[mmaimcal]Fast frequency switching in VLBI

[Al Wootten]

Folks,

Anton Zensus had described to me some work a student was doing with
fast frequency switching (FFS) in order to calibrate troposhperic phase 
fluctuations in VLBI experiments.  The April 2004 dissertation of Enno 
Middelberg from Bonn, titled 'Gas Around Active Galactic Nuclei and New Phase
Calibration Strategies for High-Frequency VLBI' appeared in our library today
and I thought some among us would find it of interest.

Paraphrasing from the summary in section 6.5:
The switching was between a reference frequency of 15 GHz and target
frequencies of 43 and 86 GHz.  The technique is used to detect sources
which are too weak for self-calibration at the target frequency within an
atmospheric coherence time, but can be reliably detected at a lower
frequency.  The 5 sigma detection limit of the VLBA at 43 GHz within 120s,
64 MHz bandwidth and 2bit smapling, ngelecting coherence loss dur to 
troposhperic phase changes, is 81.9 mJy (thermal noise of 16.4 mJy on
a single baseline).  The 1 sigma noise level using FFS with half cycle times
of 22s at 15 GHz and 28s at 43 GHz (yielding net integration times of 15s
and 21s respectively), after one cycle is 66.2 mJy, of which 39.1 mJy is
thermal noise in the raw 43 GHz visibility, 53.4 mJy is thermal noise in the
phase solutions after scaling from 15 GHz, and neglecting noise due to
tropospheric phase changes.  The 1 sigma noise level after 120s of FFS
reaches that of a conventional 120s integration at 43 GHz after 815s i.e.
16.4 mJy.  Longer integration with FFS then yields detection thresholds that
are out of reach with conventional methods.

Clear skies,
Al
+--------------------------------------------------------+
| Alwyn  Wootten   (http://www.cv.nrao.edu/~awootten/)	 |
| Project Scientist, Atacama Large Millimeter Array/US   |
| Astronomer, National Radio Astronomy Observatory       |        
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