[mmaimcal] Re: ALMA requirements

[John Conway]

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.

     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.

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.


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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