Re: back of the envelope

["Mark M. Ito" <marki@jlab.org>]

Curtis,

Curtis A. Meyer wrote:
> Hi Mark --
>
>   thanks for posting that. I gather you have assumed that there is just a 
> uniform scaling in the field everywhere. I would guess that a more likely
> problem is that because we do not know the exact locations of the coils,
> that the field profile is not what we think it is. In such a scenario, the 
> overall scale of the field may well be correct, but the local variations from
> that average may not be what we expect. 
>   
Exactly. In fact, if the "error" were just an overall scale factor, we 
could correct for that; no mapping would be needed. It could be viewed 
as a calibration of the coil current measurement. This just sets an 
upper limit on the scale of the problem.
>    I am not sure how you could access this without calcualting a field map
> from coils slightly displaced from where we expect them, and the use that
> to generate the events in GEANT. We then reconstruct them with the true
> map.  T do this, I would guess that we need to know what sort of errors 
> we might have in coil locations and alignment, and then define some sort
> of "worst-case scenario" to produce the distorted map.
>
>   
Yes, that is exactly what we discussed on Monday. We talked about when 
we should ask Floyd to produce alternate maps and how we should induce 
the non-idealness in the configuration. Apparently this is not a huge 
job. How to distort needs more thought, as you imply. Also one may want 
to try a worse-than-worst-case scenario to demonstrate sensitivity to 
the distortions.

  -- Mark

>    However, thanks for starting to look at this.
>
>      Curtis
> On Fri February 27 2009, Mark M. Ito wrote:
>   
>> Folks,
>>
>> At last Monday’s tracking meeting Eugene suggested that someone make an 
>> estimate of the position error caused by a mistake in the knowledge of 
>> the magnetic field. Here is an example: http://tinyurl.com/btp8jx
>>
>> -- Mark
>>
>>     
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