PXRR X25 microdiffractometer

The new Crystal Logic Microdiffractometer in the X25 hutch.
10 Dec 07

The Crystal Logic Microdiffractometer stands alone on a solid base. All of the mechanics required to orient the instrument to the x-ray are incoporated in the black-anodised stand. There is a "periscope" that looks in the direct beam. In addition the spindle axis has an independant vertical translation so the xtal can be placed precisely on the beam.

Collimator/aperture system

The adjustable aperture is the same as on the other Crystal Logic diffractometers, rotating "fork" arrangements. Full open is 2mm. The guard aperture also is replaceable pins with fixed apertures in them. The catch here is that to handle microcrystals, the apertures need to be put in place very carefully. Each has its own "best" orientation, which should be indicated by a mark that should be kept at the top. They should be handled only with the special tool, and should be installed with gentle pressure and slight back/forth rotation. This should assure 5-10 micron reproducibility. Get someone to demonstrate to you before you try it yourself. These pins are different from the other diffractometers -- they are tapered to give more accurate fit.

Alignment
Checklist -- Draft

For crude alignment, homing the slits will open them to full aperture, 2mm. At minimum zoom, pre-align the periscope to have the crosshair aligned to the guard aperture (Crosshair to Beam). Remove the guard aperture. Insert the phosphor on the periscope. Open the shutter.
There are CBASS tweak commands for diffractometer Horiz, Vert. Use them to center the beam on the position of the guard aperture. Note: The lift table AND the vertical axis of the omega drive will follow the optics for all of these motions.
Close down the apertures a ways, then use the CBASS Align button to do simple H/V alignment of the diff'to to the beam.
Use CBASS command-line instruction yaw_pitch to perform pitch/yaw. Again, the lift table will follow these pitch/yaw motions to keep the detector centerline parallel to the collimator axis. more on this...

Periscope	There is a crystal-alignment microscope that views straight into the incident x-ray beam. Users may control it from a web screen that provides most useful operations: http://x25-h.nsls.bnl.gov/axisvid/tryf.html. (This is accessible only inside the PXRR network firewall.) Note: The screen is always live, waiting to Center on your Click; don't click unless you know what you're doing. There are five primitive motions for the microscope, plus two more functions. The primitive motions are vertical (Up Periscope), horizontal perpendicular to the beam, and horiz. parallel to the beam, then there is motorized focus and zoom. The zoom is VERY broad, 16X. All of these five motors can be "homed" [ say how ]. In addition there is a phosphor that can be swung up into position for viewing of the direct beam, and the illumination can be controlled.
Periscope Drive
	The Up Periscope button brings the visualizer into position to view the xtal. Note: The beamstop occupies the same sapce as the periscope This command uses the vertical motor only, bringing it to the last-used vertical and horizontal position, zoom, and focus. Down Periscope simply puts it away. The +/- 90 buttons refer to incrementing the omega axis, which should move at a pace of about one second per 180 degrees of motion.
Grid	There are Display/Hide buttons at the bottom. This display contains the crosshair to which objects will be centered after clicks on the screen and grid marks along each hairline. You will need to hit CRT-reload on the browser to update this setup.
Zoom	The top row of buttons on the screen perform obvious functions, employing the motorized zoom of the 'scope. The Zoom in/out buttons change the scale by about a factor of 1.3. The grid is required to understand the magnification, and the scale of this grid is given to the right of the image in "microns per tick." The range is from about 135 to about 9 microns per tick. The image will not, in general, remain in perfect focus over the zoom range. Live with it for now.
Focus	During setup, the microscope should be focused on the phosphor -- this will employ the internal optical focus motor of the microscope. To focus on the crystal, the software uses the parallel-motion motor to keep the 'scope focused on the phosphor. The Focus +/- buttons have been scaled to the zoom level so that individual clicks will be in a range to give reasonably sharp focus with one click. Note: the Crystal/Phosphor buttons are toggled automatically by the Phosphor In/Out buttons, see below.
Lamp	There are two LED illuminators, above and below the 45-degree mirror. The upper illuminator is occluded partly by the top edge of the mirror and mostly illuminates the collimator block. The lower one illuminates both the crystal and the background. One at a time, they can be turned off or to maximum intensity, or can be brightened or dimmed in about three increments.
Phosphor control	The phosphor is driven up into position by the buttons. Two other operations are linked to this. Firstly, the focus is toggled to the phosphor (internal focus motor) or crystal (parallel-to-the-beam focus motion), depending whether the phosphor is in or out. Secondly, the crystal is moved down and away, parallel to the cold stream, before the phosphor is raised into position, then it is moved back when the phosphor is taken out. Keep track of what you're doing here. If you rotate the spindle the xtal may move out of the beam, although the software does keep track of the original centered position.
Click-To-Center (C2C):	Below the image are three buttons. The action of all three buttons is scaled to the zoom level. If something seems odd, consider re-"homing" the zoom and focus. In any event, because the zoom is a continuous motion, the motions will not be precisely accurate. One may require a couple of C2C cycles to get it right.