[mmaimcal] Query from Jaap Baars

[Mark Holdaway]

> 
> It occurs to me that an important design driver for these small 
> antennas will be the possible incorporation of a nutating 
> subreflector. If these are needed, it will have a major influence on 
> the design, and the cost, of the antennas. From a design and cost 
> perspective, we should avoid them, if at all permissible.
> 
> It is for this reason that I approach you, as Project Scientists and 
> ASAC Chairmen, with the question:
> 
> Do any, or all, of the ACA antennas have to be equipped with a 
> nutating subreflector?
> Or can we do without them, and make life a lot easier and cheaper?
> 


Three opinions, none with clear nutator recommendations, for the price of
one:


1) IF the ACA is used only for short baselines in a perfect world, you
don't need total power, as the 12m dishes will be effective here.  The 8m
or 6m dishes don't add any information you don't already have.

2) IF the ACA is used substantially for ultra-high frequency observations
where the 12m dishes cannot follow, then you need the total power from
them; and if we have decided that we need nutating subreflectors for the
12m dishes, we probably need them here as well.

One thing to consider: while there is not going to be a huge design effort
on the small dishes, we could probably do a lot better on them than the 
12m dishes WRT fast slewing: they are smaller, will have a higher lowest
resonant frequency, and can move and turn around faster, so they could
qiute possibly take total power continuum without a nutator.


3) Even if we are going to be using the ACA just as a short baseline
array, the smaller dishes would likely have better pointing and surfaces,
and hence total power collected from them would be more reliable than
from the 12m dishes, and we might want this data more than the 12m
data (iw, we would weight it not according to the thermal noise, but
accoridng to systematic noise levels).



I would rephrase the original question:

1) What will the ACA be used for? (both short baseline and high freq)
2) Do we believe we will need total power from the ACA?
	(yes, mainly based on high freq; partly based on short
	baselines, potentially larger errors on 12m dishes)
3) Do we need nutators on the 12m dishes (have we yet answered yes?),
     and on the 8m dishes (we need total power; we don't have a design
     to study to say yes or no with).

	-Mark