[Almacal] Re: [mmaimcal] Meeting today

[Al Wootten]

Hi Stéphane

Thanks.  Indeed that is a good point.  I'll talk to Skip about this but
I think it is really the magnitude which causes problems.  It is no problem
at the moment because they are using a x6 multiplier for B9.  However, I
am told that there is a x5 solution 'almost ready' for implementation, and
that for this it is very difficult to switch by 0.03%.  Since we cannot
ascertain lock at 10ms anyway it would be quite reasonable to relax that
somewhat.

And on the point you raised yesterday on the ST vane, I'm certainly in
agreement--simple is best, two loads is simple and almost certainly works,
and I fear many complications in field testing of the ST vane such as those
you mention.  Right now I still have no reports on ST vane performance,
accuracy of hot load temperature maintenance, or saturation values above
Band 6, all of which I think are ingredients to the final recommendation.
I think we're ready to make that recommendation when those reports are in
(and in fact its partially written).

Cieux clairs,
Al
Stéphane Guilloteau writes:
 > A 14:39 10/05/2005 -0400, Al Wootten a écrit :
 > >Folks,
 > >
 > >Please review the items for discussion today on the wiki agenda.
 > >https://wikio.nrao.edu/bin/view/ALMA/10May05A
 > >
 > >I have just placed there a summary of comments from the wiki and emails on
 > >a frequently misunderstood mode of ALMA observing:
 > >Frequency Switching on ALMA
 > >on which I'd like comments.
 > 
 > 
 > Al (et al),
 > 
 > I had a brief look over the document and have the following comments
 > 
 >          - yes, FSwitch is a total power mode only
 >          - yes, the small gain for OTF is not worth the strong extra complexity
 > 
 >          - now for the change of specification over the 0.03 % frequency 
 > change in 10 ms,  which is now reduced to 0.025 %, one has apparently 
 > overlooked the fact that this prohibits cancelling the first natural 
 > standing wave below 100 GHz.  25 MHz at 100 GHz is just this 0.025 %, and 
 > at 84 GHz (the lowest band 3 frequency), these 25 MHz correspond to 0.03 
 > %.  As far as I remembered, this is how the initial number came from ( the 
 > Band 1 case being considered too difficult at that stage).
 > 
 >          Any experienced observer knows that this cancellation of the 
 > standing wave is the key issue in good baselines.
 > What is less important is the speed at which the commutation is to be made. 
 > With reasonable stable receivers and good atmosphere, switching above 1 Hz 
 > does not significantly improve the result.
 > 
 >          So perhaps, rather than relaxing the 0.03 % frequency throw, one 
 > could relax the 10 ms reset time for this throw.
 > This is a detailed technical issue which only the receiver guys can respond to.
 > 
 >          Any comment ?
 > 
 >                  Regards
 > 
 >                  Stéphane
 > 
 > 
 > PS: as concerns the caption contest, the last one is obviously the winner...
 > 
 > 
 > 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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