An IRAF script called TELOFFSET has been written to assist in placing an object onto the slit using the file created in the previous subsection.
Focus the telescope with FILTER=CLEAR, since you will not be using a filter for the spectroscopy.
Go to your field and set the COSMIC base position angle to your desired angle (where COSMIC BASE POSITION ANGLE = 270 - TRUE PA; and COSMIC BASE POSITION ANGLE must be between 70 and 300).
(1) Take an image (either unbinned or binned) with FILTER=CLEAR, APERTURE=CLEAR, and GRISM=OPEN.
(2) Display the image by flipping the y-axis, i.e.
display ccd123[*,-*] 1.
(3) Edit the parameters for TELOFFSET, i.e.
Enter the appropriate filename created in the previous subsection,
the current COSMIC BASE POSITION ANGLE, and identify if this is
a unbinned image (SNAP=no) or a binned
image (SNAP=yes). Type
:go in order to run the task.
Find the object in the SAOIMAGE display, type ``a'' to identify
it, and then ``q'' to quit. The appropriate telescope offsets
needed to move this object onto the chosen slit position will be
(4) Tell the night assistant the offsets to execute.
(5) Find a guide star and start guiding.
(6) Take another image and repeat steps 1-3. If your object is very faint, or if it is a crowded field, you might have to use the unbinned mode. If the offsets are < 0.3'', then this is sufficient. If not, put the guider in ``follow'' mode, ask the night assistant to execute the offsets, then enable guiding.
(7) You should be done. If you are unsure, then repeat step 6 until you are fully satisfied.