[evlatests] Troubles with Durations

[Bryan Butler]

i thought the durations problem with holography observations had been 
fixed.  appears not to be so.  i will have a look.

	-bryan


Rick Perley wrote:
>     With some left-over 'startup' time, I tried a wide-angle L-band 
> holography last evening.  Because the start time was unknown, I used 
> durations in the file.  This revealed some significant issues with the 
> way scan start/stop times are being generated. 
> 
>     The file started with two calibrator scans, the first a five minute 
> 'dummy', to allow things to set up, and a subsequent short observation 
> to get the (hoped-for) calibration.  From there, the file contained 
> alternating holography and calibration scans.  Below is a table of the 
> durations requested, and the time actually spent. 
> 
> Scan       Requested         Got                Comments
> -------------------------------------------------------------------------
> 1               5:00                4:43              All EVLA antenna 
> amplitudes low by a factor ~3.  Phases stable.
> 2               1:20                0:06              EVLA amplitudes 
> nearly o.k. (data were flagged). 
> 3             20:20               21:23             This holography scan 
> is o.k. (except for one ten-second record -- see below)
> 4               1:20                0:03               Just a single 
> (flagged) record was recorded
> 5             20:20               22:40             First 50 seconds 
> flagged -- target antennas were moving to source!!!
> 6               1:20                0:06               Just two 
> (flagged) records recorded
> 7             20:20               22:36             Same as preceding 
> holography scan
> 8               2:40                3:33               Last 48 seconds 
> are nonsense
> ----------------------------------------------------------------------------------
> 
>     It is pretty clear what is going wrong here.  The holography scans 
> are too long by the length of time that we were supposed to be on the 
> following calibrator (which is the target source itself).  At the end of 
> each holography scan, the target (moving) antennas are left at their 
> extreme positions, and never return to on-axis position -- this time was 
> factored into the durations of the calibrator scan.  When the next 
> holography scan came, the moving antennas were in the wrong place -- 
> hence the beginning of the second and third holography scans were lost 
> due to antenna motion, as they hurried to get where they were supposed 
> to be.   The first holography scan is o.k., as the antennas were on-axis 
> at the preceding observation.  
> 
>     None of these issues occur when the file is constructed with stop 
> times. 
> 
>     In addition to these duration/timing issues, I note the following 
> three problems:
> 
>     1) The EVLA amplitudes were all low by a factor of about three for 
> the entire duration of the first calibration scan.  The next calibration 
> scan, which followed directly is (mostly) o.k. -- the amplitudes 
> magically sprang to the correct levels.  Only a single (3 second) record 
> lies in between these two.  The difference is not a matter of motion. 
> 
>     2) The last 48 seconds of the last scan are rubbish data.  There are 
> recognizable amplitudes and phases, but they are not correct.  These 48 
> seconds in fact lie after the official end of the scan, as defined by 
> the duration in the observe file.  The executor seems to be (briefly) 
> paused, wondering what to do next ...
> 
>     3) In the first holography scan, we see a curious problem which is 
> global to *all* holography runs (whether the script is set in durations 
> or stop times):  There is an initial 10-second duration of stable data 
> -- which is not recognized by the holography programs as valid 
> holography (meaning the sub-mode is apparently not correctly 
> identified), followed by a 10-second duration where the amplitudes are 
> correct, *** but ALL EVLA antennas have a different phase ***, followed 
> by a 10-second duration where the phases are back to the correct values, 
> and the amplitudes reduced -- this record is in fact the correct 2nd 
> holography position. 
>        The phase offset on the 2nd record is ** not global ** -- it is 
> different for each EVLA antenna.  No VLA antennas show this effect. 
>        To the holography programs, the (valid!) first 10-second position 
> is ignored.  The second (invalid phase, but correct amplitude) is seen 
> as the first valid holography record.  All others following are correct, 
> both in amplitude and phase. 
>        It makes no difference what the integration time is -- something, 
> operating on 10-second ticks -- is invalidating the first (good) 
> position, messing the EVLA phases of the second (but without actually 
> moving the antennas), after which, everything works as it should. 
> 
>     I hope these issues can be readily resolved!
> 
> 
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