The N&S mode involves synchronous nodding by the telescope and shuffling of charge on the CCD. In its current implementation at the Mayall, two guide stars are required: one for the object position and one for the off (or sky) position. Since both the on and off positions are guided, the most stable guiding results when both stars are of comparable magnitude. The telescope is offset between these two on and off positions in such a way that both guide stars appear in the exact same position on the guide probe -- i.e., the relative position of the guide probe and the spectrograph slitmask are unchanged between the on and off positions.
We experimented with two observation modes, which we called `BRACKET' and `ALTERNATE' respectively. In the BRACKET mode, the sequence of on and off observations are symmetric, beginning and ending with a sky observation (i.e., the sequence is sky - object - sky - object - sky - ... - sky - object - sky - object - sky). In the ALTERNATE mode, the sequence simply alternates between object and sky: object - sky - object - sky - ... - sky - object - sky. We conducted three tests comparing the two modes, and found that in all cases the BRACKET mode resulted in slightly better sky subtraction that the ALTERNATE mode.
The BRACKET mode observation takes place as follows. First, the telescope is set up on the on (object) position and the observation is begun. The array is reset and ICE sends a command to the telescope controller to offset to the sky position. The guiding is stopped, the telescope offsets, the guiding is turned back on, and the new guide star is brought into the center of the guide probe reticle. As soon as the guiding is turned back on, there is a 2 sec wait time, and the telescope control computer then sends a command to ICE. ICE opens the shutter and begins exposing. When half the exposure time is elapsed, the shutter is closed, and ICE sends a command to offset the telescope to the object position. The guiding is turned off, and the telescope reverses the offset, turns on the guider, waits 2 seconds (while it is reacquiring the guide star) and then returns a command to ICE to resume the exposure this time on the object. ICE continues with a full exposure at the object position, and then repeats the offset to sky, where it continues with a full exposure at the sky position, and so on. The bracket mode ends with a half-exposure at the sky position, the shutter is closed, the telescope is offset back to the object position and the CCD array is read out.
Starting the nod sequence on either the sky position or the object position works. The BRACKET sequence always ensures that the first exposure is on sky whether or not the telescope is pointed to object or sky.