[evlatests] Settling Times for Referenced Pointing

[Doug Whiton]

A modern ACU would have a dramatic improvement in this type of performance.

Doug

>     Ken and I continue our testing with the goal of improving high
> frequency pointing and efficiency.   One of our tests involves two
> orthogonal cuts through the beam (to determine its location and
> characteristics) -- one in azimuth and the other in elevation.   There
> is a marked -- and important -- difference between these.
>
>     When stepping in elevation,  the 'settling time' is very fast --
> amplitudes are stable with 3 or 4 seconds, although a small amplitude
> oscillation of period 4 to 6 seconds is discernible.  The amplitude of
> this small oscillation is 1 to 2 arcseconds.
>
>     When stepping in azimuth, the behavior is very different.  The
> response shows a slowly damped oscillation of remarkably large
> amplitude:  The amplitude corresponds to an overshoot of ~8 arcseconds,
> the fundamental period of the oscillation is 8 seconds.  Even after 20
> seconds, the oscillation is still visible in the data.   The damping
> time is about 10 seconds -- after this length of time, the oscillations
> are sufficiently small that useful data can be collected.
>
>     I presume the origin of this azimuth behavior lies in the servo
> system.  Certainly the characteristics dictate that we cannot speed up
> 'referenced pointing' by shortening the stepping time to less than 20
> seconds.  Reducing the amplitude and period of the azimuth oscillation
> would certainly be helpful!
>
>     I attach two postscript plots to show the effect.  The data are
> taken at K-band.
>
>
>
>
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-- 
Doug Whiton
National Radio Astronomy Observatory
Servo/MOS Group Leader
575-835-7417