Designing high-performance spectrographs for extremely large telescopes

Abstract

To answer the increasingly challenging questions of modern astronomy, larger and larger optical telescopes have been built in the last few decades, equipped with a sizable suite of focal-plane instruments devoted to observations at all wavelengths, and spatial and spectral resolutions. As a general rule, for a given spectral resolution and under seeing-limited conditions, larger telescopes require larger instruments because spectrograph dimensions scale with the slit width, which scales with telescope diameter. For example, the High Resolution Echelle Spectrometer at the Keck telescope1 has a 1.2 × 0.3m2 mosaic grating consisting of three segments, and collimator and camera mirrors of the same order of magnitude. Some attempts to reduce the dimensions of the optics have been made, for instance for the Ultraviolet and Visual Echelle Spectrograph2 at the Very Large Telescope (VLT), where a smaller 0.8 × 0.2m2 grating has been used. If we were to linearly scale such instruments to 30–40mclass telescopes, such as the 42m European Extremely Large Telescope3 (E-ELT, illustrated in Figure 1), this would result in three to five times larger components and thus very large, heavy, and expensive instruments. Moreover, manufacturing constraints have been reached in many areas. For example, the ruling engines that are used to make diffraction gratings—the main dispersing component of a spectrograph—are limited to ruled areas of around 40 × 30cm2. We have investigated a new design approach4 that uses slicing techniques (at image or pupil planes) and ‘anamorphic’ magnification at different levels to keep instrument size and cost affordable. We can use these methods because the resolving Figure 1. Model of the E-ELT within its enclosure. The main mirror has a diameter of 42m. The COsmic Dynamics EXperiment (CODEX) instrument will be situated at a stationary location below the telescope and receive its light via an optical train of mirrors and fibers. (Courtesy: European Southern Observatory.)