: The size of the Sec. mirror determines the amount of light that is obstructed by the glass on the way in and how much of the pri. mirror that can be seen and what the max. angle is for full illumination of the eyepiece. Larger Sec. mirrors cause lower contrast due to diffraction and other effects while smaller mirrors cause less light to the eyepiece when off axis and sometimes even when on axis when the mirror is small enough. The choice is yours. Some builders set themselves up with a choice for different viewing situations and for Photography work.