[evlatests] Fast band switching test

[Rick Perley]

    I ran a short test yesterday -- part of a series of tests in 
preparation for a 'flux densities' run -- to assess if we have 
established good robustness on band changes. 
   
    The test consisted of observing a single source at every band with 
the following sequence:

    L, S, C, RefPnt at X, X, Q, Ka, K, Ku

    This was repeated three times.  The observations were in 'extended 
wideband' mode, including the referenced pointing. 
    Below is a table of the duration requested, and the actual duration 
of good, steady fringes. 

    Band        Duration        Good data
--------------------------------------------------
      L               50 sec               34 sec
      S               50                     32
      C               45                     28
      XRefPnt   180                 ??  It seemed to work
      X               40                     35  (no reconfiguration needed)
      Q               50                     30
      Ka             40                     30
      K               40                     28
      Ku             45                     35
-------------------------------------------------

    The '18-second' dead zone at the end of observations, when the 
following observation switches sideband, is gone.  Also gone is the 
differing length of blank data at the beginning of a scan for different 
subbands (spectral windows) when changing correlator setup. 

    I deduce from this that the actual time it takes to setup for a new 
observation is no more than 10 seconds.  The additional time shown in 
the table above (Duration - GoodData) for some band changes is likely 
due to subreflector rotation.   This is considerably better than the VLA. 

    For multi-source observations, the relevant duration to request will 
be something like:

    Duration =  DwellDesired + 10 + Max(SlewTime, SubrRotate)