With the light source, mirror, and eyepiece set up as shown in Figure 4, and the warping harness unstressed, the image seen in the eyepiece should show only one aberration - astigmatism - but lots of that. When the eyepiece is moved toward the mirror, the image should take on the appearance of a sharp vertical line. When the eyepiece is moved away from the mirror, a point will be reached where the image has the appearance of a sharp horizontal line. Next, put a little stress on the warping harness. Now, in the eyepiece, the vertical and horizontal line-images will be shorter and will become sharp with less travel of the eyepiece. As the stress on the warping harness is increased, the images will continue to shorten and may start to rotate a little. If image rotation is observed, the mirror and warping harness should be rotated a little in the same direction. This should make the images return to the vertical and horizontal attitudes. Further increases in the stress and rotation of the warping harness should eventually result in a perfect Airy diffraction pattern in the eyepiece. Until a very good Airy diffraction pattern can be achieved in this test, there is little point in putting the mirror and warping harness into a telescope.
In order to provide good performance on deep sky objects, the Yolo reflector needs a well designed system of light baffles. Since its eyepiece looks right up at the sky through the open end of the tube, it has some of the problems of the Cassegrainian reflector. Its main problem differs from that in the Cassegrainian, however, in that the open end of the Yolo is confined to only one side of the secondary and does not extend all the way around, as it does