[mmaimcal]Re: reconfiguring

[Mark Holdaway]

Taking all data from 1996 to 2002, 


TAU:

The Bolivian Winter is very obvious in TAU, with median opacities (median
of all measurements on a given day, over all hours and years of that day)
jumping a factor of 2-5 over the regular good times.  There WILL be some
good conditions mixed in (ie, the Bolivian Winter "breathes" in and out,
and the details of how it breaths will vary from year to year). 

Opacity shows essentially no diurnal effect during the good conditions,
but shows a significant diurnal effect during the southern summer, so
there will be some good TAU times, especially at night.

It starts suddenly around Day 350 (Dec 15) and slowly comes
back to normal by day 100 (April 10).  This is like 115 days, or
16 weeks.  The WORST of the Bolvian Winter is Days 355 - 80
(95 days, or 14 weeks).  And THE ABSOLUTE WORST of the Bolivian Winter is
Days 360 - 60 (65 days or 9 weeks).  I won't tell you when the TRULY MOST
HORRIBLE TERRIBLE part of the Bolivian Winter is.

PHASE:

The Bolivian Winter is much less obvious in phase, amounting to
roughly a factor of 2 larger "median phase per day".  It begins
around day 350 and ends around day 100.  It seems to end less
gradually than the TAU.


DEMANDS of Y+

Y+ will be used at all frequencies.
There may be a bias towards higher frequencies to get the
highest possible resolution, or towards low frequencies so
that the resolution isn't so great that it resolves out everything.
I don't know.

Phase stability will be an issue, but fast switching theory basically
predicts that the residual phase will be the same as if it were a 500m
array.  One effect that goes against this is that at such high
resolution (ie, 50 mas at 90 GHz), some of the calibrator sources may
stop looking like point sources, or more importantly, may lose some
flux and not be as efficient in calibrating.  However, as the jet
components (ie, > 50 mas) are steep spectrum and the compact core is flat
spectrum, I expect this to be a minor effect.

SO MY GUESS: there is no significant reason that the Y+ array will be
more adversely affected by the Bolivian Winter than, say, the Marching
Through Intermediate Arrays will be.


DEMANDS of COMPACT ARRAY

As the resolution increases with frequency, we will be pushed to smaller
arrays (ie, THE COMPACT ARRAY) for a lot of the high frequency work.
As the opacity goes to hell at high frequencies even in the best of
conditions, I think there may be a good argument for trying to avoid
the Bolvian Winter for the COMPACT ARRAY.  

Phase stability: fast switching is not so effective for the compact array
because the crossing time is comparable to the switching time.  You won't
wreck the observations by doing fast switching, and it won't be worse than
longer baseline observations; its more like you are stuck with what the
atmosphere provides you.  I don't think the phase stability pushes us
one way or the other.

The wind speed is pretty much anti-correlated with the Bolvian Winter.
BIG WINDS begin gradually around day 100 and come down around day 330.
AS POINTING IS ALSO IMPORTANT FOR THE COMPACT CONFIGURATION (for
both mosaicing and for high frequency work), THIS seems to give some
merit to having some COMPACT array time during the Bolivian Winter.

NOTE that this analysis DOES NOT look at diurnal effects, and there are
always good calm (low wind) conditions at night, even in the windiest
winter times.



GIVEN THE FAIRLY BROAD NATURE OF THE BOLIVIAN WINTER

I don't think we can avoid putting any confifuration ever in
the Bolvian Winter.  If any configuration is justified in being spared,
I think it is the COMPACT CONFIG.
(Al should make his arguments for sparing the Y+ configuration.)
We can probably arrange for the COMPACT CONFIG to fall on the
part of the Bolivian Winter that is usually not the worst.


Take care,


   -Mark


> Hi,
> 
> (1)  Yes I think with a cycle time of 9 months you get 
> quite good properties, the time to cycle the seasons
> is 3 years, so 18 months to go to being the  opposite season 
> - but since the compact array and most extended arrays 
> are of order 6 weeks long, the time between the end 
> of one compact array and the start of another is 7.5months
> (similiary for the most extended array), so you get an approximate 
> shift  from day to night observations for your favorite 
> source on the shortest possible timesclale.
> 
> (2) A year can be divided into 8 periods of 6.5 weeks 
> With an exact 9 month schedule, the compact array 
> only every occurs in 4 out of these 8 calendar periods.
> (similary for the most extended array)
> This can be used to advantage if there were periods 
> of order 6.5 weeks long one wanted to avoid for 
> particular arrays.
> 
> If on the other the 
> object is to give every configuration a 'fair shake'
> and sample all parts of the year equally then you would like to advance
> the cycle by about 6.5 weeks over 3 years, so a cycle period of around 41
> weeks would accomplish this. This would ease transporter
> load but it would then be longer between end of a compact array 
> and the start of one (35 weeks) but still give some day/night
> cycling within a single year.
> 
>   John
> 
> 
> 
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