[mmaimcal] Summary of discussions on M51 imaging by ALMA SD ACA

[Al Wootten]

Folks,

I decided not to attend the EAEC/frontend meetings in the UK.  So we will
meet tomorrow.  We will discuss ACA science.  Toward that end, I append a digest
of discussions among ASAC members today.

Clear skies,
Al

Mark Gurwell wrote:
Date: Fri, 21 Sep 2001 16:57:04 -0400 (EDT)

Hi Min,

I was able to download the m51ha images from the iram site.  I took all
10 images:

model (ne)		model (te)
alma only (ne)		alma only (te)
alma short (ne)		alma short (te)
all (ne)		all (te)
hybrid (ne)		hybrid (te)

Where (ne) signifies "no errors" in the data and (te) signifies "typical
errors".  I've concentrated basically on the differences between all and
alma short for each of the two cases, since what we really want to explore
is the improvement brought by the ACA over alma-short alone.

Now, for the no error case there is significant improvement; this is the
same as found by the iram folks in their analysis (they find roughly a
factor of three improvement in fidelity for the highest snr points).  
However, for the noise case, there is almost no improvement.  In fact for
the images provided I find (and it appears that the iram folks concur 
given their plots of image fidelity) that the inclusion of ACA data has
very slightly degraded the image over the alma-short case.  This is
somewhat suprising an disheartening as well, though it may be a case
where the imaging techniques are masking the true limit of the data.

So, I don't know where to go with this...but it seems to me that the
m51 image case will not lead to an example of a clear-cut improvement
provided by the ACA.  What do you suggest?

Thanks,

Mark
Mark A. Gurwell                                       
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Min replied:
Date: Sun, 23 Sep 2001 17:30:06 -0400 (EDT)

Hi Mark,

I just updated my M51 test page with my new analysis.  See if you
would agree.  I would like to quantify the differences in the
ratio images better, but it is not obvious how to make it meaningful.
I'm open for suggestions.  Take a look at

	www.astro.umass.edu/~myun/aca/m51test.html.

I believe the images I analyzed are based on "typical" errors
the IRAM group considers.  I should check that.   These 
tests clearly demonstrate the danger of not having the ACA, 
but I agree with your general conclusion that M51 does not
make the critical "make-or-break" case for ACA. 


					-- Min
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Date: Mon, 24 Sep 2001 09:13:23 +0200 (MEST)

Hi Mark, Min, and Al,

Thanks for the e-mails. The effects that you are finding seem similar to
those for the HCO+ images, namely that there is an effect on the line
ratios but the improvement is small compared with ALMA + single dish. 
I am wondering whether we are too optimistic about the errors in the ALMA + SD
case, and I recall that Stephane/IRAM was going to run some cases where the
SD errors are larger. Al: have you heard anything from this yet? I think it
would be useful to organize a brief telecon with the ACA tiger team by the end 
of the week, e.g. Thursday, to see where we stand.

Cheers,

Ewine 
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Al replied:
Date: Mon, 24 Sep 2001 09:50:10 -0400 (EDT)

Hi Ewine

I haven't heard from Stephane.  I think a telecon would be very useful, as
Mark has some surface error simulations we haven't discussed.  I decided to
cancel my attendance at the RAL meetings, partly to try to get this all in
order.  I think Stephane still will attend, at least some of them; I'll try to
discover when he is available.

Clear skies,
Al
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Mark elaborated:

Date: Mon, 24 Sep 2001 09:52:09 -0400 (EDT)

Min, Ewine, Stephane, and Al,

It seems to me that the real issue is at what frequency we are going to
be performing the bulk of mosaics that can benefit from the ACA.  From
Min's analysis of 230 vs 460 GHz line ratio images 
(www.astro.umass.edu/~myun/aca/m51test.html), it seems to me that
the low frequency benefits (in the case of "typical errors") are small to
nearly unmeasurable, but that at the higher frequencies the benefits are
more tangible.  This suggests an important aspect to emphasize, namely
high fidelity imaging at the higher frequencies (650 and 850 GHz bands).

Clearly, the effects of atmospheric and antenna errors (pointing, anomalous 
refraction, phase screen, atmospheric attenuation and emission) are worse at 
these frequencies, and in these cases that we see significant improvement.
This makes sense, as the errors are proportionally larger due to the much
shorter wavelength.  As Stephane pointed out at the meeting, the inclusion
of the ACA provides a robustness to the imaging process that can overcome
some of the atmospheric limitations, allowing precision imaging of larger
fields at the higher frequencies.  Note that "larger fields" is not really
all that large, even 10" qualifies given the 7" primary beam at 900 GHz.
Therefore, it is reasonable to assume that a large percentage of the science
done at the higher frequencies will receive substantial improvement in 
image quality if the ACA is used.  

