CHESS
Align Table

Name:  align_table - optical table alignment program

Description:  align_table is a general optical table alignment program.  Its function is to maximize the count rate of a specific counter with respect to optical table position.

Preparation:  This macro will work with any optical table that has been set up in accordance with the conventions outlined in the Guide to the CHESS Optical Table Macros.  Particular attention should be paid to the following items:

• Each motor should be tested individually to ensure that the software limits are correct and that the motor is not missing pulses.  The software limits should be encountered before the hardware limits, or else a hard limit may be hit during the alignment, which will terminate the program.  If a motor is missing pulses, it will lose its calibration over time and the table parameters will become inaccurate as a result.

• The correct position of the rotation point should be given via the define_table macro.  This position should be specified in terms of its coordinates in the optical table reference frame (see the Guide to the CHESS Optical Table Macros for more information on coordinates).

• The counter whose rate is to be maximized must be declared as the detector DET.  This declaration can be made by using the macro counters.

Using align_table:  Once the above preparations have been made, using align_table involves nothing more than typing its name. align_table maximizes counts in two stages, using the x and z translations and the x and z rotations (rotations about the x and z axes).

The program runs a coarse alignment procedure first, which takes a step and then looks for a certain percent change in beam intensity. If the change is less than this amount, the step size is increased and another step is taken. The program finishes stepping when a peak is found or motor soft limits have been hit in both directions (a peak is assumed to have been passed when there has been a 10% decrease in count rate from the maximum). If a peak has been found, the table is driven to that position; if no peak has been found, the user is notified and the table is returned to its starting point. This is repeated for each motion.

Once the coarse alignment is finished, a fine-tuning procedure is done, which runs a series of small scans over each coordinate. Real-time plots are generated which help to give some indication of the size of the beam and the effectiveness of align_table in locating the peak position. Sometimes the scan may not have sufficient range to cover the peak, because the alignment may still be off somewhat (this is indicated when the maximum lies at one of the endpoints of the scan). In this case, the range of the scan is automatically increased and the scan is repeated with its center at the previous scan's endpoint. This procedure is repeated for each coordinate until a peak is clearly evident on each scan.

The program finishes by generating a summary which includes the count rate before and after running the routine, the total time time taken, and the percent improvement in the count rate. If the CESR current signal is also being read, and a counter called CESR has been defined, then these figures will also include comparisons of the count rates in counts/sec/mA.

Last Update: 2006-09-14