Quantification of the expected residual dispersion of the MICADO Near-IR imaging instrument

TL;DR

This paper analyzes atmospheric dispersion effects and residuals in the MICADO near-infrared imager for the ELT, demonstrating that current ADC design can achieve the target astrometric accuracy of 50 microarcseconds.

Contribution

It provides a detailed assessment of atmospheric dispersion models, error propagation, and residual dispersion contributions specific to MICADO's design and operational conditions.

Findings

Differential dispersion between models is less than 10 μas in H-band.

Residual dispersion is dominated by optomechanical design, totaling 0.4 mas.

ADC design is sufficient to meet 50 μas astrometric accuracy goal.

Abstract

MICADO, a near-infrared imager for the Extremely Large Telescope, is being designed to deliver diffraction limited imaging and 50 micro arcsecond (${\mu}$as) astrometric accuracy. MICADO employs an atmospheric dispersion corrector (ADC) to keep the chromatic elongation of the point spread function (PSF) under control. We must understand the dispersion and residuals after correction to reach the optimum performance. Therefore, we identified several sources of chromatic dispersion that need to be considered for the MICADO ADC. First, we compared common models of atmospheric dispersion to investigate whether these models remain suitable for MICADO. We showed that the differential dispersion between common atmospheric models and integration over the full atmosphere is less than 10 ${\mu}$as for most observations in H-band. We then performed an error propagation analysis to understand the uncertainty in the atmospheric dispersion as a function of atmospheric conditions. In addition, we investigated the impact of photometric color on the astrometric performance. While the differential refraction between stars within the same field of view can be significant, the inclusion of an ADC rendered this effect negligible. For MICADO specifically, we found that the current optomechanical design dominates the residual dispersion budget of 0.4 milli arcseconds (mas), with a contribution of 0.31 mas due to the positioning accuracy of the prisms and up to 0.15 mas due to a mismatch between the dispersive properties of the glass and the atmosphere. We found no showstoppers in the design of the MICADO ADC for achieving 50 ${\mu}$as relative astrometric accuracy.