Why do the Coronado SolarMax series filters have a central obstruction?

A noticeable characteristic of the SolarMax series of filters is the central obstruction. This is an extremely important part of the design and is, in fact, the principle of the patents issued. Ultra-narrowband filters are based on the Fabry-Perot etalon. This can be of either a solid spaced or air-spaced design. Because the solid spaced etalon is constructed of an optical material, such as glass or fused silica, the temperature coefficient of the material combined with the temperature coefficient of its refractive index, mean that the etalon is thermally sensitive and has to be kept at a constant temperature in order to maintain the tuned wavelength. Air-spaced etalons have been constructed for many years as an alternative based on their thermal stability. However, in this respect, one has to carefully define the thermal stability. For instance, if such an etalon is tuned at a temperature of 200C and the temperature is changed to 300C, it will also be tuned, and have the same characteristics, at this new temperature WHEN THE WHOLE ETALON HAS COME TO THERMAL EQUILIBRIUM at the new temperature. However, it will NOT be tuned during the time that the temperature is changing and its bandwidth will be considerably BROADENED, due to thermal gradients within the components, causing distortion of the air-gap. Such an etalon, when used as a Solar filter, will have unacceptable, variable characteristics and, therefore, performance. As with the solid spaced filter, it would have to be temperature controlled. It was this dilemma that prevented the use of air-spaced etalons as Solar filters until the breakthrough brought about by the patented Coronado design. The reason for this, of course, is that, in use as a Solar filter, facing the Sun,- the filter is never in thermal equilibrium due to continuous incident radiation. In the Coronado design, an extra "spacer" in the etalon, controls the shape of the etalon components during temperature changes and maintains all the etalons characteristics. Thus the filter is kept tuned to the correct wavelength, and maintains its proper bandwidth, at all temperatures and UNDER CHANGING CONDITIONS. This CAN NOT be the case for an etalon that does not have this controlling central spacer. Because of this central spacer, a mask has to be put over the center of the filter to prevent direct sunlight from getting to the image plane. This is the same situation as the central obstruction in a Cassegrain type telescope (or a SCT or Newtonian or Maksutov/Cassegrain). However, whereas a typical cassegrain has a central obstruction of approximately 35%,- giving an area obstruction of ~12%,- that in the SolarMax filters is only 2.5% to 5.5% depending on the model. This is well within the obstruction generally considered to be completely unnoticeable to image quality.