[mmaimcal]Re: Comment in ALMA memo 489

Tue Apr 6 12:40:07 EDT 2004

Hi Mark

My own "On the fly" comments, so some of them will be wrong...

----- Original Message -----
From: "Mark Holdaway" <mholdawa at tuc.nrao.edu>
To: "Michael Rupen" <mrupen at aoc.nrao.edu>
Cc: "Stephane Guilloteau" <Stephane.Guilloteau at obs.u-bordeaux1.fr>; <mmaimcal at nrao.edu>; <almasci at nrao.edu>
Sent: Tuesday, April 06, 2004 5:38 PM
Subject: Re: [mmaimcal]Re: Comment in ALMA memo 489

>
>
> The way I look at it:
>
> If you had just a single pointing of interferometric data plus a single
> total power point, that total power point will only raise or lower the
> entire map like a piston -- it isn't making the image any more realistic.
> In this case, it is just the (0,0) point. In order to get out the higher
> spatial frequencies in the single dish data, you need multiple pointings,
> and knowledge of the beam.  So, instead of just that one pointing, lets
> make a 3 x 3 map of single dish pointings.  Now, the adjacent pointings
> also have their (0,0) measurements, and those measurements, combined with
> the beam, tell the image how the extended emission must "bend". So, those
> adjacent pointings are telling the total power data in the central
> pointing how better to distribute itself (ie, higher spatial frequencies
> than (0,0), but information you could never get if you didn't have more
> pointings, so it isn't just doing a piston thing.

    Sounds a priori reasonable, but when short spacing is missing, what you see is not a negative offset, but a negative
bowl... I never found any decent explanation for that. Any idea ?

>
> The interferometer data has all the multiple spacings (down to the
> shortest baseline) already, so it KNOWS how to distribute the flux inside
> a given pointing without recourse to additional neighboring pointings.
>
    But what is done in the joint deconvolution of ALMA+SD   DOES take that into account !.
The knowledge of the location of the more "compact" sources found by CLEAN directly tells where to locate the single
dish flux. Remember, we are deconvolving the mosaic all at once, and NOT making a linear mosaicing after deconvolution.

>
> Now, the observation described above is sort of a 1 pointing mosaic with a
> guard band of total power data about it.
    We never discussed the case of a single-field in this way. Comparisons were made on Mosaics only...

> Generalize it in your mind to a
> multi-pointing mosaic with a guard band in total power around that.
>
> In the simulations, the ALMA+ACA+SD case benefits from this guard band in
> that an OTF map was used (though that case doesn't need it as much, as the
> shortest ACA baselines also have some of the same information) --
    Actually, not really. The OTF map was used as an intermediate step, but I don't believe we take any information from
a guard band around the ACA data. Need to go back to details to check that.
    I agree however that no guard band for ACA still implies SOME guard band for the ALMA fields, since the ACA beam is
larger. But the ALMA antennas did not observe that guard band in interferometry.

> but the
> ALMA+SD simulation case didn't benefit from this guard band since it was
> never included.  I originally made this point to Pety in 2001, but it
> apparently wasn't properly understood.
    I remember quite well you made this point, but the fact is that there is no way to implement such a guard band in
the
CLEAN algorithm we used.

> That was during the rush to the ACA, which seemed unstoppable.
    I disagree both with "rush" and with "unstoppable", which to me seem to imply we expected a pre-definite answer.
    There was a clear demand from the ASAC to make simulation to ASSESS the usefulness of the ACA, not to PROVE it. Time
was short, and the available software too. Simulations were made with MEM which showed ACA improve things, we (Pety et
al) made simulations with other techniques based on CLEAN which show a similar result.

    It was in particular the convergence of the two approaches which convince the ASAC. As I remember, somebody (can't
figure out who) finally said "anyhow, it is certainly better to measure these spacings than to entirely rely on software
to recover them".

> It seems that we are currently able to be more thoughtful about all of this.
>
    Algorithms which use the "guard band" along the lines you propose remain to invent, or at least to be used for this
application. I still believe you (or somebody else charged of that job) must make simulations proving the assertion.
Remember that one of the problems in ALL deconvolution techniques is the amplification of errors. You argue that you can
use the information better (and I tend to agree with the rationale, although there are some contradiction with the
sampling arguments), but I believe it remains to be seen whether this improved information is sufficient in realistic
cases, i.e. cases with observational errors.

In short, we have some good reasons to believe that
    - perhaps the CLEAN based method is not optimal
    - and perhaps other methods can do better
but both assertions remain to be proven. The second one also includes the first, anyhow, so working on the second is
best for every body...

        Stephane

>    -Mark
>
> > Hello all,
> >   a quick response to one of Stephane's points --
> >
> > > Mark also says a guard band in Single-Dish mode ONLY also improves the
> > > image quality. If I have followed correctly the discussion, I believe
> > > the only demonstration of that (if any) is based on data where there was
> > > NO EMISSION outside the mosaicked area. May be I am mistaken, but in
> > > case this is true, I believe it is important to check whether a guard band
> > > also helps when the source is more extended than the mosaicked area,
> > > because this is likely to be the actual situation.
> >
> > I've done this in practice, if not in theory, in several cases, and the
> > guard band does seem to help, mainly in modeling the extended structure
> > (errors in which tend to dominate current maps).  This is VLA + 140-foot
> > HI stuff.  Others must have similar experience, since it's so much more
> > expensive to get interferometric observations, and basically trivial to
> > get a little extra single-dish time to observe the guard band; and as
> > Stephane says emission extending beyond the area of interest is likely to
> > be the rule when mosaicking, rather than the exception.  Does
> > this match other people's current experience?  Of course this may not be
> > relevant to the infinitely better data ALMA will produce.
> >
> > Cheers,
> >
> >        Michael Rupen
> >        NRAO/Socorro
>
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