Beam Autocenter Setup Dialog

This dialog box allows you to set the beam intensity, exposure time and binning that will be used to acquire an image for centering the beam automatically. Essentially, you need to set the beam to a size appropriate for the method of autocentering to be used, then you need adjust the exposure time and binning to avoid saturating the camera, where you can take pictures from within this dialog to check both of these adjustments. The autocentering routine can use two methods: finding the edges of the beam, as the Center Beam command does, or simply measuring the average location (centroid) of all the counts in an image containing the whole beam. The edge-finding method is preferred unless the beam edges are not sharp enough for it to work, because the centroid method requires a greater intensity change and requires two images to avoid an error if part of the beam is out of the image. The centroid will also be off from the beam center if the sample causes large gradients in intensity.

To use the edge-finding method, adjust the beam so that you can see a majority of its edge, or until it is just inscribed in the camera field of view. There should be no need to make it smaller than that. To use to centroid method, you need to make the beam substantially smaller than the camera field, with the diameter perhaps half to two-thirds the size of the camera.

The program can store the settings to control the autocentering separately for each spot size, magnification, and camera. In addition, separate settings can be stored for intensities on either side of condenser crossover. This does not mean that you always need to set up parameters for every set of conditions. To a limited extent, the program will derive settings for a particular mag and spot size from the settings for a nearby mag and spot. However, it cannot transfer settings from one camera to another or from one side of crossover to another.

You can set up these parameters by adjusting the beam to a likely size on the viewing screen, then taking an image with 'Test Acquire'. Adjust the beam size as needed, and reduce the exposure time or binning to avoid camera saturation. These latter values are originally based on the Trial camera parameter set.

You can open this dialog while in Low Dose mode.  In Low Dose, autocentering images will be taken with Trial area parameters, except with the intensity needed for autocentering.  Thus, when you open the dialog, the program will set the area to show when the screen is down to Trial; this original area choice will be restored when you are done.  The program will also turn off continuous update.  If you drop the screen with the dialog open, the Trial area will be shown with the autocentering intensity.

Setup beam on scope and track scope state

This option links the settings shown in the dialog to the state of the microscope. Turn it on to have the state specified by the current parameters imposed on the scope, and to be able to adjust the intensity setting. With the option on, changing the spot size or magnification in the dialog will cause the corresponding change on the microscope, and vice versa. In addition, these changes will switch the dialog to a different parameter set, and scope intensity will be set to the intensity setting of the new parameter set. With the option off, you can change the mag or spot size selectors to look at different parameter sets without affecting the microscope; you can also change binning or exposure time but not intensity.

Spot size and Mag readouts and selectors

The controls let you see and change the current magnification and spot size of the dialog, which determine which parameter set is displayed.

Intensity readout

The intensity setting in the current parameter set is displayed but can only be modified when the 'Track scope state' option is on.

Smaller - Bigger

The buttons will decrease or increase the beam diameter by about 10%.

Binning

Use the spin buttons to set the binning of the exposure. Binning can be reduced to avoid saturation from a bright beam when exposure time is already as low as feasible.

Exposure

The exposure time is shown and can be changed in this text box.

Beam centering method

These radio buttons allow you to choose between finding edges and using centroids, as described above.

Source of settings

This line reports whether the currently shown settings are stored values specific to this spot size and magnification, or whether they have been derived from a different spot size and/or magnification. If settings are derived, then as soon as you modify them in any way (i.e., change intensity, binning, exposure, or centering method), they will be stored and shown as directly set.

Test Acquire

Use this button to take an image with the current settings and see how large the beam is and whether it is saturating the camera. The dialog will close and reopen.  The screen will be restored to its state before taking the picture unless you are in Low Dose mode.

Delete Settings

The button can be used to delete stored settings, which might be useful if you need to change settings substantially and want to start with one derived from another mag or spot size where you have already changed the settings.