Align with Rotation Dialog

This dialog allows you to align two images taken before and after rotating the specimen. The microscope image shift can then be changed so that a new image will be centered on the reference image, just as when doing an ordinary autoalignment. You can also use this alignment to transform Navigator items into the new registration, although this transformation will be of limited accuracy because it is based on measurements at a single point. You may not be able to trust the transformation farther than ~500 µm from the point being aligned, unless you have very large maps for realigning to (~30 µm). It is more reliable to use a transformation based on registration points that span the area of interest; even one based on only two registration points. Thus, the main value of this procedure might to assist in accurately locating registration points after rotation. In tests on an F20 in Boulder, the main source of error (after accounting for both a stretch in the image and a stretch in the stage coordinate system) was that the image rotation was not a good enough estimate of the true grid rotation as measured by registration points.

To run this procedure, there must be a map in the buffer where images are read back from file, the current registration must differ from that of the map, and there must be either an image in buffer A or a montage overview in buffer B taken at the current registration. When the dialog opens, a single image is copied to buffer B, so that buffer B contains the original image regardless of its source. At this point, you should place the approximate rotation in the 'Starting angle to rotate by' text box. This is the angle by which the image needs to be rotated to align with the map, which is the opposite of the amount by which the grid was rotated. Enter a positive angle if the image needs to be rotated counterclockwise to align with the map.

When you press Align to Map, the program will search a range of angles centered on the starting angle and with the total range given in the 'range of' text box (e.g., a range of 20° will search to 10° on either side of the starting angle). The program finds the rotation that gives the highest cross-correlation, makes a rotated copy of the image in buffer A and shifts it into alignment. You can assess the alignment by toggling between the rotated image and the map with the 'Home' and 'Delete' hot keys. You can also shift the image with the mouse at this point to improve the alignment, if necessary.

A message will be printed in the log window if the best correlation occurs at the end of the search range. If so, you should either change the starting angle or increase the search range, and press Align to Map again.

If the search fails to find the right rotation, you can turn off the 'Search angles over range of' check box and adjust the starting angle to give the proper alignment.

To have the microscope image shift adjusted to match the alignment of the image, press Apply Image Shift on Scope. You can then take a new image and align it to the map. In fact, you are always free to start with a new image in A or montage overview in B after repositioning the specimen.

Press Transform to compute a transformation from the rotation and shift used to align the image and the stage positions of the image and map, and to transform Navigator items from the old registration to the new one.

Two calibrations will make this procedure work better. First, if there is significant stretch along one axis in the imaging process, the rotated image will not line up well with the map unless this stretch is taken into account. The alignment routine will do so if there is a camera property entry 'RotationStretchXform' from which the stretch can be derived. Second, if the stage axes are not precisely perpendicular or do not have the same scaling, then the transformation of positions will not be accurate unless this stretch in the stage coordinate system is taken into account. The transformation routine will do so if the stage stretch has been calibrated by doing a transformation with registration points then running the Stage Stretch command in the Calibration menu. See Stage and Imaging Stretch for instructions on these calibrations.

The transformation will also be more accurate if differences in image rotation between the map and the image being aligned are minimized by setting the eucuentric height and focusing at this location both before and after the rotation.