[mmaimcal] Query from Jaap Baars

[Al Wootten]

Mark Holdaway writes:
 > 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).
Thanks, Mark

I agree with this.  I think that the ACA will be used primarily at the
higher frequencies, perhap even beyond the scope of ALMA proper.
ALMA has planned for 4 with 1 spare nutators for the 12m antennas.

Stephane has weighed in with his opinion (similar to your No. 1):
'Dear Jaap,

    My first reaction would be to get rid of the nutators on the ACA
antennas. The reason for nutators is eventually to
get the short spacing information from single-dish observations. Obviously,
we want to do that with the 12-m antennas,
whose single-dish spacing information will provide some overlap with the ACA
array, but not with the ACA antennas
themselves.

    Similarly, we don't need all 12-m antennas do be equipped with nutators.
4 or so is enough.

   If any sort of atmospheric cancellation is desired in "total" power mode
for the ACA antenna, fast scanning across
the sky will have to do it. Since the antenna will have to support fast
switching anyhow, that should be possible.

        Stephane'

However, I think your thoughts are somewhat more substantive.  Let's discuss
how to proceed tomorrow.

Al