# Inverse Chop Addition: Thermal IR background subtraction without Nodding

## TL;DR

This paper introduces 'inverse chop addition', a novel thermal IR background subtraction technique that replaces classical nodding, improving observational efficiency for large telescopes without sacrificing background reduction quality.

## Contribution

The paper proposes and tests a new background subtraction method using symmetries of thermal background, suitable for extremely large telescopes where classical methods are infeasible.

## Key findings

- Achieves comparable background reduction to classical methods
- Increases observational efficiency during IR observations
- Validated through VLT/VISIR technical observations

## Abstract

Due to the large size and mass of the secondary mirror on next generation extremely large telescopes it will not be possible to provide classical chopping and nodding as is used during mid-IR observations today. As a solution to this we propose an alternative approach to thermal background reduction called `inverse chop addition`. Here we use the symmetries of the thermal background to replace nodding, which allows us to get nearly identical background reductions while only using a special chopping pattern. The performance of this method was tested during technical time observations on VLT/VISIR. With this method, a higher observational efficiency can be obtained than with `classical chopping and nodding`, while achieving equally good reduction results. These results suggest that `inverse-chop addition` could be a good alternative for classical chopping and nodding on both current and next generation ground-based facilities.