The AMiBA platform debate
(16/feb/2001)
The AMiBA design team is firming up the instrument specifications. One major item
which is yet to be defined is the platform size. A platform is needed to house all 19
antennas of the CMB experiment. The hope is to use the same platform for the SZ
experiment. That is, the CMB would use 19 antennas, each 30 cm in diameter,
grouped in a tight hexagonal cluster, occupying the central 2 m of a platform. It has
been suggested that a 5 m platform would provide a sound basis for the SZ
experiment, with 19 antennas, each 1.2 m in diameter.
This note identifies a number of issues which would drive the design process, and
help decide whether such a concept is viable. In effect, it attempts to identify those
matters which bear on the question : what array type/configuration best attacks the
SZ goals? Is the straw-man design (1.2 m antennas; 5 m platform) a good
approximation to the preferred option?
I think that all the issues have been aired – in particular in the memos by Haida Liang
and Ravi Subrahmanyan. (AMiBA memos #1, 3, 4, 15).
Note: some instrumental specifications are already largely fixed:
19 Dual-polarisation receivers, with system temperatures ~ 100 K;
16 GHz Bandwidth, 8 channel correlator.
I. Cluster Survey.
The observables are the population statistics : the distribution by size and flux
density. Modest resolution imaging of detected clusters.
The instrument will only be interesting if it has enough sensitivity and resolution to
provide statistically meaningful numbers for a plausible investment in observing time.
The array should also be significantly better than current instruments.
The question to be resolved is this: Is there an antenna size/platform size that makes
sense? The current working model has a primary antenna of 1.2 m diameter, and a
platform of 5 to 6 m in size. Is that interesting enough?
The interlocking isues seem to be:
1. Resolution. Too large an array will lead to a reduction in efficiency of the
instrument, with little/no signal on the longer baselines. A number of different
arguments point to a typical scale size of 1 to 2 arcminutes. One could point
to the current SZ detections (Carlstrom et al); or the SZ-simulations (Ue-Li;
Liddle) that Haida has been examining.
2. Confusion. A measurement will only be significant if it stands clear of the
noise – the receiver noise and the confusion noise. We need to be comfortable
that the survey threshold stands several  above all other sources. There are
two quite different problems:
Confusion from foreground sources. This is somewhat unknown territory,
as we need to extrapolate from lower frequency observations. Holdaway,
Subrahmanyan, Toffolatti suggest a density of
N ( S ) ~ 5S 0.75
(counts per square degree, S in mJy)
This is significantly lower than the expected cluster density distribution and
since we expect less than one cluster for each pointing, it suggests that this
form of confusion is manageable. Note that both the cluster predictions and
the point source distribution predictions have significant uncertainties, but I
don’t think the conclusions change – confusion seems manageable. We
would need high resolution observations if it were a serious issue, and this
would require baselines exceeding 10 m to be useful.
Confusion from other clusters. This problem will set in as the survey
threshold is lowered. Higher resolution observations are unlikely to help as all
these sources are comparably extended. Pushed to the limit, this merges with
the second SZ experiment – the baryon distribution search.
There is some suggestion that clusters are themselves clustered. This would
exacerbate the problem, as it would make the simple population statistics
suspect. I don’t see any simple resolution to this at this stage – one needs the
data. The Carlstrom data suggests that it isn’t a problem.
Point sources within the clusters These, if present, would act against the SZ
signal and corrupt the mass estimates. Estimates (eg, Carlstrom) indicate that
these will not be an issue. High resolution would be the only remedy.
Will additional studies add more light? We could explore the point source confusion
issue by adding sources to the Ue-Li/Liddle simulation fields, and then repeating the
Miriad analysis.
.
II. SZ-filaments
This experiment explores, via the SZ effect, the large-scale baryon distribution. In
practical terms, it will look at the spatial power spectrum at l-values around 3000.
The bandpower seems likely to be just a few K (Holzapfel; the sz simulations).
The platform is well suited for the experiment, but since the bandpower amounts to
sub-mJy map rms, we will need to worry about foreground point source confusion.
The l-space signatures are probably not different enough to separate the two. This
experiment is likely to require many hours of integration on a small number of fields;
I suggest that high resolution observations with a large array (eg, BIMA; ATNF)
would offer a useful defence against the point sources.
M.Kesteven’s conclusions:
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A 5m-class platform would answer the SZ experiment goals, being well
matched to the source scale sizes.
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No compelling case can be made for a “somewhat larger” platform in order to
provide protection against confusion in the SZ cluster count experiment : it
seems that confusion will not be an issue, and secondly, if it were, then a
“seriously larger” platform would be needed.
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The SZ-filament distribution experiment will be a very challenging
experiment. Confusion is just one of its difficulties.