If you know the d-spacing, q-value, or 2theta of one or more Bragg
rings from your calibration sample, then Datasqueeze can automatically
find the detector parameters (with a little help). Make sure that you can identify which Bragg ring(s)
on your false color image
you will use for calibration, and that you know the relevant momentum transfer q,
Bragg angle 2theta, or d-spacing d for each ring.
- To start, click on "Start" underneath "Run Least-Squares Calibration Wizard".
A new window should appear. Within this window, you follow instructions in a linear sequence.
(You can always back up using the "Previous" button).
- Verify that you have the correct beam wavelength entered, and retype
into the box labeled "Lambda" if necessary.
- Choose the number of Bragg rings that you will use to
calibrate the instrument parameters. If you do not have
an entire circle showing, you will need to use at least two.
In general, the more rings you use, the better the fit, but also the longer it takes.
- Next, you are asked if you already have the Bragg ring positions
in a standard Bruker calibration file. These files are
actually quite simple, and you can create one yourself using most standard text editors
with the provided file (agbe.std) as a template. The first line is a title, and the each remaining
line consists of a d-spacing (in angstroms) and an intensity (which isn't used for anything).
Note however that this is one place where the number format is not locale-dependent; the
d-spacings should all use the American format. That is, a d-spacing that is a little over
45 Angstroms should be written as 45.3 rather than 45,3.
If you answer "Yes" to the
query in this box then you will navigate to the file you want.
- Enter the
values of momentum transfer q,
Bragg angle 2theta, or d-spacing d for each of the rings that you
will use for calibration. If you have read in a calibration file,
they will already be entered in editable boxes; if not, a default
value will be entered.
For the calibration to work properly you need rings, not sharp peaks from
single crystals or fiber diffraction patterns, so your calibration material
should be a powder.
- Set the approximate position of the
beam center. You an do this by clicking on the beam center position
on the false color image, or by typing in the "X-Center" and "Y-Center" boxes.
- For each ring, you will identify the feature in the data
corresponding to the ring that will be analyzed.
Drag the blue dots on the false color image so that the inner and outer rings span the Bragg ring of interest,
just as if you were going to make a q-chi plot of that ring.
This will tell Datasqueeze over what region it should do the least-squares fit.
You want to make sure that the inner ring is everywhere at smaller radius, and the outer ring
everywhere at larger radius, than the feature of interest, and also that no other strong features
are included within that region.
- Datasqueeze will now attempt to refine the detector parameters.
Depending on the size of your image,
and the reflection chosen, this may take some time. When it is done, the white box will display the results.
If you wish you can cut-and-paste the information to another document.
If the parameters look reasonable to you, and the rings on the false color image look centered, click "Next."
Otherwise you can either back up using "Back"
or cancel the whole process and revert to previous parameters using "Restore".
- You are given the opportunity
to save the d-spacing(s) you used in a calibration file. If you
already got the parameters out of a .std file there is probably no
point to this, but if you typed them in by hand then it may be convenient
to save them for future use.
-
After the best calibration parameters (wavelength, q-range,
tilt, etc.) have been determined, they can be saved in a file
for subsequent use. Previously saved
parameters can be retrieved in Batch mode using the
"RETRIEVEINSTRUMENTPARAMETERS" command or in user mode
using the Retrieve
Parameters button in the Calibration window.
|