The Infrared Imaging Spectrograph (IRIS) for TMT: the ADC optical design

TL;DR

This paper details the optical design of the IRIS ADC for TMT, addressing challenges from increased imager size and pupil size, and balancing residual dispersion, distortion, and pupil displacement to optimize image quality.

Contribution

It introduces the final optical design of the IRIS ADC, considering new constraints and trade-offs to achieve minimal residual dispersion and distortion.

Findings

Residual dispersion less than ~1 mas achieved

Design addresses distortions causing image blur

Pupil displacement managed to reduce wavefront error

Abstract

We present the current optical design for the IRIS Atmospheric Dispersion Corrector (ADC). The ADC is designed for residual dispersions less than ~1 mas across a given passband at elevations of 25 degrees. Since the last report, the area of the IRIS Imager has increased by a factor of four, and the pupil size has increased from 75 to 90mm, both of which contribute to challenges with the design. Several considerations have led to the current design: residual dispersion, amount of introduced distortion, glass transmission, glass availability, and pupil displacement. In particular it was found that there are significant distortions that appear (two different components) that can lead to image blur over long exposures. Also, pupil displacement increases the wavefront error at the imager focus. We discuss these considerations, discuss the compromises, and present the final design choice and expected performance.