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more single dish data than the ALMA+SD case did.  However, from your
words above, I do not get that clear impression.  Is it true or not?


> 2. Band guard: my recollection is that simulations show that observing a
>    band (either SD or interferometry) around the field-of-view we want to
>    image always improve fidelities. Depending on the processing of the SD,
>    it may be mandatory (ie to avoid aliasing effects). For the
>    interferometric map, I don't think it is mandatory but it
>    helps. However, I doubt that program committees will allow us to observe
>    around the field-of-view of interest (which may be empty!): it will be
>    seen as a lost of time.

I suggest two different philosophies for these simulations:

A)  Make images with the ALMA+SD and ALMA+ACA+SD cases with the
    exact same data (except for ACA, obviously).    (Presumably,
    no extra guard band is provided for the ALMA+SD case in this
    philosophy -- the discussion of the guard band in Memo 489
    is intended mainly to point out that the extra SD data in
    the ALMA+ACA+SD case would really have helped the ALMA+SD
    case).

B)  Ask a different question:  what data is required to make a
    really good image for the ALMA+SD case?  and for the 
    ALMA+ACA+SD case?  If we need to spend significantly more
    time to do one or the other (ie, to get a guard band),
    we just state those facts.  We don't throw up our hands
    in advance and say "We'll never get this" -- rather, we
    look at what extra we might need to get and understand
    that as a tradeoff -- for example, it might be more
    efficient to use some extra time with the full ALMA
    than to use more time with the ACA in some case.  But
    we can't answer that tradeoff question if we never
    ask the right questions in the first place.

> 3. Declination range: To optimize the use of ALMA, it will probably be used
>    as much as possible when sources are high and for high sources. In
>    addition, what will be the smallest elevation authorized at high
>    frequency where the atmosphere plays such an important role? So this
>    limitate the effect of baseline projection. But anyway, declination
>    effects must obviously be studied in more details.

Very few sources will be observed right at zenith.  
Yes, there is a tradeoff between opacity & phase stability on the
one hand and zenith angle on the other, but people will push to
observe sources at all possible declinations -- though they will
push less and be less succesful for high zenith angle observations
at the highest frequencies.

Morita-san is correct in stating that more extreme declinations will
require N-S stretched configurations, and at the first declinations
you switch over, you will have the problem of not-so-many short
spacings all over again.

So yes, declination is something complicated that needs to be further
studied, after we take care of some of the above problems.

> 4. Simultaneous observations are always desireable. However, I expect that 
>    PIs will merge ALMA configurations taken at different times. Is that as
>    much different than taking ALMA and ACA observations at different times?
>    What must be carefully planed is that the ALMA and ACA observations are
>    done in similar weather conditions.

Unlike the VLA, the ALMA configurations are designed to support 
"single configuration imaging" by including some very short baselines
in every configuration.  These will not always be enough to provide
the astronomer what he/she needs/wants, but I suspect more than
half the time, single configuration imaging will suffice.  

A related problem that may be more problematic to my original argument is
that dynamic scheduling will often break up an observation into
small bits over several days, in which case part of the benefit
of "simultaneous observations by the homogeneous array" evaporate --
ie, we still need to combine data taken under different atmospheric
conditions.   However, it probably isn't as systematically bad if
both the total power and the interferometric data sample those
dvarying conidtions together.

Take care,

    -Mark
 
> Best regards,
>    Jérôme.
>