An off axis solar filter blocks light into the telescope by only allowing light through a small portion of glass that is off center. A full aperture filter allows the most light into the telescope because it allows the full aperture of your telescope to be viewed. This allows the best possible daytime viewing when there is minimal atmospheric turbulence. If turbulence is present, a mask can be put over the end of the filter to effectively reduce the aperture. With a mask you can effectively change the aperture range to anything less than full aperture depending on the size of the mask. An off axis solar filter cannot be used on a refracting telescope.