Instead of using the Calibration Wizard or retrieving previously stored calibration parameters,
the user can manually enter calibration parameters. This is sometimes down in conjunction with the
"Show Bragg Ring Positions from File" option which allows you to compare
the results
with positions calculated using a standard Bruker calibration file.
Click the Enter button to
bring up the window for manual calibration parameter entry.
- Centering:
This feature allows you to change the center of the image. In SAXS mode this is the position where the
direct beam would have hit if there were no beamstop, and in WAXS mode it is the position
where the beam would hit if Detector 2Theta were zero. For a detector with N x N
pixels, the X-Center and Y-Center
values are often somewhere close to (N/2, N/2), although sometimes
data are taken in a mode where the beam zero is close to the edge of the detector, and
sometimes the beam zero is completely off the detector. In Batch mode see the CEN command.
To start recentering, click on "Enable Center Change" (nothing will happen until you do this).
The X-Center and Y-Center boxes in the right should change from italic to roman
font.
At this point, you can change the center either by retyping the desired values in
the
two boxes, or by dragging the X in the image to the new position.
Very important: you must click the "Disable Center Change"
button when you
are done. Otherwise, mouse clicks to center may still be enabled and you may
get
confusing results in other parts of the program.
The windows will change back to
italic when
you have done this, and the image will be recalculated so that
the center of the diffraction pattern is at the center of the image (depending on the Zoom option
you have chosen).
- Detector Tilt, Tilt Azimuth
It commonly happens that the detector is not quite perpendicular to the
beam normal. In that case, what should be a circular pattern is elongated
perpendicular to the tilt axis. The combination of Det-Tilt and Azimuth correct for this.
(These options are only used in SAXS mode.)
Det-Tilt is the amount that the detector is tilted (in degrees) and Azimuth is
the angle with which the pattern is tilted. For a pattern that appears to
be stretched along the x-axis, you should set Azimuth=0 and play with Det-Tilt
until the pattern is circular. For a pattern that appears to be stretched along the
y-axis, Azimuth should be 90. In fact, any value of Azimuth between -360 and 360 is
allowed.
- Wavelength: This is the wavelength of the radiation chosen.
The default is the wavelength for Cu-Ka radiation, as is used in
most tabletop diffraction units. In Batch mode see the LAMBDA option.
- Maximum Q:
The "Q-range" corresponds to the maximum momentum transfer at one edge (not corner)
of the image, assuming that the beam zero is in the center of the image. It is one way of
characterizing the angular range of the instrument, the others being "2theta max" and the
ratio of "Detector Radius" and "Sample-Detector Distance".
For small-
angle scattering the
Q-range will be small while for wide-angle scattering it will be large.
2-theta max and the
Sample-Detector distance are
automatically updated every time the Q-range is changed, and vice versa. In Batch mode see the QRANGE option.
- 2theta-max
This is the scattering angle corresponding to the
Q-range; it is the
2-theta at one
edge (not corner)
of the image assuming that the beam center is in the center of the image. It depends on the camera
length. For small-
angle scattering 2theta-max
will be small while for wide-angle scattering it will be large. The Q-range and
Sample-Detector Distance are
automatically updated every time 2theta-max is changed, and vice versa. In Batch mode see the TWOTHETARANGE option.
- Sample-Detector Distance
is the distance from the sample to the camera, in whatever
physical units you prefer (e.g. cm).
2-theta max is the arctangent of this distance divided by the
Detector Radius.
The Q-rangeand
2-theta max are
automatically updated every time Sample-Det is changed, and vice versa. In Batch mode see the LENGTH option.
- Detector Radius
is the distance from the center to the edge (not corner) of the
detector, in the same units as the camera length.
2-theta max is the arctangent of the sample-detector distance divided by the
Detector Radius. The Q-range and
2-theta max are
automatically updated every time Det-Rad is changed, and vice versa. In Batch mode see the RADIUS option.
- Detector Azimuth, Detector 2Theta: These options are only used in Wide Angle model. We assume that
the detector is mounted on a 2theta arm which has been rotated away from the primary beam. The Detector Azimuth
establishes the direction that the 2theta arm has been rotated; an azimuth of zero corresponds to a rotation in the
horizontal plane (common to laboratory tabletop units) and an azimuth of 90 corresponds to a rotation in the vertical
plan (often used in synchrotron diffraction. systems). The Detector 2Theta
is the amount by which the detector has been rotated away from the beam center.
- Chi Offset
Chi is the azimuthal angle angle that defines the orientation of a feature in an image.
By default, chi=0 is defined to be in the usual place, at the bottom of the first quadrant.
This is fine for powder diffraction patterns or other cases where the orientation does not matter.
However, suppose you have a fiber that is almost but not quite aligned with the horizontal or vertical axis.
In that case you might want to make qx-qy plots, or to define chi=0 to be along the fiber axis.
Datasqueeze deals with this by redefining the meaning of "chi=0". The "chi offset"
is the amount in degrees by
which the experimental equator differs from the nominal horizontal direction. To set it:
- Click on Set Chi Offset. A crosshair pattern should appear in the false color image.
- You can either type your best estimate for the chi offset into the box, or drag the
control points on the cross hair until a horizontal or vertical line is aligned with an
interesting feature in your data.
- Click on Done.
In Batch mode see the CHIOFFSET option.
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