The DIS:Drift script allows you to take a time series of spectra by moving the object along the slit in the +X direction during an exposure. The range of motion is centered on your starting position, so typically you should begin by putting your object in the center of the slit. ... In detail, the script performs the following sequence: . Slew to the starting position (actually a bit farther because the script allows some time for the telescope to ramp up to speed). ... Take an exposure. ...