[mmaimcal]forwarded message from Stefano Stanghellini

[Jeff Mangum]

Hi Al,

Stefano makes a very good point.  Isn't it repeatability that matters?
If he can deliver an antenna with the axis non-intersection spec he
describes, do we really need such a large OTT?

Note that I of the belief that OTT is not necessary, or even desired,
for pointing.

Cheers,

Jeff


"Al" == Al Wootten <awootten at nrao.edu> writes:

Al> Hi Bryan and Jeff, and everyone
Al> Stefano has a different take on this--specify the axis non intersection
Al> to be very small and don't go over the top.  His perspective is different
Al> from those discussed so far so I circulate it here.  I'll have to go to the
Al> ASAC telecon for the rest of the morning (I did dig up my leader passcode
Al> Bryan).

Al> Clear skies,
Al> Al
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Al> From: Stefano Stanghellini <sstanghe at eso.org>
Al> To: Al Wootten <awootten at nrao.edu>, jbaars at nrao.edu, jbaars at eso.org
Al> CC: demerson at polaris.cv.nrao.edu,
Al>    Ewine van Dishoeck <ewine at strw.leidenuniv.nl>, dsramek at nrao.edu,
Al>    rkurz at eso.org, mrafal at eso.org, Kraus Maximilian <mkraus at eso.org>,
Al>    Massimo Tarenghi <mtarengh at eso.org>
Al> Subject: Re: Antenna RFP
Al> Date: Wed, 24 Sep 2003 14:32:49 +0200

Al> Dear Al,

Al> Thank you for your prompt answer.
Al> Still, I do not agree yet to the various reasoning, neither I found in the
Al> Calibration paper sufficient information to convince me.

Al> The point is that an engineering specification shall specify what we really
Al> want and not what we want as a safety basket.
Al> This is the more true as every specification comes with a price tag.

Al> My understanding of the issue is that we want long term stability of the offset
Al> because this is equivalent to the change in distance between the antennas.
Al> Today, apart from the three millimeters maximum offset, there is in the
Al> Calibration paper no value of stability, neither in the prototype specs.
Al> In addition the paper refers to slow variations (16 months interval measurement
Al> at the VLA). You refers to measuring at every repositioning of the antenna on a station.

Al> This is contrary to my understanding of the issue: A good manufactured
Al> antenna will have fast variations due to run-out and these variations are
Al> changing permanently. Is this what you want to measure? Or it is only the
Al> non-repeatable slow variations which you want to measure?

Al> Now some side information: The AEC antenna has an axis offset in the
Al> range of  0.16 mm. The one of vertex (I need to check) should be
Al> around 0.5 mm, but measured with a lower level of accuracy)
Al> As I have understood the issue, this axis offset does not necessary
Al> be so small, if it is well known.
Al> This offset can be measured very precisely during acceptance testing
Al> and I intend to specify this measurement. (Typical precision lower than
Al> 100 micrometers).

Al> The variation of this offset has to be kept small by design. The reason
Al> for this offset variation are mainly:

Al>  - thermal gradient (but the antenna will be insulated) Here  you have
Al> no control on it and it will change at least twice daily, so you are not
Al> solving the problem measuring every week or so;

Al> - Elastic deformation. This will be very small, and it needs to be specified
Al> in order to maintain it small. Imagine you have an elevation structure which
Al> is not balanced: you will have a moment and this moment will bend the yoke
Al> arm. You will have a variation  of the offset of say, x micrometer, depending
Al> on the elevation angle. This is however small and repeatable. Furthermore it is
Al> independent from the interval within your measurements. It can be computed
Al> rather exactly, and possibly measured.

Al> - Wind induced elastic  deformation. This cannot be seriously measured
Al> by going over the top. It can be only computed.

