title: Stigmatic spectrometers for extended sources: design with toroidal varied-line-space gratings.
abstract: Performances are presented of three classes of imaging slit spectrometers for extended sources with aberration-corrected gratings. A general analytical expression for minimizing off-axis grating aberrations is obtained, and it is demonstrated that these aberrations are minimized when the spectrometer is operated at a magnification higher than unity. Classical designs with toroidal uniform-line-spaced (TULS) or spherical varied-line-space (SVLS) gratings are compared with a new class of designs that utilize toroidal varied-line-space (TVLS) gratings. Although TULS and SVLS designs with two stigmatic points can be designed to operate at near-unity magnification with excellent on-axis spectral and spatial resolutions, they cannot be made to satisfy the general off-axis condition, and so their off-axis performances are not optimum. On the contrary TVLS designs with two stigmatic points can be operated at almost any magnification, thus satisfying the off-axis condition perfectly. Such designs are suitable for imaging spectrometer observations that require an extended field of view.
authors.affiliation: Laboratory for UV and X-Ray Optical Research, Department of Information Engineering, Instituto Nazionale per la Fisica della Materia, University of Padova, via Gradenigo 6/B-35131, Padova, Italy. poletto@dei.unipd.it
authors.forename: Luca
authors.initials: L
authors.lastname: Poletto
authors.forename: Roger J
authors.initials: RJ
authors.lastname: Thomas