[evlatests] Referenced Pointing Tests

Rick Perley rperley at nrao.edu
Thu Nov 18 16:06:19 EST 2010 

    I suggest Ku band for this proposed test, for two reasons:

    1) It's my belief that, in the end, we'll be using Ku band for 
future primary referenced pointing. 
    2) We have enough antennas now (6) to make meaningful tests over a 
full 'extended OSRO' style bandpass.  We even have known rubbish (TV 
downlinks) at the bottom 1 GHz or so, to help us develop algorithms to 
ignore this. 

    A third reason would be there are still a small number of antennas, 
so we won't be oversaturated with data ...

    Rick

Claire Chandler wrote:
> Not yet, I think we have some tests to do first: take some data in 
> extended OSRO mode and compare the median and range with what subband 2 
> gives, for different frequency bands.  C-band would be the most extreme 
> (with a few subbands affected by RFI).  It's not clear that X-band is a 
> good test at the moment with so many of the antennas still with VLA 
> receivers.
>
> Claire
>
> On Thu, 18 Nov 2010, Bryan Butler wrote:
>
>   
>> i've always thought the median is the simplest good filter - i suspect we'll 
>> find no better.  should i ask keith to implement that for testing?
>>
>> 	-bryan
>>
>>
>> Claire Chandler wrote, On 11/18/10 13:41 PM:
>>     
>>>  The trouble is that telcal might be able to get a solution but it might be
>>>  rubbish.  I was thinking we should perhaps use the median of all subbands,
>>>  but there's clearly some work to be done to establish what algorithm is
>>>  best.
>>>
>>>  Claire
>>>
>>>  On Thu, 18 Nov 2010, Barry Clark wrote:
>>>
>>>       
>>>>  How about letting telcal try another subband if it can't get a solution
>>>>  on its first choice?
>>>>
>>>>  Bryan Butler wrote:
>>>>         
>>>>>  yes, all the solutions are written into the pointing file.  i don't 
>>>>>  know
>>>>>  that we will ever have a truly robust way of avoiding RFI, other than
>>>>>  trying to avoid where we know it exists.  we had been thinking at one
>>>>>  point of at least allowing the user to specify which subband was used 
>>>>>  -
>>>>>  if we do that is it enough?  maybe.  certainly somebody should look 
>>>>>  into
>>>>>  whether trying to take the results from all the subbands is better 
>>>>>  than
>>>>>  one, but maybe we should just go ahead now and change the 
>>>>>  recommendation
>>>>>  for RSRO/ECSO observers so that they can use the same setup for
>>>>>  reference pointing as for observing.  i suspect that the problems
>>>>>  encountered from that will be less than the problems that arise from
>>>>>  changing configurations, but maybe i'm missing something.
>>>>>
>>>>>   -bryan
>>>>>
>>>>>
>>>>>  Claire Chandler wrote, On 11/18/10 12:18 PM:
>>>>>           
>>>>>>  Hi Rick,
>>>>>>
>>>>>>  You need to define what you mean by "works for wideband".  Recall 
>>>>>>  that at
>>>>>>  present the default algorithm in TelCal is to take the second 
>>>>>>  subband from
>>>>>>  the bottom of each baseband for the solution that is fed back to the
>>>>>>  online system.  Therefore it relies on that subband being RFI-free. 
>>>>>>  I
>>>>>>  believe that solutions are being written for all subbands for 
>>>>>>  testing
>>>>>>  purposes at the moment, and what we really need to do is to 
>>>>>>  establish how
>>>>>>  to combine the results from all those subbands to obtain a solution 
>>>>>>  that
>>>>>>  is not affected by RFI, and can potentially be used to enable 
>>>>>>  pointing on
>>>>>>  weaker sources.
>>>>>>
>>>>>>  Claire
>>>>>>
>>>>>>  On Thu, 18 Nov 2010, Rick Perley wrote:
>>>>>>
>>>>>>             
>>>>>>>       I ran a test of referenced pointing this morning.    There 
>>>>>>>  were two
>>>>>>>  goals:
>>>>>>>
>>>>>>>       1) Does 'wide-band' referenced pointing work as well and 
>>>>>>>  reliably as
>>>>>>>  'narrow-band'?
>>>>>>>
>>>>>>>       2) Does a 15-second cadence work as well as 20-second?
>>>>>>>  (Explanation:  In the past, the system used 10 seconds to move 
>>>>>>>  from one
>>>>>>>  position to the other, and 10 seconds to integrate.  But from 
>>>>>>>  holography
>>>>>>>  we know that the move-and-settle time is less than 5 seconds, so a
>>>>>>>  15-second (5+10) regimen should work).
>>>>>>>
>>>>>>>       To test these, I used 3C147 (~5 Jy) to measure pointing 
>>>>>>>  offsets at
>>>>>>>  X-band, with four combinations:
>>>>>>>
>>>>>>>       1) Narrow-band with 20 seconds.
>>>>>>>       2) Narrow-band with 15 seconds.
>>>>>>>       3) Wide-band with 20 seconds
>>>>>>>       4) Wide-band with 15 seconds.
>>>>>>>
>>>>>>>       The intention was to observe each mode sufficiently long for 
>>>>>>>  3
>>>>>>>  consecutive pointing solutions.  Due to a blunder on my part, the 
>>>>>>>  'fast'
>>>>>>>  tests were observed long enough for only 2 solutions.  But that is 
>>>>>>>  still
>>>>>>>  sufficient for the following conclusions:
>>>>>>>
>>>>>>>       A) Wide-band mode works as well as narrow-band.
>>>>>>>
>>>>>>>       B) Fast mode appears to be as good as 'slow'.
>>>>>>>
>>>>>>>       There were a few failed solutions for all four trials -- the
>>>>>>>  statistics are not good enough to claim that any one of these four 
>>>>>>>  is
>>>>>>>  better or worse than the others.  Comparison of the actual 
>>>>>>>  solutions has
>>>>>>>  been done for only one antenna (so far) -- there is no significant
>>>>>>>  differences, other than an expected change in elevation offset as 
>>>>>>>  the
>>>>>>>  sun rose.
>>>>>>>
>>>>>>>       By looking at the detailed log, the sequence of operations is 
>>>>>>>  easily
>>>>>>>  discerned.  (Ken claims this may be unreliable, and he intends to 
>>>>>>>  peer
>>>>>>>  deeper after lunch).  But, be this as it may, I deduce that the
>>>>>>>  *absolute minimum* time needed for a single referenced pointing
>>>>>>>  measurement is (presuming the antennas are on source) is:
>>>>>>>
>>>>>>>       124 seconds for 'slow' mode,
>>>>>>>       94 seconds for 'fast'.
>>>>>>>
>>>>>>>
>>>>>>>
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