Min