Fast F/1 reflective Maksutov microscope objective

Published 6.2.2015

A very fast solid reflective objective is easy to design and achromatize for a wide spectral range.

Originally suggested by Maksutov, a Maksutov microscope objective (or eyepiece) is a reverse Gregorian type reflective objective. It has high numerical aperture and has corrected spherical, coma and color aberrations. It requires a field flattener to correct the field curvature, but for on-axis observations, it has a long working distance and short total track.

I inspected an f = 10 mm, F/1 and NA=0.45 reverse Gregorian objective that has two reflective surfaces with low conics. In order to bypass the necessary spider construction needed to hold the smaller mirror (or shade, in case of the Shwarzschild configuration), I designed it as a solid singlet lens with a donut shape reflective surface on the infinite conjugate side, and a small reflective surface on the image side, called a Maksutov configuration. Reflective systems have no color aberration, so in order to avoid it with this "single lens", I also designed it with CaF2 glass.

A Maksutov configuration with a 2.3º FFOV, f=10mm and F/1. Glass CaF2 can be diamond milled. Total length is less than 50 mm with a free working distance of 22 mm.

A Maksutov configuration with a 2.3º FFOV, f=10mm and F/1. Glass CaF2 can be diamond milled. Total length is less than 50 mm with a free working distance of 22 mm.


The stop size is 10 mm in diameter, and the designed full field of view of 2.3º enables diffraction-limited 0.4 mm image circle with almost full bandwidth. Maximum distortion is -0.05%. The MTF curves retain their resolution easily to the edges of the field.

To manufacture a Maksutov microscopic objective, there are two surfaces with two curvatures - front has spherical center and conic outer edges, whereas the back surface has spherical outer edges with a conic center. There is vignetting due to the smaller mirror, but the obstruction ratio is only 23%, very tolerable. The mirror surfaces can be protected to a high degree, and there is no problem with dust. Computer-controlled diamond milling is necessary. After polishing, the conic areas must be coated with a reflective coating. The optomechanics of the objective are a simple enough matter, and no further assembly or alignment is necessary, as there is no spider rack.