Figure 13.1 Result of running Find Image Centers with points not found
When finding the position of the image centers, the routine uses a Center of Mass algorithm to locate each center point. Noise can cause the routine to incorrectly locate the true center of the peak points. There is a mechanism to quickly scan through the points, and verify the routine found the correct center. Enable the Pixel Zoom tool, and click on the image anywhere to bring up the Pixel Zoom Window, which shows an 8x magnification of the screen.
In the Dynacal window, in the Find Centers tab, after you have found the image centers by clicking on the Find Image Centers button, you can click on a row, and the program will do the following. It will scroll the CCD image to bring the highlighted center into view. If the Pixel Zoom window is open, it will move the zoomed area to the coordinate highlighted and selected in the Geometric Correction Window, and a red box is drawn about the point in the main CCD view.
If the routine is missing points, first try to tweak the finding routine so that it successfully finds all the points. If that doesn’t work, or if there is just one or several missing points, a second option is to edit the points manually. In more detail:
- Option 1: Adjust the threshold. A lower value will find dimmer points but may track to noise. Use the advanced option box and enter the ‘d’ command to set the bounding box used when looking for peaks. The ‘d’ option is most useful when the grid is set to every second or every third point, in which case the guess value for the bounding box area will be incorrect.
- Option 2: Select a point from the list that is marked in red, then click the ‘Edit Value’ button or ‘Pick New Point’ button. See Software Reference Manual for details.