HARMONI at ELT: tolerance analysis and expected as-build imaging performance of the infrared spectrograph

TL;DR

This paper analyzes the tolerance and expected imaging performance of the HARMONI infrared spectrograph for the ELT, demonstrating that target image quality can be achieved within feasible tolerances through detailed error analysis and Monte Carlo simulations.

Contribution

It provides a comprehensive tolerance analysis and error budget for HARMONI's spectrograph, ensuring the desired performance is attainable with realistic manufacturing limits.

Findings

Wavefront error remains within limits across configurations

Monte Carlo analysis confirms feasibility of target image quality

Error correction strategies effectively mitigate technological limits

Abstract

HARMONI is the first light visible and near-IR integral field spectrograph for the ELT. It covers a large spectral range from 470nm to 2450nm with resolving powers from 3300 to 18000 and spatial sampling from 60mas to 4mas. It can operate in two Adaptive Optics modes - SCAO (including a High Contrast capability) and LTAO - or with NOAO. The project is preparing for Final Design Reviews. The integral field spectrograph is a key sub-system of HARMONI instrument, which forms the 2D spectral image and projects it onto the scientific detector. It has 40 operational modes with different platescales and gratings covering the band of 811-2450 nm with three resolution grades. In each of this configurations the as-built spectrograph wave-front error is strictly limited. We perform the sensitivity analysis for measurable and unknown errors and build the errors budget on this basis. Then we correct the values for the actual technological limits and perform a three-stage Monte-Carlo analysis combined with simulation of a few specific effect as the holographic grating wavefront error. Eventually, we show that it is possible to reach the target image quality in terms of the wavefront error and spectral resolution for the entire sub-system with practically feasible tolerances on design parameters.