[alma-config] [Fwd: Re: Memo 538 draft]
Frederic Boone
frederic_boone at yahoo.fr
Fri Sep 30 07:23:57 EDT 2005
Dear Alwyn and John,
Thanks for the news and the draft.
> A 50 element ALMA array will not enable the full
> Level 1 science requirements,
This is indeed an important change that implies some
modifications in the design.
More quantitatively if we assume (for illustration
only!) Nyquist sampling (2*lambda/D) is required and
the observations cannot last more than 8 hours then:
1. according to Eq. 24 of A&A 386, 1160, the
configurations should not exceed 3.5 km.
2. according to Eq. 25 it is possible to have a
gaussian distribution of samples with a 10dB cutoff at
the edges provided the configurations do not exceed
1.2 km.
The largest configuration (3.5km in this particular
case) should look like a ring or Reuleaux triangle
(top of Fig.4 in A&A 386, 1160), the configurations in
case 2 above (<1.3km) should be centrally condensed
(bottom of Fig.4) and the configurations in the range
1.3--3.5 km should have an intermediate design (see
Fig.4, kind of rings with antennas in the center).
These values are given for illustration (I am not
claiming we should not have configurations larger than
3.5 km!), if a sampling at 4*lambda/D (i.e. 2*Nyquist)
is declared to be enough for the sience requirements
then these values should be multiplied by 2 (and so
on...).
Applying these simple considerations to John's design,
a spiral configuration with a maximum baseline of 3.5
km implies a sampling at the edges=
3.5/1.3*Nyquist=2.7*Nyquist. But this is assuming a
10dB cutoff, and the spiral produces a much lower
cutoff so that the sampling is most likely >4*Nyquist.
This choice might be justified but it should be clear
that it does not favour imaging of extended complex
sources (if the source is known a priori to be 4
times smaller than the primary beam there is no
probem). And we are not yet in the highest resolution
regime (these are not the largest configurations).
Other choices are possible (e.g. more antennas at the
edges of the configurations) that would give a better
sampling of the uv-plane.
Some comments about the memo draft.
------------------------------------------
1. About the re-weighting method mentioned to force
the beam shape it would be nice to give the
reference(s). To my knowledge it was first introduced
in the appendix of ALMA memo 400 (also presented at
the CDR held in Socorro january 2002).
2. About the sentence in Appendix A:"It is certainly
not the case (as has been claimed in Boone2002) that
the output of any such deconvolution is always
equivalent to the noise increase due to a re-weighting
of the data so that the dirty image main lobe equals
the clean beam.".
There must be a misunderstanding.
Indeed, this paper is entirely dedicated to estimating
the effects of deconvolution and minimizing these
effects with appropriate sample distributions. It is
clearly written that interpolation between the samples
is part of the deconvolution process and that it
introduces some noise. This effect is even detailed in
the Appendix B for the case of Shannon interpolation.
In the Section 2 of the paper it is said that,
although
it is difficult to give a universal (i.e. true for all
the deconvolution methods) estimate of the errors it
can be reasonably expected that the behavior will be
similar for all methods: the further away the samples
the larger the error.
Then, as the goal of the paper is to minimize the
errors with appropriate distribution of samples, it is
proposed that for a given scientific goal there is a
spacing (more accurately one should talk in terms of
density of samples) that should not be exceeded (which
depends on the method) to keep the errors close to the
instrumental errors. Then, when we are in this
regime, additional errors come from the reweighting.
Here it should be emphasized that "reweighting" does
not refer to the deconvolution method actually used:
"reweighting" refers to a process that is hidden or
not but present in *all* the deconvolution methods
*without exception* as long as the output is a clean
image.
3. Then in the next sentence John, you seem to claim
that deconvolution does not necessarily increase the
noise like reweighting does (which again I never
claimed because the interpolation process also
contributes) and give the example of a point source.
I don't understand the argument: for a point source
there is no imaging process and therefore no
reweighting. For a point source all the samples carry
the same information their distribution does'nt matter
a all!
The title of my paper is "Distributions of Fourier
samples for imaging" not for "model fitting"...
All the point in this reweighting discussion is that
the distribution of samples matters. By improving the
distribution (even if sampling is already ideal) we
can improve the image. I guess we agree on that point
(we would probably not care so much about the
configurations otherwise...).
4. Then you seem to associate the "reweighting" with
the "restoring" process.
But again "reweighting" is always there (hidden or
not) as long as one produces a clean image (not a
model fit) with a finite resolution. In MEM the beam
shape can vary acrross the map but the resolution is
never infinite.
There is a beam so there is "reweighting".
I would appreciate if these comments can be taken into
account in the memo.
Best regards,
Frederic.
> but the Executives requested an
> analysis of this baseline,
> based on currently available funding. Accordingly,
> the JAO asked the
> Science IPT for a design for a 50 antenna array
> which could provide
> excellent imaging along with the possibility of
> extension to 64 antennas.
