Improving telescope objective with N-BK7 accessories

Published 25.9.2015

Giving new life to an old telescope design is quite possible and not necessarily expensive either.

I designed a generic Fraunhofer doublet telescope in order to design two accessories for it using nothing but Schott N-BK7 glass. Both accessories address the astigmatism in a doublet, flattening the image plane for a wider field of view of the original doublet.

Fixing induced aberrations is generally not the correct approach, but in this case the doublet is considered a commercial product for star viewing, alone quite adequate for visual viewing, and the accessories are for enabling large sensor cameras without the doublet's characteristic blur at the edges.

The base objective is an 80 mm aperture, 480 mm focal length (F/6) Fraunhofer doublet using ED glass. Astigmatism was a predicted problem. First thoughts for an accessory was either a singlet meniscus lens for fixing astigmatism, or a singlet field flattener. Singlet field flatteners are positioned too close to the image plane for a commercial product, so I designed a meniscus lens to fix the astigmatism. This resulted in a very curved, very weak lens which would have considerable manufacturing difficulties, so I abandoned it and bypass its description here.

Second accessory was a two lens field flattener which can be positioned much further from the image field than a singlet. Positive and a negative all-spherical menisci are curved enough to have little manufacturing problems, while keeping the FFOV spot size consistent throughout the Full frame image plane. This approach is very attractive, because if the aperture allows for it, there is a high chance one could use pre-manufactured stock lenses for creating this type of field flattener, and the focal length would not change much. With the Full frame field flattener, the focal length is 475 mm (F/5.9), an insignificant reduction from the base objective.

Third accessory is an improvement to the previous with more curved surfaces and the last (convex) surface being an even asphere with only 4th and 6th order aspheric coefficients utilized. There is no considerable change in spot size from previous design, except that the image format is now Medium format, or 645, and focal length is increased from 480 mm to 510 mm (F/6.3).



I designed two types of two-lens field flatteners using only N-BK7 glass, that would flatten an F/6 doublet image for astrophotography. The resulting spot size is not diffraction limited but is small and consistent throughout the whole field, which is increased to 44 mm image diameter (Full frame) with all-spherical surfaces, and to 70 mm diameter (645) with one even asphere. The base doublet was generic, meaning the designs can be adapted to any objective that has enough working image space.