The dialog presents a rectangular grid over which one clicks the left mouse button to move the telescope. An arrow drawn between the center of the grid and the mouse cursor shows the direction that the optical camera image will move on the TV. Similarly, the distance away from the center of the grid at which you click sets the distance to be moved on the TV. The effective size of the grid on the sky can be is set by the contents of the "Grid Interval" entry field below the grid. The number of arcseconds that you enter there, is reflected in the separation of neighboring lines on the grid. The same separation is used for the displayed concentric circles. The actual number of arcseconds that would be moved in the X and Y directions of the TV, is displayed textually above the grid.
When the "Snap to grid" toggle button is depressed, only offset positions that lie on the displayed grid vertices will be allowed, and the cursor arrow will appear to jump from one to the next as you move the cursor over the grid. The repeatability delivered by this option can be useful if you want to move the telescope first one way then back to its starting position.
The "Reverse arrow" toggle button reverses the direction of the arrow that is drawn between the center of the grid and the mouse cursor. This also reverses the direction that the telescope moves when one clicks over a particular location of the grid.
Pressing the button that says, "Press here when the star is centered in the crosshairs" does two things. First it executes a "mark one, f0" command to place a mark in the archive indicating that the star is centered. Then after this has had time to propogate through to the control system and get recorded along with the coordinates and centering offsets of the current star, it executes a "signal done" command to tell the currently executing schedule that it is safe to move to the next star. The scheduling script can use the "$signaled(done)" function to wait for this signal. Without the delay between placing the mark in the archive and moving on to the next star, the next star would erroneously get marked as centered. During the delay, the star-centered button is shown in red.
If you don't know what this angle should be, then you can work it out empirically by noticing what direction a star image on the TV monitor moves when you click in a given direction over the grid.