in the Cassegrainian. The eyepiece and eyepiece tube are, or course, pointed right at the secondary. By designing the Yolo so that the tilt angles are a little greater than the minimum required to make the light path just clear mirrors of the minimum required diameters, the central illuminated part of the secondary is moved a short distance away from the open end of the tube. This helps very appreciably in the solution of this problem and is a device which cannot be used in the Cassegrainian.
Having the eyepiece located at the bottom of the tube and looking up instead of at the top of the tube and looking across, as is the situation in the Newtonian reflector, is both a disadvantage and an advantage. It is a disadvantage in that it makes the light baffle system necessary in the Yolo; but it is an advantage in that once the system of baffles is installed, the field of view in the Yolo is much darker than that which can be obtained in Newtonian. In the Newtonian, the eyepiece is pointed squarely at the diagonal mirror, but it also "sees" the far side of the telescope tube surrounding the diagonal. This part of the tube is quite close to the open end and thus is fairly well illuminated by direct starlight, general sky light, and perhaps light from other sources. Thus the eyepiece in the Newtonian is looking right at a surface which receives quite a lot of illumination. Since the eyepiece in the Newtonian is quite close to the diagonal, it gets a fairly good look at the mirror cell of the primary and any surface roughness or baffles located in the lower part of the tube, all of which are looking right up at the sky and are, therefore, fairly well illuminated. This feeds more unwanted light into the eye of the observer and spoils his view of faint objects.
Once the baffles, B and G, in the Yolo (see Figure 5) are properly adjusted, all direct starlight, and any other light coming down the tube, is