The optical design is based on gregory-maksutov or spot maksutov telescope, and often called as CAT ( catadioptric) lens which is a combination of mirrors and refractive lens most of these lens are manual focus. The aperture is fixed. It is virtually free ( marketing bluff beware) of chromatic aberration, coma, and off-axis aberrations. However, due to central obstruction of secondary mirror, contrast is normally lower. However, lateral color aberration ( different image size for different colors) still exist, except on the Zeiss mirotar, which use dual corrector plates The overall image quality depends mainly on the secondary mirror aberrations. So in lower priced models, if the secondary mirror is a separate element, it has more room for improvement. Exposure: The secondary mirror blocks about 1/3 of the area of the lens aperture. This in itself would result in the loss of about 2/3 stop in effective lens speed (i.e. a T-stop that was 2/3 stop slower than the f-stop). The actu...