The prime-focus spectrometer: A new design concept

In a reflector, the light loss is at a minimum at the prime focus, but it is too awkward to mount a spectrometer of any size there. In this paper, I propose to place the slit at the prime focus to be followed immediately by a collimating mirror as shown in Fig. 1. The collimated beam diameter can be as large as one-ninth of the telescope aperture, and a maximum spectral resolving power of 10⁵ can be achieved. This new design can replace with advantage the conventional Cassegrain spectrometer in all types of work. It is eminently suited for use in the next generation of telescopes such as the MMT and the single telescope array.