[Halld-tagger] goni resolution

[Ken Livingston]

Hi All,
I had a look at this today. I think Richard is right - we should aim for 
slightly better resolution than the goniometer spec for Mainz.

We'd just about get away with it for coherent brem at 9.0 GeV out of 
12GeV beam, but if we ever wanted to put the peak further down the 
spectrum it would be tricky.

I believe they want specs in round numbers of degrees. Let take 5urad 
and round down to 0.0002 deg. That's 5 times better precision than Mainz 
on the 2 critical axes Pitch and Yaw.

For a beam of 12GeV that allows placements of the peak in increments of 
the following:

Edge(GeV)      Increment(MeV)
9Gev               2.4
6GeV               9.5
3GeV              21.0


Also, we discussed in Jlab how much vertical (and horizontal) 
translation is required to put the farthest diamond in the wheel into 
the beam at any desired azimuthal angle. Since there is 4 fold symmetry 
about the beamline (azimuthal or roll azis) the most any crystal needs 
to be rotated by is 45deg. Hence, for one at a distance of r, the max 
tranlation required is r*cos45 = r/sqrt(2). With a 25mm radius wheel 
that gives about 17.7mm. Allowing some extra for misalignment of the 
beam and translating to newer spots on the crystal, 0-25mm is plenty for 
vertical translation (which is expensive).

Here's a spreadsheet with the numbers I suggest.
http://nuclear.gla.ac.uk/~kl/GlueX/goni_res.xls

We can discuss these at monday's meeting, and If we can agree, I'll send 
the details to Newport and get a quote.


Cheers,
Ken



On 11/29/2011 04:48 PM, Richard Jones wrote:
> Hovanes,
>
> Rather than obsolete I would say, "in need of review".  That is the correct order of magnitude, but it needs to be reviewed.  One microradian is overkill and 10 microradians is marginal.  I would be willing to stand and defend the 5 microradian number at this point, because it is the natural width (FWHM) of ideal diamond 2,2,0 so it sets a scale below which one is not very interested.  But we all need to sit together and review it with the arguments in front of us.
>
> Of course step size is not the same thing as accuracy.  Reproducibility is a related spec, but not the same as step size.  One of us needs to put all of the arguments for target angle resolution together on one wiki page, in preparation for that discussion.  Is that something you would be prepared to do?
>
> -Richard J.
>
>
> On 11/29/2011 10:18 AM, Hovanes Egiyan wrote:
>> Hi Richard,
>>
>> Is Table 4.2 in the Hall D CDR obsolete or these are our current specs?
>> The specs for
>> the angular precision there are 10urad, which is 0.57x(10^-3 deg), about
>> factor
>> of two better than the number for the Mainz goniometer. I think
>> the number you gave during the last meeting was 1 arcsecond, which
>> is 4.8urad, which is factor of ~ 2 better than the spec in Table 4.2, and
>> factor of  ~3.5 better than the Mainz spec.
>>
>> I am pretty sure that Ken is working with an assumption that 17 urad is
>> enough for
>> GlueX. If we want better step size then we probably should let him know
>> about it.
>>
>> Hovanes.
>>
>>
>