[d2n-analysis-talk] BPM/Harp Scan calibration (fwd)

[Diana Parno]

I see that Vince copied this to Brad, as well, but I thought the rest of 
you folks would be interested in Vince's response regarding the BPM 
calibrations (below). Below Vince's response, I have also copied the 
emails from Arne Freyberger, which Vince kindly sent along to me last 
night.

So, we've got a few outstanding questions: what kind of precision do we 
need on the beam position? Is it worthwhile to try to reproduce Vince's 
results that the harps are not trustworthy during d2n? What do y'all 
think?

Diana

---------- Forwarded message ----------
Date: Thu, 4 Feb 2010 20:18:07 -0500 (EST)
From: vasulk at jlab.org
To: Diana Parno <dseymour at andrew.cmu.edu>
Cc: Brad Sawatzky <brads at jlab.org>
Subject: Re: BPM/Harp Scan calibration


Hi Diana,

I only very reluctantly came to the conclusions in my earlier email.  I
would prefer to use the Harp information, but it is very clear that the
Harps have become unreliable.  Brad does have a point that the information
feed to the ADC's and EPICS read back have a common source.  But MCC has a
separate calibration procedure from how we have typically done the
calibration.  So we are using MCC calibrated numbers to calibrate our ADC
readout.  Not ideal, but I do not think we have any other options that
will give us the absolute beam position.

I had several email correspondences with Arne Freyberger, who used to be
an expert on the Harps.  I can forward you those conversations, since all
the information I have provided you came from him about the BPM and their
calibrations.  The claim is that the BPM girder, which includes the harps,
has not been disturbed and the BPM offsets have been fairly stable for
years.

Arne indicated to me that the BPM's are routinely calibrated on MCC's
side.  I can also send you that email as well.

As an aside, back when Harp B had it's coupler problem.  I
cross-calibrated the MCC's EPICS readings versus the Harps positions.  The
cross-calibration was fairly good.  I have various plots that I can send
to you.  Then I have the data from the beginning of Transversity where the
calibration began to breakdown and from d2n, which is rather bad.  The
good agreement from before Transversity is one of the main reasons I am
willing to use the EPICS readings.

Finally how well do you need to know the beam position for d2n?  Arne
claimed we should know the absolute position to 50 microns.  I am not as
convinced about this fact.  Typically you only care about this level of
accuracy when you need great momentum resolution (100's keV).  You do not
have the resolution on the BigBite side.  The HRS on the other hand you
do, but you are in DIS kinematics.  The resolution is not important here
for the cross section.  I think by using the EPICS information, you can
get at least 0.5 mm accuracy in beam position.  On top of it, we were also
using a fairly large raster size.

Perhaps you should do a simulation to test how much a 0.5 mm or even 1 mm
beam position uncertainty will affect your absolute cross-section
measurement.

I will find the emails from Arne and send them to you.

Vince

-------------------------

---------------------------- Original Message ----------------------------
Subject: Re: BPM Calibration in Hall A
From:    "Arne Freyberger" <freyberg at jlab.org>
Date:    Sat, October 24, 2009 11:26 am
To:      "Vincent Sulkosky" <vasulk at jlab.org>
--------------------------------------------------------------------------

Vincent,
      The BPMs are quite stable, and well within 0.5mm.  Probably stable
within 0.05mm,  as long as:

    1. They are not physically bumped or moved
    2. the software offsets (SOFF, GOFF) are properly restored after an
       ioc reboot.

BPMs physically moving is a rare event associated with beamline work.
Aside from the compton region your beamline has been relatively
constant.   The SOFFs and GOFFs   history can be tracked via
archiver/allsaves to insure that they have been stable as well.  In
general OPS would notice a large problem with SOFFs and GOFFs changing
since the relative orbit would not restore after a reboot.

Yes, the harps have not been re-surveyed.   This should be scheduled in
the Winter2010 down.

If you want detailed information on positions, SOFFs and GOFFs, give me
a list of BPMs and date ranges, I'll ask an operator to dig the data out
for you.
Arne

---------------------------
---------------------------- Original Message ----------------------------
Subject: Re: BPM Calibration in Hall A
From:    "Arne Freyberger" <freyberg at jlab.org>
Date:    Mon, October 26, 2009 10:44 am
To:      "Vincent Sulkosky" <vasulk at jlab.org>
--------------------------------------------------------------------------

Vincent,
I've attached a spreadsheet that went through our twice-daily snap files
of the machine.  The entries make it into the spreadsheet only if the
values are unique, so you can see that:

    1. The GOFFs are consistently zero, which is what I expected
    2. the X soffs do not change
    3. the Y soffs were not in the "snap" script until jan 7th 2009, but
       since the X soffs did not change I would be surprised if the Y
       soffs did change
    4. The snap files between March 25th 2009 and March 30th  read back
       zeros, but I don't think A was taking data at that time.

So the values appear to be stable during this time period.