This is important, because of the limited time available for high frequency
work we want to provide every reasonable measure to allow for good imaging
at these frequencies...its not like we can just go back to each project 
until we get the perfect weather needed.

To demonstrate this, it might be useful to have someone compile a 650 or
850 GHz image of the same source to see the improvement that the ACA adds
at the high frequencies, including a "line ratio" test ala Min.

The downside of this is that we seem to have a limited use for the ACA at
the lower frequencies, at least given the simulations that I am aware of
so far.  Given that the lower frequencies, particularly 230 and 345 bands,
will be the 'workhorse bands' that will likely have the highest overlap of
proposals and available weather conditions, this means that a lot of science
may not benefit from the ACA.  

We also need to think about this question:  if a high percentage of high
frequency projects need the ACA (e.g. more than 25%), will the ACA be fast 
enough to provide the uv data?  I recall that Stephane calculated (Al, I 
think you came up with a similar number) that 25% of all projects would
need the ACA, and that the ACA would be spending essentially 100% of its
time doing the uv spacing observations for those projects (the factor of
four is to match the sensitivity of the base array with the ACA).  

What if it seems that 50% (by time) of the high frequency projects need 
ACA data? I assume that for the base array, there will almost always be 
a high frequency project done as long as the weather permits its, e.g. 
a 100% use of high frequency weather to do high frequency observing.  This 
suggests that we will not have enough good weather to obtain the ACA data
needed to satisfy the 50% of the projects that need it.  I don't know if
this is a problem, in that I don't have a feel for how much of the high
frequency projects will require the ACA.  However, the simulations that 
show significant improvement mostly at the high frequencies, and the small
field of view of the array in a single pointing, suggest that a fraction 
larger than 1/4 may not be unrealistic.  If so, what do we do? I mean 
besides doubling the size of the ACA?

Sorry for the long email.

Mark
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Min replied:
Date: Mon, 24 Sep 2001 11:00:18 -0400 (EDT)


I think all estimates on the projects requiring the ACA 
are just guesses, and it will be hard to justify any number.
Regardless, I concur with Mark's assessment that the most
significant contribution by ACA will be made at high frequencies.
It was implicit in my understanding of the simulations, but
I was focussing more on the mosaic technique than the
effects of the error arising from the atmosphere and
hardware earlier.  

Frederick and Jerome earlier mentioned that producing larger
mosaics would take a very long time.  On the other hand,
the 230-vs-460 test suggests that the high frequency image
dominates the effect, and it would be worth doing 19 field
mosaic simulation at 650 or 800 GHz, using a scaled down
version of the model image.  

Ewine's question on the estimate of total power error is also
quite valid.  It is dangerous to approach the technical 
people with this question, but we might query a sensible
person and ask what might be a conservative estimate of TP
error we can safely assume (as opposed to what we really like
to see).  Needless to say, the ACA will provide a real
security blanket for the imaging performance if we end up 
nowhere near the target TP performance.
					-- Min
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Al wrote:
Date: Mon, 24 Sep 2001 10:52:36 -0400 (EDT)

Hi Mark

I agree.  After all, the specifications were set to make imaging very
good at 300 GHz and below; I think the simulations suggest that that goal
was achieved.  Mark has the surface error simulations calculated
and is in the process of evaluating them.  Bryan is working on the
debris disk science, including those images.  I think Mark is still 
working on the M51 images, which require more computation owing to the 
number of point sources which must be cleaned.  I think that if we had
examined a CI 809/492 GHz line ratio image of M51, the benefits of the ACA
would have been more marked.

Mark addressed the issue of 'what frequencies the ACA observes' somewhat in
his protomemo on surface errors of 20 July.  That wasn't circulated outside
the tiger team I think but it is at www.tuc.nrao.edu/~mholdawa
and it basically reports the philosophy behind Mark's simulations.

I suspect that the 25% estimate is low for the ACA but I expect that for
some of the time when the 12m array is in an extended configuration, the ACA 
will not be used as often with data collected in that configuration.  During 
good weather, it may be playing catchup.  There will be some periods when the
ACA can make good use of high frequency weather but during which the large
array will be better employed at lower frequencies.  This needs a more
careful examination, to be sure.  

Clear skies,
Al