Where are the collimators?

Obsolete Results from August/September 2000 see DTA812 and 820 for the latest information

Old Results of Measurements of the Locations of the Sieves and Large Collimators for the HRS Spectrometers (use these for experiments before August 2002)

(based on JLab Alignment Group DT #'s: A627 & A629, measurements were made during August-September 2000)

	Δx (mm)	Δy (mm)	z (mm)
Right (formerly Hadron) Sieve	0.92	-1.46	1175.5
Right Large Collimator	2.44	-0.17	1101.0
Left (formerly Electron) Sieve	-0.38	3.46	1184.3
Left Large Collimator	-1.86	4.06	1109.9

Δx is the horizontal displacement of the Sieve or Collimator from its ideal position. +x is to the left when facing the spectrometer.

Δy is the vertical displacement of the Sieve or Collimator from its ideal position. +y is up.

z is the distance from the ideal target position to the sieve or collimator.

For further discussion read on.

Vacuum Loading Corrections

?x & ?y as reported above are corrected for vacuum loading. The original measurements were made with no vacuum in the spectrometer. Then the spectrometer vacuum was activated and the relative motion of the sieves and collimators in x and y were measured (DDx and DDY). This was done several times in order to test the repeatability of the vacuum induced shifts. The results are summarized below. (units are mm)

RIGHT	Vacuum motion of sieve
	test1	test2	test3	avg.	s
DDx	0.21	0.21	0.18	0.20	0.02
DDY	-0.24	-0.33	0.05	-0.17	0.20
	Vacuum motion of large collimator^†
	test1	test2	test3	avg.	s
DDx	0.11	0.18	0.14	0.14	0.04
DDY	-0.26	-0.29	0.05	-0.17	0.19

LEFT	Vacuum motion of sieve
	test1	test2	test3	test4	avg.	s
DDx	0.14	0.18	0.18	0.15	0.16	0.02
DDY	-0.2	-0.17	-0.37	-0.16	-0.23	0.10
	Vacuum motion of large collimator^†
	test1	test2	test3	test4	avg.	s
DDx	0.04	-0.01	0.01	0.00	0.01	0.02
DDY	-0.26	-0.26	-0.41	-0.18	-0.28	0.10

As can be readily seen the observed reproducibility, indicated by the s's, of the vacuum induced ??x is much better than that for ??y.

^{†In the measurements the motion of the sieve and the slit box were measured. There was no direct measurement of the motion of the large collimator. In the table above the motion of the large collimator is assumed to be the same as that of the slit box.

Historical Considerations
The figures below show a comparison between the sieve to target distances as reported on the web (our "standard values") and as measured by the JLab Alignment group in 1998 and again in 2000 (the latter being the newest values as reported above).

Electron = Left

Hadron = Right

In the following plots the history of the measurements of the horizontal and vertical offsets of the sieves and collimators is shown.

For the Electron (beam left) arm the ?x value is reasonably stable, but ?y is not. The ?y measurements come in two distinct groups, '97 & '00 or '96 & '99. The '96 & '99 measurements involved measuring the location of the sieve or collimator with respect to Q₁ and then using previous measurements of the location of Q₁ relative to the dipole to determine the offsets (by convention the "Spectrometer Axis" is defined by the location of the dipole). For '97 & '00 measurements were made directly of the location of the sieve or collimator with respect to the dipole. It's very hard to believe that the "known" location of Q₁ is almost 3 mm in error. The alternative is that the vertical positioning of the sieves and collimators has some uncertainty. This could be due to backlash in the mechanical system or drifts in the calibration of the position encoder. Measurements of the vertical displacement of the Hadron (beam right) Sieve and collimator are much more stable as can be seen in the figures below.

Please send comments or questions to John LeRose: lerose@jlab.org
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