Case study: Using an achromatic objective as its own focal reducer

Published 6.9.2018

Commercial telescopes are more often than not provided with a focal reducer, which enables the customer to use the telescope also for astrophotography. Here I introduce two cases where a second objective unit identical to the primary unit serves as the focal reducer to the first unit which reduces not only focal length but also design and manufacturing costs.

A focal reducer, which is a finite-conjugate lens system inserted by the user of the telescope to the end of a telescopic objective, needs additional lens design and manufacture. Due to the finite-conjugate nature of the reducer, they must be designed to the objective or they perform very poorly. For slow relative apertures (F/10 or slower) a generic design might be found, because the errors it introduces are too small to detect, but for faster telescopes they are unique. When the number of lenses in a telescope objective increases from two, the needed amount of lenses for the reducer can even double that, and the manufacturing costs add up quickly.

My two case studies describe achromatic doublets, F/10 and F/8 and their focal reducers, F/8 and F/6.4, respectively. In both cases, the objective is an identical air-spaced achromat as its focal reducer.

Layout of F/10 achromat and x0.8 reducer (to F/8)

Layout of F/8 achromat and x0.8 reducer (to F/6.4)

The above layouts enable manufacture of only two distinct lenses, their assembly into a single unit, then using a second identical unit as the reducer for the first unit.


Above is the performance of the F/10 to F/8 telescope. On the left column, there are the ray fan plots (top) of center, 0.7 radius and edge of the 3ᵒ FFOV field, and field curvature and distortion (bottom). The center of the image is diffraction-limited to 1.25ᵒ FFOV in both cases. On the right, with the reducer attached (respectively).


Above is the performance of the F/8 to F/6.4 telescope. The center image is diffraction-limited to 1.2ᵒ FFOV without the reducer (left column). With the reducer, the spot size is less than 12µm in diameter within 1.34ᵒ FFOV (right column).

In both examples, there is practically no distortion, making the cases very good for generic commercial telescope, including deep-space astrophotography. Also in both cases, the back focal length is over 190 mm, enabling the use of adapters for various DSLR camera adapters to be designed without difficulty.

The idea of using existing components for altering the design is also explored in our publication "A telescope objective with symmetrically shaped lenses".