[asac][almanews] ALMA Memo 490 Released

[Stacy Oliver]

ALMA MEMO #490 

Effects of Atmospheric Emission Fluctuations and Gain Fluctuations on
Continuum Total Power Observations with ALMA


M.A. Holdaway 

2004-03-30


Atmospheric water vapor emits strongly at millimeter wavelengths. Fast
temporal and spatial fluctuations in the atmospheric 

water vapor results in problems in removing the atmospheric emission,
which is done by very quick position switching or beam

switching. Most phase fluctuations seen with the 11.2 GHz site testing
interferometer are due to fluctuations in water vapor, 

so we can use the statistics of the phase fluctuation measurements to
infer how well the cancellation of variable water vapor 

emission will be for a given observational strategy and a given
atmosphere. In fact, we have found a (non-optimal but sufficient)

way to match observations at all ALMA bands (1-10) to atmospheric
opacity and stability conditions such that OTF total 

power continuum observations are essentially always thermal noise
limited and never limited by atmospheric fluctuations. 

In addition to the problem of atmospheric fluctuations, the total power
continuum observations also must contend with 

gain fluctuations, which in fact will limit these observations. Gain
fluctuations of 1e-4 in one second will just barely limit

total power continuum observations. However, gain fluctuations of 1e-3
will result in residuals which are an order of

magnitude worse than the thermal noise limit. While other considerations
might drive the decision for the receiver stability

specification, it should be noted that a specification of 1e-3 in one
second, or worse, will cause problems for ALMA 

continuum observations of large objects. All simulations were performed
in AIPS++, and several reusable glish tools have been

written. 

 

 

View a pdf version of ALMA Memo #490. 

http://www.alma.nrao.edu/memos/html-memos/alma490/memo490.pdf

 

 

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