Abstract
Achieving diffraction limited imaging with future ground-based optical telescopes will require adaptive optics for correction of atmospheric turbulence and also efficient techniques for atmospheric dispersion compensation. We study the benefit of using a linear atmospheric dispersion corrector (ADC) coupled with a deformable mirror on a 42-m Extremely Large Telescope (ELT) operating in the VIRJ spectral bands. The ADC design consists of two identical thin wedges made of F5 glass. The amount of dispersion introduced by the ADC is adjusted by translating one of the wedges along the optical axis so that it always cancels atmospheric dispersion as it varies with telescope elevation. We show that the ADC working in conjunction with a deformable mirror provides diffraction-limited image quality over a 1-arcmin field.
Highlights
The Earth’s atmosphere has various chromatic effects on light passing through it
We consider here an adaptive Extremely Large Telescope (ELT) featuring a 42-m ellipsoidal primary mirror (M1), which in conjunction with a 4-m concave ellipsoidal secondary mirror (M2) constitute an f/15.2 Aplanatic Gregorian (AG) system, which is shown in Fig. 1 together with a flat tertiary mirror M3 and a folding flat beamsplitter M4 delivering the final image onto a gravity stable Nasmyth platform
We have studied the benefit of using the linear atmospheric dispersion corrector (ADC) on ELTs equipped with adaptive optics (AO) systems
Summary
The Earth’s atmosphere has various chromatic effects on light passing through it. Most of the optical effects are refractive and we shall consider their relevance for high resolution imaging with ground-based optical telescopes, but there is a diffractive effect. Assuming a Hufnagel-Valley turbulence profile, this effect is of order 2% reduction in Strehl ratio for wavefront sensing at 0.5 microns and correction at 1 micron [6] It will become worse if the turbulence vertical profile contains strong, high-altitude layers. On a 50 m telescope with a 30 m outer scale, this effect would lead to a reduction of Strehl ratio of 4% at 0.9 microns if the wavefront sensing is carried out at 0.589 microns (Na laser wavelength) [6]. For a broad bandwidth (0.4-0.9 microns) the effect would typically lead to a reduction in Strehl ratio of 3% at a zenith angle of 30 degrees [6] It depends on the vertical distribution of turbulence, and does not depend on the telescope diameter
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