[mmaimcal] stretched E and D arrays

[Min S. Yun]

In case it has not been made clear, I had always imagined that
reconfiguration of 64 element donut array would be done in
16 antennas at a time -- a full reconfiguration takes moving
32 antennas, but we can visit 2:1 (or 3:1 if needed) elongation 
hybrid array in between.  So, no extra moves are required for 
the hybrid array in this case.

What I have not had time to investigate in detail
is the imaging performance of these hybrid arrays, but I
suspect they will be reasonable for low elevation sources 
because of foreshortening of the N-S projection.  I will try to
do this before heading out to Grenoble, but I have only one day
before leaving for ASAC....


				-- Min

On Wed, 14 Feb 2001, Mark Holdaway wrote:

> Dear All,
> 
> I am changing my comments made at yesterday's Img/Cal meeting.
> After reviewing Old MMA Memo 155, I see that the E configuration
> (and pretty much the D as well) have little sensitivity loss
> due to shadowing between dec +20 and -70, which is very similar
> to Memo 155's D1 array, which went from +24 to -72.
> 
> So, I take back what I said about this configuration's shadowing
> performance.
> 
> This points to another problem, though: both the E and the D
> configurations (D array shadowing is very similar for both the
> Donut and the Spiral arrays) will need to have at least one
> more hybrid configuration to permit far Dec observing, unless
> we decide we can tolerate the shadowing and the highly elongated
> beams.
> 
> Quantitatively:
> 
> E array transit observing:
> (Note: the situation degrades rather smoothly as we go off transit)
> 
> dec	%baselines	% sensitivity
> 	remaining	remaining
> 
> +50	.04		.2
> +40	.28		.53
> +30	.61		.78
> 
> -70	.77		.88
> -80	.45		.67
> 
> 
> D (spiral) array transit observing
> (D donit is a bit worse)
> 
> dec	%baselines	% sensitivity
> 	remaining	remaining
> 
> +50	.56		.75
> +40	.69		.83
> +30	.80		.89
> 
> -70	.88		0.94
> -80	.71		0.84
> 
> 
> -------------------------------------------------------------
> 
> For the loss in sensitivity, we can do a simple cost-benefit analysis,
> trading the capital investment for more antenna stations and time lost in
> moving antennas against the cost of the sensitivity lost (considering the
> fraction of time ALMA would observe in each DEC range in the two compact
> arrays). To come up with a cost of the sensitivity that is lost due to
> shadowing, we could take the capital cost of the ALMA, add to it its
> lifetime of operating costs, and divide by its lifetime to get a cost per
> unit time.
> 
> This cost-benefit analysis would not address any losses to the scientist
> for not having a circular beam, which is more difficult to assess
> (unless we were take a brute force solution such as to taper the (u,v)
> coverage to have the same maximum extent in all directions).
> 
> For the compact spiral configurations, there are antenna pads everywhere,
> so the only costs in going to stretched configurations will be logistical
> (ie, paying for the antenna moves) and lost observing time.  To some
> extent, we will need to move those antennas out ANYWAY (though the order
> may be different, and we may endup moving some back in, too).  It seems
> pretty clear to me, without doing the math, that making stretched
> E and even D configurations is a big win for the Spiral array case.
> For the donut array case, there would be more cost in stations, so I won't
> make such a confident guess for it.
> 
> Just taking observational density as proportional to sky area,
> 
> D array loses 4% of its integrated sensitivity to shadowing (assuming
> here that all is transit observations; it will be higher for more
> realistic cases),
> 
> and E array loses 11% of its integrated sensitivity.
> 
> Lets say we are in E array 17% of the time, in D array 17%.
> Also, lets say ALMA is $600M + $20M * 20 yrs = $1 Billion.
> 
> So, the lower limit to the cost of the shadowed sensitivity, over the
> lifetime of the ALMA, is
> 
> E:   .17 * .11  * $1 Billion   =    $18.7 Million
> 
> D:   .17 * .04  * $1 Billion   =    $ 6.8   Million
> 
> Which I think is well worth saving by dealing with the
> logistical issues of the hybrid arrays.
> Again, this doesn't consider the additional benefit of having more
> circular beams, or the cost of any addition infrastructure,
> lost observing time due to the moves, etc.
> 
> 
> On the other hand, the loss is only $1.3  Million per year, small
> compared to the operating budget.
> 
> 
> 
> 
> 	-Mark
> 
>