[mmaimcal] Antenna specification question

Mon Nov 6 15:01:08 EST 2006

Stéphane Guilloteau wrote:

>Hi all,
>
>         The need for nutators is one of the unclear problems of ALMA. 
>Nutators were introduced because the foreseen stability of the receiving 
>system (receivers and atmosphere) was not high enough to provide suitable 
>performance combined with OTF mapping. Simulations showed this stability 
>issue could be a limiting factor.
>
>  
>
Note that the nutators cannot really improve the 3e-3 stability spec 
very much - we
will still be hosed by the stability problems even with nutators.
We might actually come out ahead by installing nutators on ALL antennas
and averaging down the 1/f noise with bulk observations?

   -M

>         However, nutators have their drawbacks too. One of the main issue 
>is that the optical path is different between the two (or three) nutator 
>positions, leading to a different primary beam at some level. To what level 
>this influences the final imaging result has not been investigated into 
>details so far. So, we opted for a compromise: equipping the 4 antennas 
>required for total power measurements with
>         - the most stable receivers
>         - nutators
>4 antennas were shown to be sufficient in terms of S/N to provide the total 
>power, under the assumption they observe 4 times longer than the main ALMA 
>array (as the rest of ACA). In some way, these antennas should be better 
>than the others !
>
>It is clear that some gain can be obtained by optimising the quadrupod 
>without the nutator for antennas which don't need it.
>At least, there could be a cost saving (with a lighter quadrupod), but 
>there could also be a performance enhancement. For example the stability of 
>the subreflector positioning is one thing which could not be assessed to 
>the required accuracy in the antenna evaluation (pointing was tested mostly 
>optically, and the limited radio pointing measurements were at the margin 
>in terms of precision).  It would be good to obtain from the manufacturer 
>what improvement they have in mind...
>
>         Regards
>
>                 Stephane
>
>
>A 17:11 03/11/2006 -0500, Al Wootten a écrit :
>  
>
>>Hi Rob
>>
>>Thanks for the comments.
>>
>>In fact, we originally had no nutators on any of the MMA telescopes.
>>At a review in July? 1999 we were told that we needed to have nutators on all
>>the telescopes.  However, they are prohibitively expensive.  From our
>>early estimates of how much total power continuum ALMA would do we set the
>>spec you cited.  I think the five which ALMA is purchasing cost (total)
>>~$850K or so ballpark.  Fifty more would exceed the cost of an antenna.
>>
>>The four Total Power antennas are also equipped with the best receivers,
>>    
>>
>>from a point of view of 1/f noise.  ALMA-JP are doing a study of how
>  
>
>>much integration the Total Power and ACA arrays really need to do to
>>provide good images for different sorts of projects.
>>
>>I think that in the best of all possible worlds, all antennas should meet
>>all specifications.  In order to deviate from this rule, there should be
>>a quantifiable benefit.  If we hear what that benefit might be we can
>>compare benefit with loss.
>>
>>Clear skies,
>>Al
>>
>>Rob Reid writes:
>> > Hi,
>> >
>> > At 2:27 PM EST on November 3 Al Wootten sent:
>> > > Q:  Does the ALMA production antenna have to meet all specifications
>> > > when equipped with a nutator?  If not, the design of the quadripod might
>> > > be optimized for performance without the nutator.
>> >
>> > If the demand for total power exceeds what the ACA can provide, it 
>>would be
>> > nice to be able to upgrade a few ALMA antennas with nutators.  This is
>> > especially true if OTF mapping with ALMA can't be done fast/stably 
>>enough to
>> > get total power, given that the two different types of ALMA antennas might
>> > drive differently.  Admittedly putting nutators on some of the 
>>antennas would
>> > probably add to that problem.
>> >
>> > > The specifications are unclear on this, but the point of view so far 
>>is that
>> > > it must meet all specifications when equipped with a nutator.
>> >
>> > I'm new at digging through the ALMA specs, but according to
>> > SCI-90.00.00.00-0230-00, "at least 4 antennas must be equipped with 
>>wobbling
>> > subreflectors".  "at least" implies that more might be needed if 4 
>>turns out to
>> > not be enough.  If the production companies are really keen to further 
>>split
>> > their designs into nutatored and nutatorless, then presumably they'd 
>>need an
>> > agreement on how many nutatored antennas each company is responsible for.
>> >
>> > --
>> > Rob Reid             A NRAO NAASC ALMA PDF           @   @  @ @  @   @
>> > 1-434-244-6822               http://www.cv.nrao.edu/~rreid/  ^   ^  ^ 
>>^  ^   ^
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>>
>
>Dr. Stephane GUILLOTEAU
>L3AB, Observatoire de Bordeaux
>2 rue de l'Observatoire
>         F-33270 FLOIRAC,  France
>
>Tel: (33) 557 77 61 68
>Mobile (33) 683 84 66 70
>E-Mail: guilloteau at obs.u-bordeaux1.fr
>
>
>
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