Al> - bearing run-out. I expect bearing run out to be in the order of 30 microns
Al> for azimuth axis and << 10 microns on the elevation. This run out however
Al> has a repeatable fast moving component (twice /over 360 degrees azimuth,
Al> 0.5 times over 90 degrees elevation) and a slow varying non repeatable effect.
Al> (I would judge this to be less than 15% of the other)  This is probably the
Al> only one of relevance, and it looks like being very, very small compared to
Al> the overall accuracy requirement of 65 micrometer.
Al> Note that I will specify the max acceptable bearing runout and get it measured
Al> in the frame of the acceptance testing, so we will have all the antenna around
Al> the value I mention above.

Al> In conclusion:

Al> - There is a lot that can be done at engineering level, both in terms
Al> of specification and acceptance testing;

Al> - I still have not fully understood the "real" requirements and I do not agree
Al> to put  in the spec "safety basket" unless I am convinced there is not better
Al> method of clearly specifying what ALMA needs. (On this I would kindly ask our
Al> colleague Jaap, who is my safety basket, to give me some help on the matter);

Al> - specification comes with a price tag and I feel authorized to challenge
Al> them in this phase;

Al> - last but not least (?) Going over the top is a considerable source of hysteresis
Al> in the system, (releasing stresses, closing gaps, changing the direction of friction)
Al> and I am against it in such a precise machine unless it is absolute necessary.
Al> (ESO avoids to do this in telescopes like the VLT.)

Al> Issue to be  followed. regards,


Al> Stefano




Al> Al Wootten wrote:

>> Current prototype spec on non-intersection of axes is ~3mm; current draft
>> RFP spec is ~2mm and some would argue to make it <1mm.
>> 
>> Current spec for 'over the top' motion is 125 degrees and we were asked
>> if this could be made smaller.
>> 
>> Currently, non-intersection of axes is <1mm at OVRO and BIMA.
>> The preponderance of opinion
>> within the science IPT is that as long as it is <3mm the absolute value
>> is not critical but it must be stable.
>> 
>> What we need to know is how accurately we can measure it
>> and how stable the axis non-intersection remains over the measurement
>> interval.
>> 
>> We need to measure the axis non-intersection to the same precision as the
>> baseline accuracy, which is 65 microns in the current calibration document
>> Calibration of ALMA (see http://almaedm.tuc.nrao.edu/forums/alma/dispatch.cgi/docapproval/docProfile/100684/d20030917184242/No/t100684.htm
>> 
>> Each antenna will move about every two weeks, after which the measurement
>> of axis non-intersection may be made with the antenna on a new pad.  It
>> should be stable to 65 microns over at least that time period.
>> 
>> It is possible to measure the axis non-intersection with antennas which
>> do not go 'over the top' but it can be tedious.  With ALMA the tedium is
>> relieved by the abundance of calibrators revealed by its sensitivity,
>> the excellent weather on the site, and the very high frequencies to which
>> it will operate.  However, there are also many antennas!
>> 
>> Many observatories with antennas which do go 'over the top' have found the
>> ability to do that useful.  OVRO, BIMA and the VLA do; IRAM does not.
>> Most believe that the ALMA antenna should provide the capability to go
>> over the top to secure the measurement of axis non-intersection.
>> 100 degrees is not enough owing to the trigonometric dependencies of the
>> equations involved; 125 degrees gives a much better measurement.
>> 
>> Some cited the scarcity of clear zenith weather as supporting over the top
>> motion, to relieve long antenna slews while tracking a source.  However
>> the preponderance of opinion is that pointing is seldom accurate 'over the top'
>> and this was not a driving reason for the spec.
>> 
>> In summary, the Science IPT thinks that
>> 
>> *the absolute accuracy of the non-intersection is not as important as its
>> stability; it must be measured to the same accuracy as baselines, or
>> 65 microns and should remain stable over periods of many weeks to months.
>> 
>> *it is much easier to measure the axis non-intersection if the antennas
>> go over the top by about 125 degrees.
>> 
>> Clear skies,
>> Al