[mmaimcal] Meeting today

[Stéphane Guilloteau]

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