>
> John Conway has provided this design, which is
> described in a document
> located at the following location (userid and
> password necessary):
>
http://edm.alma.cl/forums/alma/dispatch.cgi/docapproval/docProfile/103106/d20050829201752/No/t103106.htm
> or at:
>
http://www.oso.chalmers.se/~jconway/ALMA/OUT/ALMA-90.02.00.00-005-A.SPE.pdf
>
> A separate document in preparation by M.Holdaway
> defines the positions
> and reconfiguration scheme for the approximately 35
> additional pads
> which are needed for the outer configurations of
> ALMA. This will be
> ready by year's end, after some iteration with
> environmentall concerned
> vizcachas and site road and network design.
>
> Also, I'd like to draw your attention to a new ALMA
> Memo on the
> configuration for the compact array, No 538.
> Currently it is on edm
> only but it will appear at www.alma.nrao.edu soon.
>
> Your comments are invited on these documents.
>
> Thanks,
> Clear skies,
> Al
> > Date: Thu, 29 Sep 2005 14:33:44 -0400
> De: Carolina Lizama <clizama at alma.cl>
> À: "MORITA, Koh-Ichiro" <morita at nro.nao.ac.jp>,
> mholdawa at nrao.edu
> CC: Al Wootten <awootten at nrao.edu>, cwhite at nrao.edu,
> alma_edm at eso.org
> Objet: Re: Memo 538 draft
>
> Dear Koh-Ichiro,
>
> Memo # 538 has been approved and posted.
>
> Title: *Array Configuration Design of the Atacama
> Compact Array*
>
> Authors: *Koh-Ichiro Morita (NAOJ)*, *Mark Holdaway
> (NRAO)*
>
> The link is:
>
http://edm.alma.cl/forums/alma/dispatch.cgi/almamemos/showFolder/100569/def/def/592372
>
> Regards,
> Carolina
>
>
>
>
>
>
> MORITA, Koh-Ichiro wrote:
>
> >Dear Sir/Madame,
> >
> >Here are PDF copy, PS copy, and abstract files for
> >ALMA memo 538.
> >
> >Cheers,
> >Koh-Ichiro
>
>======================================================
> >Koh-Ichiro Morita
> >Nobeyama Radio Observatory,
> >National Astronomical Observatory, Japan
> >Minamimaki, Minamisaku, Nagano 384-1305, Japan
> >Email: morita at nro.nao.ac.jp
> >Phone: 81-267-98-4396
> >Fax: 81-267-98-4339
>
>======================================================
> >
> >
> >
> >
>
------------------------------------------------------------------------
> >
> > ALMA Memo # *538*
> >
> > *Title*
> >
> > *Array Configuration Design of the Atacama Compact
> Array*
> >
> > *Koh-Ichiro Morita (NAOJ)*,
> > *Mark Holdaway (NRAO)*,
> >
> >
> > *Date(15/09/2005)*
> >
> > Keywords: *Atacama Compact Array, Array
> Configuration*
> >
> > *Abstract*
> >
> > In this memo, we present a design concept of the
> array configuration
> > of the Atacama Compact Array (ACA) and strawperson
> plan of the
> > configuration design. Basic parameters of the
> array configuration of
> > ACA were discussed from the analysis of the
> sensitivity in {\it uv}
> > plane of mosaicing observations with the ACA and
> the ALMA. For
> > detailed design, it was shown that the compact
> spiral concept is
> > appropriate to meet major requirements for the
> ACA, which are to
> > obtain higher {\it uv} response at the short {\it
> uv} spacings and
> > better sidelobe performance. To satisfy the sky
> coverage requirement,
> > the north - south elongation is needed. We propose
> that the ACA system
> > consists of two configurations, one (Inner Array)
> is a compact spiral
> > array with small north - south axis ratio (x 1.1)
> and the other (NS
> > Array) is a dedicated configuration with large
> north - south axis
> > ratio (x 1.7). In current actual plan, inner 6
> pads are shared by both
> > configurations.
>
> > begin:vcard
> fn:Carolina Lizama
> n:Lizama;Carolina
> org:ALMA Project Santiago Office;Joint ALMA Office
> adr;dom:;;El Golf 40 piso 18 Las Condes
> email;internet:clizama at alma.cl
> title:EDM Specialist
> tel;work:56 2 4676134
> tel;fax:56 2 467 6103
> version:2.1
> end:vcard
>
> > _______________________________________________
> Alma-config mailing list
> Alma-config at listmgr.cv.nrao.edu
>
http://listmgr.cv.nrao.edu/mailman/listinfo/alma-config
>
--
Frédéric Boone
MPIfR (Max-Planck-Institut für Radioastronomie)
Auf dem Huegel 69
D-53121 Bonn
Germany/Deutschland
Tel. : 49-228-525471
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