[mmaimcal] Re: ALMA requirements

[Al Wootten]

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
 > > 
 > > 
 >