[mmaimcal] Re: ALMA requirements
Bryan Butler
bbutler at nrao.edu
Wed Jan 4 15:55:04 EST 2006
the antenna location determination should be made in a single SB, so al's
recommendation of doing it once per SB is still OK.
-bryan
On 1/4/06 05:58, Jeff Mangum wrote:
> 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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