[asac] [almanews] ALMA Memos 403 and 404 Released

[Stacy Oliver]

ALMA Memo #403

Fast Switching Phase Correction Revisited for 64 12 m Antennas

M.A. Holdaway (NRAO/Tucson)

2001/12/17

Fast switching phase calibration has not been investigated for the ALMA
telescope since ALMA has been defined as 64 12 m antennas. Furthermore, the
logic chain which picked the optimal calibrator in past investigations was
approximate. In order to better understand the requirements which are placed
on the current ALMA design by fast switching, we have rewritten the fast
switching simulation code in AIPS++, including a more complete optimization
with fewer assumptions, using updated sensitivity, antenna slewing, and
atmospheric information.

We find that when the observing frequency is matched to the phase stability
(ie, high frequency observations are always carried out during the most
stable phase conditions), the Chajnantor site is good enough to permit fast
switching observations of the expected frequency range (ie 30 to 950 GHz) to
succeed with high efficiency. Typical observing efficiencies, including both
time lost to the phase calibration cycle and decorrelation losses, range
between 0.80 and 0.90 for sources above 45 deg elevation angles. The
observing efficiency decays very gently at lower elevation angles, with a
typical efficiency of 0.70 at 20 deg elevation.

The extra sensitivity provided by 64 12 m antennas does not help as much as
might be expected with fast switching because the time spent integrating on
the calibrator is very small compared to the entire cycle and is a
moderately small portion of the calibration phase of the cycle. The 1.5 s
delay due to changing frequencies is pretty well matched to the slew times
for typical objects. The slew profiles provided by Vertex are sufficient.

The residual phase errors resulting from fast switching will cause
baseline-dependent decorrelation. Some minor algorithmic work should proceed
on fixing this decorrelation. It seems likely that the phase information
gleaned from observing the calibrator will be sufficient to accurately
estimate the decorrelation correction on a per baseline basis.

View a pdf version of ALMA Memo 403.
http://www.alma.nrao.edu/memos/html-memos/alma403/memo403.pdf

Download a postscript version of ALMA Memo 403.
http://www.alma.nrao.edu/memos/html-memos/alma403/memo403.ps
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ALMA Memo #404

Atmospheric Dispersion and Fast Switching Phase Calibration

M.A. Holdaway (NRAO/Tucson) and J.R. Pardo(Inst. Estructura de la Materia
Dpto. Fsica Molecular Serrano)

2001/12/19

The differential atmospheric phase of an interferometer has an approximate
linear variation with frequency up to about 300 GHz. However, near
absorption lines, and especially in the sub-millimeter wavelength
atmospheric windows where the absorption lines are very strong and always
near, the assumption of a non-dispersive atmosphere breaks down markedly.

We present simulations performed with the ATM atmospheric transmission model
(Pardo et. al, 2001), tailored to the specific observing conditions at the
Chajnantor site and we propose specific observing strategies to employ for
the ALMA telescope. While the *absolute* wet and dry dispersive phase (ie,
the part of the phase which deviates from the phase which is linear with
frequency) can be very large through the atmosphere, the *differential*
dispersive phases (ie, the difference in the dispersive phases above two
antennas paired in an interferometer) are much smaller. We find that the
differential dry atmospheric dispersion is essentially zero at all
frequencies of relevance to the ALMA for the expected pressure fluctuations
within the area covered by the interferometer. The differential wet
dispersion can be large enough to be of concern in the 350, 400-500, 650,
and 850 GHz windows.

In fast switching, we expect to observe a calibrator source at 90 GHz and
scale the phase solutions to the target frequency. If time dependent wet and
dry phase errors occur, ALMA has a potential problem because the wet and dry
phases will scale differently with frequency in the sub-millimeter windows.
Separation of the phases into wet and dry components may be possible, but
this sounds very messy and uncertain, requiring multi-frequency calibrator
observations or associated radiometric measurements and good atmospheric
modeling. If dry phase errors are negligible and the phase errors can be
split between electronic and atmospheric components, then the
frequency-dependent phase scaling factor can be determined by a model such
as ATM to accurately account for the dispersion. As we do not have a good
handle on the magnitude of dry phase errors, we cannot estimate the success
of such a strategy. A worst case scenario would be to assume that the dry
phase errors are larger than the dispersive phase. By using the ratio of the
frequencies to scale the phase solutions to the target frequency, we correct
for the dry errors, but miss the differential wet dispersive phase. The
differential wet dispersive phase will manifest itself as some fraction of
the phase errors which are just not calibrated. These residual phase errors
will be larger during unstable atmospheric conditions, at the edges of the
transmission windows, and on longer baselines. During the 10th percentile
atmospheric stability conditions, on baselines of 1000 m, the fast switching
residual phase will be dominated by the uncompensated dispersive phase at
the edges of the sub-millimeter windows (ie, at frequencies where the
transmission is less than 50% of the peak transmission for that window).
This will markedly affect the ability of fast switching to correct
atmospheric phase errors for sub-millimeter observations. Longer baselines
could be accommodated by observing during better conditions or by observing
near the window center where the dispersive phase is close to zero. If dry
phase fluctuations are smaller than the dispersive phase, as will almost
certainly occur far from the window centers, the dry phase can be ignored
and a correct accounting for the dispersive phase from a transmission model
such as ATM can be applied.

If radiometric phase correction were used, a differential dry delay could be
quite damaging for sub-millimeter observations. However, if the dry phase
were very small, the differential dispersive phase could be calculated from
transmission models and applied to correct the phase more perfectly, just as
in fast switching with a negligible dry term.

View a pdf version of ALMA Memo 404.
http://www.alma.nrao.edu/memos/html-memos/alma404/memo404.pdf

Download a postscript version of ALMA Memo 404.
http://www.alma.nrao.edu/memos/html-memos/alma404/memo404.ps

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