| | | Selecting function 3 from the main menu enables you to run any of the four wavelength programs. The first option selects the wavelength program, or it allows the program number to be controlled externally. Before running a program we need to specify Whether the program is to be repeated for a specified number of cycles or indefinitely. Each position (if included in the program) is visited in numerical order, but since each position can correspond to any wavelength, the | | |
| | | actual wavelengths can be visited in any desired sequence. To start a wavelength program select option 6. If the system is not set to wait for a trigger pulse the program will run immediately. Position changes can be set to start on receipt of a trigger-input pulse (via the run/stop-input pin on the computer connector suppliers) and using option 3 from the menu. The major control feature to be considered in relation to wavelength changing is the control and display of transition times between wavelength positions. It is important to switch off signal detection during wavelength transitions, since the matting will be scanning across intermediate wavelengths during this time. The transition time between positions clearly depends on the angle through which each galvo needs to move, so it will be different for each transition in the program cycle. However, the time for any given transition will be essentially constant from one cycle to the next. This has the advantage that the overall cycle times are entirely regular and predictable, but a key question now arises as to whether or not the transition time should count as part of the total time spent at each position. What does this mean in practice? Imagine that the monochromator is programmed to spend 10.0msec at each of three positions, and that the transition times to reach each position are 0.3, 0.5 and 0.6msec respectively. If we include the transition times within the time at each position, then the actual sampling times are 9.7, 9.5 and 9.4msec respectively, and the total cycle time is 30.0msec. On the other hand, if the transition times are not included, then the sampling times at each wavelength are all exactly 10.0msec, but the total cycle time is extended by the sum of the transition times, to 31.4msec. The decision on which is the most appropriate is up to the user, as the software allows either mode of operation, although our preference is to count the transition time as part of the total time. Be aware that if the transition time (however specified) exceeds the total time at any position, it will be impossible for the system to operate correctly. It is the responsibility of the user to avoid such a situation! Further control is added with the facility for transition time extension. This allows a minimum transition time value to be specified, which is the same for all positions. In the above case, we could specify a minimum transition time of 1.0 msec. Depending on whether or not transition times are included within the times at each position, this would give either 9.0msec sampling time at each wavelength and a 30.0msec cycle time, or 10.0msec sampling time at each wavelength and a 33.0msec cycle time. However, in both cases both the sampling times and the cycle time are now explicitly specified. Although this would appear to give the best of both worlds, it does have the disadvantage that one has thrown away a total of 1.6msec of usable data during the cycle. No method is perfect! The above discussion has presumed that there is some way of determining the actual transition times, so that an appropriate value for the minimum transition time can be specified, and indeed there is. After a wavelength program has run, there is a menu option to show the actual transition times for each position change, to microsecond precision. These times will of course include the specified transition time. | | |