A new broadband atmospheric dispersion corrector for HROS-TMT

TL;DR

The paper presents a broadband Rotational Atmospheric Dispersion Corrector (RADC) for the HROS-MOS on the TMT, designed to correct dispersion from $0.31$ to $1.0\,\mu$m with high transmission and minimal beam deviation. It introduces a two-Amici-prism ADC using Nikon 7054 and CaF$_2$, optimized to maintain light within a single fiber diameter while achieving robust correction across the band; dispersion modeling is enhanced by implementing the Filippenko 1982 atmospheric refraction model in Zemax via a DLL. Performance analyses show residual dispersion below ~250 mas and beam deviations reduced to meet 1-fiber requirements, with throughput significantly improved after correction, even at large zenith angles. A comprehensive tolerance study indicates the design remains robust against manufacturing and alignment errors, and micro-lenses are used to enable a ~f/3 fiber coupling, making the solution practical for MOS operations on HROS. The work also demonstrates the value of integrating an accurate blue-end atmospheric model into optical design to achieve reliable broadband correction for high-precision, fiber-fed spectroscopy.

Abstract

Atmospheric dispersion causes light from celestial objects with different wavelengths to refract at varying angles as it passes through Earth's atmosphere. This effect results in an elongated image at the focal plane of a telescope and diminishes fiber coupling efficiency into spectrographs. We propose an optical design that incorporates a Rotational Atmospheric Dispersion Corrector (RADC) to address the broadband dispersion encountered in the multi-object mode of the High-Resolution Optical Spectrograph (HROS) on the Thirty Meter Telescope (TMT). The RADC corrects the dispersion across the entire wavelength range (0.31-1 micron), using Amici prisms optimized for over 90% transmission efficiency and minimal angular deviation of the beam from the optical axis after dispersion correction. For enhanced accuracy, particularly in the blue region, we have, for the first time, implemented the Filippenko (1982) model in Zemax via a custom Dynamic-Link Library (DLL) file.

Figures (14)

• Figure 1: (a) The TMT optical layout, and (b) The spot diagrams for a reference wavelength of 0.45$~\mu\text{m}$. The top panel shows the spot diagram for the on-axis position, while the bottom panel displays the spot diagram for the off-axis position.
• Figure 2: Optical layout of the atmospheric dispersion corrector (ADC), where the Amici prisms are counter-rotated by 90 degrees at Zenith 0$^\circ$, along with the fore optics. P1, P2, P3, and P4 prisms belong to both Amici prisms. Here, $S$ denotes a surface, and S1 and S2 represent surface 1 and surface 2 of the prisms, respectively. In Amici 1, S1 and S2 correspond to P1, where S2 of P1 is S1 of P2, and S2 of P2 is the last surface of Amici 1. Similarly, in Amici 2, S2 of P3 is S1 of P4, and S2 of P4 is the last surface of Amici 2. The first inset shows the ADC at a Zenith angle of 60$^\circ$, incorporating flint glass Nikon-7054 and crown glass Ohara-CaF$_2$. The second inset displays the micro lens for the red and blue channels, which converts the f/15 beam to f/3.
• Figure 3: Transmission of materials used in designing the ADC (Nikon-7054 and CaF$_2$), as well as the collimator and camera (S-FPL51Y), for a thickness of 10 mm.
• Figure 4: The image quality on the TMT focal plane. The circle represents a 1 fiber, corresponding to 2180 microns in linear scale at an f/15. (a) Spot diagrams of on-axis objects at different zenith angles without dispersion correction, where aberrations are minimal. (b) Spot diagrams of off-axis objects at different zenith angles without dispersion correction, where astigmatism predominantly affects image quality.
• Figure 5: The image quality on the TMT focal plane. The circle represents a 1 fiber, corresponding to 2180 $~\mu\text{m}$ in linear scale at an f/15. (a) Spot diagrams of on-axis objects at different zenith angles after the dispersion correction (b) Spot diagrams of off-axis objects at different zenith angles after the dispersion correction