[mmaimcal]forwarded message from Stefano Stanghellini
Al Wootten
awootten at nrao.edu
Mon Sep 29 09:34:17 EDT 2003
Hi Jeff
The OVRO antennas do not go over the top. I understand that the
inability therefore to separate the axis non-intersection from the 'up'
term when antennas are moved is one of the primary problems with data
from OVRO--there is always a phase drift across the bandpass. If the
antenna has an axis non-intersection which is very small, such that it
does not affect the baselines significantly, then the phase drift will
be minimized. But we move antennas every four days and approving a
specification which we know will complicate baseline determination, and
which will introduce phase errors, should only be done if the cost of
implementing over the top is large. Repeatability matters but only if
repeatable within the baseline errors. What should the stability be? I
proposed a change less than the baseline error over the two weeks
between antenna moves in the baseline plan. I haven't heard much
discussion of this.
Clear skies,
Al
Jeff Mangum wrote:
> 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
> Al> MIME-Version: 1.0
> Al> Content-Length: 6689
> Al> Content-Type: text/plain; charset=us-ascii
> Al> Return-Path: <sstanghe at eso.org>
> Al> Received: from cv3.cv.nrao.edu (cv3.cv.nrao.edu [192.33.115.2])
> Al> by polaris.cv.nrao.edu (8.11.6/8.11.6) with ESMTP id h8OCXIg24686;
> Al> Wed, 24 Sep 2003 08:33:18 -0400
> Al> Received: from eso-wall-ext.hq.eso.org (firewall-user at eso-wall-ext.hq.eso.org [134.171.69.199])
> Al> by cv3.cv.nrao.edu (8.11.6/8.11.6) with ESMTP id h8OCXD512077;
> Al> Wed, 24 Sep 2003 08:33:13 -0400
> Al> Received: (from uucp at localhost)
> Al> by eso-wall-ext.hq.eso.org (8.10.2+Sun/8.10.2) id h8OCXBh06502;
> Al> Wed, 24 Sep 2003 14:33:11 +0200 (MEST)
> Al> Received: from mercury.hq.eso.org(134.171.7.20) by eso-wall-ext.hq.eso.org via csmap (V6.0)
> Al> id srcAAAY7aOSm; Wed, 24 Sep 03 14:33:09 +0200
> Al> Received: from serapis.hq.eso.org (serapis.hq.eso.org [134.171.7.10])
> Al> by mercury.hq.eso.org (8.12.6/8.12.6) with ESMTP id h8OCWrKm003885;
> Al> Wed, 24 Sep 2003 14:32:53 +0200 (MEST)
> Al> Received: from eso.org (pc003513.hq.eso.org [134.171.24.104])
> Al> by serapis.hq.eso.org (8.11.7+Sun/8.11.7) with ESMTP id h8OCWrw03131;
> Al> Wed, 24 Sep 2003 14:32:53 +0200 (MEST)
> Al> Message-ID: <3F718EF1.A2ABB716 at eso.org>
> Al> X-Mailer: Mozilla 4.79 [en] (WinNT; U)
> Al> X-Accept-Language: en
> Al> References: <16240.49949.949604.622363 at polaris.cv.nrao.edu>
> Al> X-Virus-Scanned: by amavisd-milter (http://amavis.org/)
> Al> X-MailScanner-Information: Please contact postmaster at cv.nrao.edu for more information
> Al> X-MailScanner: Found to be clean
> Al> X-MailScanner-SpamCheck: not spam, SpamAssassin (score=-6.3, required 7,
> Al> BAYES_01 -5.40, EMAIL_ATTRIBUTION -0.50, QUOTED_EMAIL_TEXT -0.38,
> Al> REFERENCES -0.00, REPLY_WITH_QUOTES 0.00,
> Al> USER_AGENT_MOZILLA_XM 0.00)
> 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
>>
>
More information about the mmaimcal
mailing list