[mmaimcal] ImCal meeting today

[Mark Holdaway]

Some results of simulations.

	-M

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We have performed a series of simulations similar to the simulations of
J?rome Pety, Fr?d?ric Gueth & St?phane Guilloteau.  The same basic model
images were used, though we first regridded the images to a 256 x 256
size.  

Currently, only pointing errors were included.  Simulations with both
surface errors and pointing errors are churning in the background.  We
calculate the effects of the pointing errors on the visibilities in a
different way than the French team: we perform this computation in the
image plane, and we do not need to truncate the beam at the 20% level
as the French team do (our Gaussian model goes down to about the 0.2%
level).  However, our method of calculating the effects of pointing
errors on the visibilities is more costly than the French method.
Pointing errors of 0.6~arcsec were used for both the 12~m ALMA dishes
and the 7~m ACA dishes.

Only the CLUSTER and the DEBRIS models were used due to time
constraints.  In the DEBRIS model, the bright point source in the
image center resulted in very bad sidelobes obscuring the features of
the debris disk.  This is a defect of MEM.  Hence, we do an initial
cleaning of the compact central source from all fields and mosaic the
residuals.  This resulted in excellent quality images, though the
image fidelity with a 1, 3.3, or 10% cutoff is only sampling the
inner core (only the 0.33% cutoff fidelity samples the extended disk).

We have considered three different instrumental options:
1) ALMA + ACA (total power from 4 ALMA antennas)
2) ALMA HA (homogenous array, ALMA plus total power from 4 ALMA antennas)
3) ALMA + ACA TP (interferometric data from ALMA and ACA, total power from ACA)

The inclusion of the third option was based on the assumption that the larger
fractional pointing errors on the 12~m dishes might affect the imaging worse
than the errors on the 7~m dishes, in spite of the Fourier coverage gap being
located at shorter baselines in the 7~m dish case.

9 pointings of the ALMA interferometer were taken at Nyquist sampled
positions for options 1, 2, and 3.  81 pointings of Nyquist sampled
ALMA total power data were taken for options 1 and 2.  9 pointings of
Nyquinst sampled ACA interferometer data were taken for options 1 and
3.  49 pointings of Nyquist sampled ACA total power data were taken
for option 3.

We performed simulations at 115, 230, 345, 490, 650, and 850 GHz.  As
the French group did, we scaled the image size inversely with
frequency so that the same number of pointings were used for each
simulation.

The results are qualitatively similar to the results obtained by the French group,
in that the ALMA + ACA (option 1) is superior.  In the case of the CLUSETR model,
the improvement with the ACA is marginal (typically 10-30%), while in the
case of the DEBRIS model, the improvement is substantial (more like a factor
of 2 for a clip of 0.33%, the only clip value which samples most of the disk).

Our option 3 is not as good as option 2 (presumably because the
effects of the Fourier gap is occuring at shorter baselines where
there is more power).





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