Preliminary optical design for a 4.0-m f/2.19 prime focus field corrector with a 2.0-deg. field of view

Direct-imaging surveys require telescope designs which maximize the product of the primary mirror diameter times the field of view. While a 4.0-m telescope would not be considered "large" by modern standards, the provision of a 2.0-degree field corrector would make it comparable in imaging information-gathering capacity to an 8.0-m telescope with a 1.0-degree field corrector which is at or near the present state-of-the-art. We have explored plausible optical design options for such a 4.0-m telescope within the confines of a set of specifications which emphasize practical issues. These include realistic image quality requirements, mechanical simplicity, ease of construction, minimum cost and freedom from restrictive constraints on the primary mirror shape and the telescope structure, such that it could potentially be used in other optical configurations. We present a mechanically robust optical solution which uses a 4.0-m f/2.08 parabolic primary mirror and a refracting field corrector containing 4 powered lens elements with 2 mild aspherics located on softly curved surfaces, ample room for filter(s) and a shutter, with a flat vacuum dewar window and a flat 2.0-degree diameter field of view. We discuss or specifications and we provide a fully-quantitative system prescription as well as our analysis of the system's expected direct-imaging performance in the traditional Umod, B, V, R, (V+R) and I passbands. While we recongize that other optical designs may prove to be superior to ours by some standard(s) of measure, we believe that our solution provides a useful and realistic baseline design which is competitive for the intended purposes.