[mmaimcal]Re: Comment in ALMA memo 489

[Mark Holdaway]

> > Mark's level
> > I suggest that you have an effective algorithm for dealing with the +ACA
> > data case, and that you are dealing with the homogeneous array (ALMA+SD)
> > case less effectively.  I am calling for a fair comparison.
> 
> Koh-Ichiro's level
> Do you mean that ALMA+SD mosaic should use OTF scan same
> as ACA?  However, the dump rate of the baseline correlator
> for cross-correlation is different (much slower) from that
> for auto-correlation.  So, I do not know how to realize
> an interferometric observation and an OTF single dish scan
> simultaneously with the ALMA 64-element array.
> Can you give me your detailed idear?

Mark's new level
Yes, I am suggesting that to be a fair comparison, we need to use the
same total power data.

I have been informed by Larry that all the 12m (the ALMA 64 and the ACA 4)
antennas have the the same electronics for total power.  There are square
law analog detectors that can be used for fast total power continuum, or
the autocorrelation spectra from the correlators, which can still work
pretty fast.  The 4 antennas in the ACA are special in that they are the 
only ones which will have nutators.  

Also, if we have to use the correlator (ie, for spectral line), it is not
all that slow, and if we are doing spectral line we don't need to be
zooming across the source so fast as atmospheric stuff won't be a big
problem for spectral line.

   -Mark

> > 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).
> 
> As I mentioned above, we need to know observing details for
> homogeneous mosaics with the 64 element array.
> 
> > 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.
> 
> I fully agreed with you.
> 
> Take care,
> Koh-Ichiro
>