[mmaimcal] Re: ALMA requirements

Jeff Mangum jmangum at nrao.edu
Wed Jan 4 07:58:24 EST 2006 

Hi John,

John Conway wrote:

>Hi,
>
>
>Please  remember that 'normal' astronomical oberving is not the only mode
>one has to design for - one important requirement is the ability to do do
>observations to solve for antenna positions after antennas are moved.
>This  requires 'geodetic' observations involving rapidly switching between
>calibrators all over the sky, During this process it is vital that
>the instrumental delay stays relatively stable.
>
>In memo 503 it was estimated that this antenna calibration process
>would take 30minutes -60minutes using the four antennas that had been
>moved plus a couple from the rest of the array. I think therefore
>from the point of view of antenna position determination setting
>the  requirement for these 'stretcher' induced delays at tens of minutes
>is  therefore probably too loose and 60 minutes seems a safer target.
>  
>
Then the stretcher reset interval should be greater than 60 minutes. 

>     John
>
>P.S On a more general front the information about antenna based changes in
>delays of tens of  microns  on timescales 3 -30 minutes is new to me, and
>assuming this 'stretcher' reset is on large enough timescale not to be a
>problem this  may set the accuracy to  which antenna positions can be
>found.
>  
>
Note that the antenna-based pathlength variations over this period will 
be ~50 microns or so, but they are a mixture of repeating and 
non-repeating components, so this value is a worst-case for the antenna 
pathlength stability.

>There was also some disussion about one year ago about the level of
>Azimuth induced phase errors due to cable wrap (another critical area
>for antenna position determination), a goal was determined and ICD
>written. Has this  goal been achieved, what is the expected level of Az
>induced delay errors and what are the  implications for antenna position
>determination? Maybe it is time again to look again at the position
>calibration issue.  Unfortunatley I am no longer funded to work on
>ALMA - so I guess someone else has to be assigned to look at this
>problem.
>  
>
Can you send me the draft-ICD and other background material?  Thanks.

-- Jeff

>
>On Tue, 3 Jan 2006, Al Wootten wrote:
>
>  
>
>>Recall that Jeff will be speaking this pm at 4 on the AEG prototype antenna
>>tests.  Jeff pointed out to me that the antenna is supposed to be stable to
>> variations which could result in path length changes on timescales of
>>3-30 minutes or so.  The AEG tests show that there are changes of tens
>>of microns over these timescales.  Thus, I think we would want to do an
>>instrumental calibration on timescales of tens of minutes.  That's in
>>accord with our previous comments.
>>
>>For further details, tune in at 4pm.
>>
>>Clear skies,
>>Al
>>Al Wootten writes:
>> > Hi Bill
>> >
>> > If it is more than one phase wrap I think this would not be a good thing
>> > to do.  I gather from what you write that the number of phase turns could
>> > be deduced from the before and after stretcher voltage.  Perhaps this
>> > should only be done between schedule blocks--not a well defined interval
>> > but probably about a half hour or so.  It would be good if disaster didn't
>> > strike if it had to be an hour.
>> >
>> > I see Mark and Darrel just sent a note with a similar comment, though
>> > more detailed.
>> >
>> > Clear skies,
>> > Al
>> > Bill Shillue writes:
>> >  > Right.  thanks for reply.
>> >  >
>> >  > So I think the question is, can I reset some or all of the stretchers at
>> >  > every instrument cal?  In so doing, there is a *big* phase jump. The
>> >  > phase jump will be different for every antenna, between zero and maybe 5
>> >  > millimeters (many fringes).  The exact size of this phase jump will not
>> >  > be known exactly but should correspond well with the before and after
>> >  > stretcher voltage, maybe to within ten microns.
>> >  >
>> >  > Bill
>> >  >
>> >  >
>> >  >
>> >  >
>> >  > Mark Holdaway wrote:
>> >  > > Bill Shillue wrote:
>> >  > >
>> >  > >> Mark, Darrrel,
>> >  > >>
>> >  > >> I have a question that nobody seems interested in, and which thought
>> >  > >> one of you might be able to help with.
>> >  > >>
>> >  > >> Short version:
>> >  > >>
>> >  > >> How long do the 1st LO reference line correctors need to stay active
>> >  > >> and in-range?
>> >  > >>
>> >  > >> Long version (sent to a list 3 weeks ago and no reply)
>> >  > >>
>> >  > >> I cannnot find reference to a requirement on the LO phase stability
>> >  > >> for time periods greater than 300 sec.  And yet I seem to remember
>> >  > >> Larry D saying, "Oh no, you cannnot rely on calibration to take out
>> >  > >> all phase drifts and thus you need to make the LLC range cover much
>> >  > >> longer time period, ideally forver."
>> >  > >>
>> >  > >
>> >  > >
>> >  > > Bill,
>> >  > >
>> >  > > I cannot fully respond to this at this moment -- I am busy preparing for
>> >  > > URSI today.
>> >  > > I probably don't understand the issue sufficiently to issue this
>> >  > > statement -- but we
>> >  > > were counting on doing an instrumental calibration every 5-10 minutes.
>> >  > > You should
>> >  > > probably ask the horse himself.
>> >  > >
>> >  > >    -Mark
>> >  > >
>> >  > >> I need a NUMBER from 300 to infinite seconds!!!  What is it!?  This
>> >  > >> requirement will impact our final LLC stretcher design.  We cannot
>> >  > >> make further progress without it.  I am concerned that if we pursue
>> >  > >> this then someone will say, "yes, forever sounds good, please design
>> >  > >> for that." But that ignores that more stretch means more cost, greater
>> >  > >> size and complexity, and possibly worse performance.  So I need
>> >  > >> someone at Science/Systems/Cal to really think this through and
>> >  > >> determine the frequency that we will be allowed to reset the LLCs.
>> >  > >>
>> >  > >> One more thing - if we decide that 30 minutes is suitable, then I need
>> >  > >> to design so that under expected worst case environments (sunrise on
>> >  > >> Chanantor) - that I have enough dynamic range that all EIGHTY line
>> >  > >> correctors will stay in range with some level of certainty.
>> >  > >> Otherwise, astronomers will get messages like this
>> >  > >> "Warning : line length corrector #31 just went out of range"
>> >  > >> The going-out-of-range can easily be accompanied by a RESET to the
>> >  > >> mid-range but there will be a phase jump at that moment in time and
>> >  > >> the size of the phase jump will probably have an uncertainty of ten
>> >  > >> microns.
>> >  > >>
>> >  > >> Bill
>> >  > >
>> >  > >
>> >  >
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