[asac] Message from Neal

[Min Yun]

Hi Karl,

The following comment by Neal has been brought up several times
previously, and we still don't seem to have an explanation as what this 
15 minute spec for changing bands is and how it is derived.  As I
state below, the ability to scan thru continuum bands quickly
will allow us to determine the redshifts of high-z dusty galaxies
and to understand their evolution.  We should ask the receiver
group to clarify the situation and investigate ways to cut down
on the switching time.

> 
> On other items, I agree that selecting and tuning to a new band should
> take less than 15 min, unless I misunderstand what this means. Presumably,
> there will be look up tables and fast iterative procedures to optmize
> at a particular frequency.
> 

The latest number from the Large Millimeter Survey conference 
at U Mass last week is that 65% of all SCUBA sources are too 
faint to be identified at optical or NIR.  The mm/submm SED 
modeling plus detections of CO transitions may be the only way 
to obtain redshifts and understand their
evolution.  Since even ALMA will have to search a wide range of
frequencies to find the CO lines, quickly mapping out the dust SED
would be a useful first step in the redshift determination (my
presentation at the conference).  While the 15 minute switching
time for receiver bands is not completely limiting of this
sorts of work, it is easy to see that this switching time dominates
the total observing time by an order of magnitude.  In fact, this
would be true for any observation trying to characterize the
mm/submm continuum spectrum (e.g. stellar photospheres, circumstellar
disks).  It would be good to avoid this situation if possible at all.




					-- Min