Setting of the slit width is done by Observer Support and not the observer. Slit location should always be verified by illuminating the fibers from the telescope using the quartz source, putting a Questar focussed at infinity in the beam, pointed at the collimator looking back at the fiber. With this arrangement, it is easy to see when the slit is properly centered on the fiber. This is a relatively broad spectral feature which is eliminated by the flat fielding process and after the data has been reduced can only be detected through careful measurements of absolute efficiency on standard stars. Click to zoom it to a larger size so that you can see the details.
Light comes from the telescope to a fiber head which holds the fibers at the telescope focus.
The fibers are mounted in a stainless steel post which is polished to mirror smoothness along with the fiber tips. Light from the telescope passes through a pellicle mirror before forming a focus on the tips of the fibers. Reflected light returning from the stainless steel mirror surface is reflected off the underside of the pellicle and reimaged onto a television camera. A clear tv image is seen of the tips of the fibers and a small area around them. The telescope is then pointed at a target object.