screened off; and the eyepiece can see only the back sides of the baffles. Since these surfaces are not directly illuminated through the open end of the tube, they provide a very dark background for the eyepiece. Since the secondary mirror in the Yolo is relatively far from the eyepiece, the eyepiece does not get to see much of the primary mirror cell reflected in the secondary; and, since the primary in the Yolo is located at the bottom of a much longer tube than in the Newtonian, it receives many times less illumination (roughly inversely as the squares of the tube-lengths) than its counterpart in the Newtonian. Very little light from this source, therefore, gets into the eyepiece of the Yolo.
The light-baffle system for the Yolo reflector is designed as shown in Figure 5. First, the two mirrors and the field lens of the largest eyepiece (or photographic plate) are laid out to scale. Next, the outline of the cone of rays from a point source is laid out (light solid lines). Then the outline of the cone of rays from an extended source which will just fill the field is drawn in (light dashed lines). The next step is to locate baffle G. If the telescope is intended for visual observation only and the field lens of the largest eyepiece is no more than about 2 inches in diameter, baffle G can be located as close as two primary mirror diameters in front of the tube proper. If, on the other hand, the instrument is to be used in photography, with a plate as large as 5" by 7", then baffle G may need to be located as far out as four- or five-diameters.
With the location of baffle G decided upon, lay a straight edge on the drawing so that it will pass through points O, B, and G. With point O (inside edge of eyepiece or plate holder) as a pivot, adjust the straight edge until both points B and G lie about the same fraction of the distance between