Gravity correction

[NickDraper]

From RKH:

Due to the wider range of wavelengths (poss 0.5 to 14 angstrom) and longer distances on SANS2d than on LOQ we really do need to put gravitational droop in. The drops are frequently much more than the precision with which we can determine the beam centre, and significant for the Q value calculation at the longest wavelengths. I think it should go fairly easily into the wavelength to Q calculations. Would help to have a global variable to turn gravity on or off.

Drop ~ 0.000313 lambda^2 L^2 with lambda in angstrom and sample to detector L in metres. (see attached file)

We won’t worry about the very fine detail of the parabolic paths between sample and detector being slightly longer than the straight line one.

There is an issue as to what wavelength the vertical coordinates on the detector correspond to. We could say they are correct at zero angstrom (as per a tape measure survey) or at some given wavelength, say 4 angstrom, which makes it easier to relate to displays of data.

The latter would suit SANS but I suspect the former would be more logical and transferable to other beam lines ?

[NickDraper]

This issue was originally trac ticket 755

[NickDraper]

http://trac.mantidproject.org/mantid/raw-attachment/ticket/755/gravity1.pdf
(uploaded by @NickDraper on 2009-10-09T15:20:17)

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