The possibility of building a spectrometer based on a flat two-dimensional (crossed) grating is being considered. The most suitable layout for this is the Czerny - Turner, where a spectral image lies in the plane. In this paper an attempt to compensate for the transverse aberrations is made by using a diffraction grating with variable spacing grooves in both sections and aspheric mirror elements of layout, including ones having a freeform surface. Using crossed grating greatly simplifies the device layout and may be particularly effective when used in the ultraviolet and infrared regions of the spectrum, due to a small choice of transmissive materials for manufacturing spectral prisms. This paper gives examples of such case.
In this paper a well-known approach is used for calculation of off-axis three-mirror telescope. It includes usage of conic cross-sections properties, each of the sections forming a stigmatic image. To create a compact optical system, a flat mirror aberration corrector is introduced, which is at later stage transformed into a free-form surface in order to compensate field aberrations. Similarly, one can introduce such a corrector in finalized layout for its further optimization and getting a suitable form, including the conversion of multimirrors axial optical system into decentered one. As an example, off-axial Gregory telescope embodiment is used for infrared waveband region, due to the fact that, unlike the Cassegrain telescope, it provides a real exit pupil, and usage of the mirror corrector brings several advantages. Firstly, this feature may be used to include cold stop or adaptive mirror in the exit pupil, wherein corrector is introduced into a converging beam before the focus of the first mirror. Secondly, when placing corrector in the exit pupil of the optical system it is possible to eliminate high and low order aberrations of center point, which in turn improves optical system f-number, and minimize field aberrations. As another example, off-axial Ritchey-Chretien telescope embodiment is used as a good fit for visible region systems. Analysis and calculation results of optical systems with free-form correctors with surfaces, defined by Power polynomial series are presented in this paper. Advantages of different freeform surfaces usage depends on optical system layouts specifics.
Existing off-axial optical systems aberration theories are based on series expansion of optical path function (eikonal
function). Numerical evaluations, conducted in this paper, have shown, that the most accurate results are obtained with
theory, based on series expansion of exact ray-tracing equations, where expressions for high-order aberration coefficients
include expressions for low order ones. We have used that very method in our search of optimal decision for off-axis
optical reflective telescope systems.
General approaches to the solution are shown on the example of one of the alternative variants of UV long-slit spectrometer. It is suggested using concave aberration-corrected holographic diffraction grating in FUV spectral range (102 – 175 nm). In NUV spectral range (175 – 310 nm), where mirror coating reflecting factor is higher, it is suggested using a beam of zero order of this grating with subsequent spectral decomposition performed with additional grating. It is known that diffractive grating efficiency depends on the form of grooves profile, in the first approximation step profile echelett grating fits quite well for this task.