[mmaimcal]Re: Comment in ALMA memo 489

[Mark Holdaway]

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.

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.


Now, the observation described above is sort of a 1 pointing mosaic with a
guard band of total power data about it.  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) -- 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  I originally made this point to Pety in 2001, but it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.  That was during the rush to the
ACA, which seemed unstoppable.  It seems that we are currently able to be 
more thoughtful about all of this.

   -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