Publications

Are curved focal planes necessary for wide-field survey telescopes?

Ackermann, Mark R.; McGraw, John T.; Zimmer, Peter C.

The last decade has seen significant interest in wide field of view (FOV) telescopes for sky survey and space surveillance applications. Prompted by this interest, a multitude of wide-field designs have emerged. While all designs result from optimization of competing constraints, one of the more controversial design choices is whether such telescopes require flat or curved focal planes. For imaging applications, curved focal planes are not an obvious choice. Thirty years ago with mostly analytic design tools, the solution to wide-field image quality appeared to be curved focal planes. Today however, with computer aided optimization, high image quality can be achieved over flat focal surfaces. For most designs, the small gains in performance offered by curved focal planes are more than offset by the complexities and cost of curved CCDs. Modern design techniques incorporating reflective and refractive correctors appear to make a curved focal surface an unnecessary complication. Examination of seven current, wide FOV projects (SDSS, MMT, DCT, LSST, PanStarrs, HyperSuprime and DARPA SST) suggests there is little to be gained from a curved focal plane. The one exception might be the HyperSuprime instrument where performance goals are severely stressing refractive prime-focus corrector capabilities.