Date: Thu, 09 Sep 2004 13:07:18 -0400
From: John M. Finn 
Reply-To: finn@physics.wm.edu
To: carlini@jlab.org
Cc: 'Neven Simicevic' ,
     'Jim Birchall' ,
     'Dave Mack' , 'Juliette Mammei' ,
     'Klaus Grimm' , armd@jlab.org, 'Mark Pitt' ,
     'Allena Opper' , 'Greg Smith' ,
     'Norman Morgan' , 'Mike Finn' ,
     'Yongguang Liang' ,
     'Shelley Page' ,
     'Tony Forest' 
Subject: RE: Qweak working group - next meeting
                                                                                
Dear Roger et al.
                                                                                
                                                                                
Obtaining a good optical focus is a matter of constraining the first order
transport properties of the magnet. Even when the best fit is obtained,
there is a acceptance range of validity of the results.  Simply changing
parameters in an ad hoc fashion is not likely to lead to good insight to
what is possible or desirable. We are hampered in our efforts by the lack of
a good ray trace model of the magnet. This should have been generated before
the magnet design was finalized, but it is still useful to have. A ray trace
analysis will tell us what is truly possible. It doesn't surprise me that
that minimizing the transverse defocusing at a given scattering angle
changes the focal length or may require changing (lowering/raising) the
magnet's current to compensate.
                                                                                
Since the Qweak baseline design meets our experimental goals, I don't
seriously expect that a last minute design change will be implemented. Last
minute changes invariably involve major time delays and must go through a
thorough vetting process. The same is true if you wish the reopen the
rationale for not having the sectors overlap or not protecting the
electronics from the beam. There are tradeoffs in opening up the acceptance,
meeting the other goals of experiment, and employing good engineering
practices.  If Dave Bowman were still active in this project, I suspect that
he would be going through the roof about now.
                                                                                
                                                                                
Regards,
Mike