Performance
Sensitivity
At close-to-diffraction limited scales, HARMONI will capitalise on the D4 sensitivity gains of the ELT, providing unprecedented gains in sensitivity and spatial resolution, which when put together, will transform the landscape in observational visible and near-infrared astronomy. Even in seeing limited conditions (or when AO cannot be used to provide high Strehl ratios), HARMONI provides impressive gains with respect to the current generation of VLT instruments, e.g. a gain of ~25 in speed relative to MUSE at the ESO-VLT. Its high throughput (35% average) ensures that scientists can make the most of the precious photons collected by the enormous collecting area of the ELT.
The table below gives the limiting magnitude for 5σ, per pixel, 5 hours, LTAO, point source. Assumes a 2 × 2 spaxel, extraction aperture. SCAO 0.5 mag better, No AO 2 mag worse.
It can be seen that at near-infrared wavelengths, the best sensitivity for point sources is at the 10 and 20 mas spaxel scales. Starting with 30 × 60 mas spaxels, as one moves to finer spaxel scales, the sky background contribution gets smaller, but, at least initially, the source flux does not decrease. However, detector noise starts to play a bigger role. Once the telescope PSF is resolved (4 mas spaxels), the source flux per spaxel drops too, and sensitivity is lowest. However, for extended sources, the largest spaxels give the highest sensitivity, as would be expected. R band point source sensitivity also shows the same behaviour as extended sources, because the sky background is not so important in the R band. Although 10 and 20 mas spaxel scales have very similar sensitivities, two scales are provided so as to best match the sampling and the FoV to the properties of the source being studied.