[mmaimcal] Antenna specification question

[Stéphane Guilloteau]

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.

         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