DACs: Sample mounting, centering and collection

1. Sample mounting

1. Open the hutch, move Omega to 90 degrees. And on the Mini-Kappa tab on the QEGui, make sure Kappa is at Zero.

2. Put the DAC on the goni with the alignment pins facing down, and made sure it is completely flat with the mini spirit level. It is very important that it is flat, otherwise it will be very difficult to center.

3. Close up the hutch and move Omega to 0 degrees.

Picture of refraction and goni + spirit level

2. Sample centering

1. Roughly center the opening onto the cross hairs.

2. Open the DAC centering tab of the QEGui

3. Adjust Sample X until the sample comes into focus. Will be movement of ~500 um. Note down the Sample X position, or enter it in the "Focus at Omega = 0" box on the GUI

4. Rotate by 180 degrees to Omega = 180 and adjust Sample X until the sample comes into focus. Note down Sample X position or enter it in "Focus at Omega = 180" box on the GUI.

5. Split the different between the Sample X positions (will be shown in the GUI) and change it to this position. This will be the position you need for collections. 

6. Adjust the focus of the camera via the DAC centering GUI. It will be a value somewhere around 14700 - 14800. The focus should be adjusted until the sample is in focus at 0 and 180 degrees.

7. If the sample does not stay centered in the X axis between 0 and 180 degrees, it is likely because the diamonds are slightly misaligned. If it does not stay centered on the Y axis, it may be because the cell is not perfectly perpendicular to the beam direction. It is a good idea to reset the camera focus, and repeat the sample mounting and then centering. 

Picture of DAC GUI

Picture of focus

3. Screening

If starting at 0 degrees, do a 20 degree wedge starting at -10 degrees. Adjust attenuation or sample exposure time as necessary to get enough data. There will be significant shadow from the cell, rings from the gasket, and diffraction spots from the diamonds. You don't want any overloads apart from diamond reflections. If there are overloads, you need to decrease exposure time or increase attenuation. It will help to scroll through the frames on ADXV to check when overloads are coming up. When you are happy with your settings, click Copy and the screening options will be copied to the dataset tab.

Picture of typical diffraction pattern

Picture of screening GUI

4. Data collection 

For maximum completeness and redundancy (and because each run doesn't take long), it is a good idea to collect as much data as possible. 

After runs 2 and 4, when you change kappa or phi, you will need to repeat the Sample centering steps. Although you probably will not need to change the camera focus. If you leave the camera focus at the same value, the adjustment of Sample X will be much smaller. But make sure that the sample is in focus with Omega at 0 and 180 degrees.

After data collection, it is a good idea to return Omega to zero and recenter the DAC, and then change Kappa and Phi back to Zero. This way, when you put the next DAC on the goniometer, it will be in approximately the right place.