The Direct Imaging Mirror has experienced problems with its limit switches. As a result, the mirror may not come back to the exact same position in the beam once it has been moved out. This, in addition to some play in the slit wheel motor, means that the slit may not appear in the same column on the array each time the slit is imaged with the DI Mirror in. The movement of the slit is generally less than a few columns, although it can be as much as 5 columns (usually this happens at the very beginning of the night, the second time the mirror is replaced in the beam). The effect of this apparent movement is the following: if the slit is imaged once at the beginning of the night, and later during the night (after several movements of the DI Mirror to image targets) an object is imaged and moved to the previous location of the slit on the array (x axis, or column), the object may not be perfectly aligned with the slit. This movement should have no effect on the y-axis location (or row) of subsequent spectra.

For observers using the smallest slit width and those who require precise information about the location of the slit on their targets, we suggest that the slit be imaged with the DI Mirror in the beam before each observation. The suggested procedure is the following:

Move to the object
Put the DI Mirror in the beam (DI mode)
Put the slit in the beam and image the slit
Record the column (x axis location) of the slit
Remove the slit and insert a filter
Image the object
Move the object to the same column as the slit
Re-insert the slit - the object should be visible through the slit
Remove the DI Mirror

After this procedure, the object should be aligned with the slit. Note that there is currently an offset of about 27 pixels in the y position of the object as imaged with the DI Mirror and the position of the object's spectrum obtained with the grating.

John Rayner (rayner@irtf)
Last modified 18 Sept. 1998