ExoMOS is a proposed instrument for the 3.5m ESO New Technology Telescope (NTT) specifically designed to study the atmospheres of extrasolar planets via transmission spectroscopy.


Hundreds of extra solar planets have been found by the dip in brightness (transit) of their parent star as they pass in front of it. Detecting this dip itself is relatively easy -- if you know where and when to look -- and something that we do as an undergraduate project here in Oxford. The size of the dip tells us how big the planet is, but looking more closely and measuring the size of the dip at different wavelengths allows probe what the planet is made of! That, however, is not very easy. It means measuring changes in the brightness of the star of around 1 part in 10000. This is possible with existing instruments, but only for a few objects and with great care. ExoMOS is a proposed instrument specifically designed for making these kinds of sensitive observations, and should let us measure the properties of hundreds of Exoplanets.

Instrument design

ExoMOS will be a multi-object spectrograph, capable of measuring up-to 6 objects simultaneously. For each one, it will take a low resolution spectrum from 0.4 to 2.4 microns. Measuring multiple objects allows us to detect and correct for effects in our own atmosphere, and hence improve the accuracy of the transit measurement. As exoplanet host stars are bright, suitable comparison stars are relatively rare. ExoMOS has a very wide field (20-arcminute ; 2/3 of the full moon) to give us the best chance of finding comparison stars. ExoMOS is also designed to have as few moving components as possible, making sure the instrument stays as constant as possible during the long (5 hour+) observations needed to cover the transits.

Each of the six pick-off arms -- based on technology developed for the 'MOONS' instrument -- will relay a large (20x45-arcsecond) sub-field into the instrument. The arms can be moved to select any 6 objects over the 20 arcminute field. This allows ExoMOS to have a very wide slit, of up-to 20-arcseconds, as this has been shown to be the best way to get very accurate spectrophotometry of exoplanet transits. The slit can be stopped down to a more conventional ~1-arcsecond width for other observations. The ExoMOS spectrograph has three cameras, fed by dichroics, to cover the entire 0.4 to 2.4 micron wavelength range in a single shot.

The whole instrument is contained in a cryostat to keep it cold and stable, which is in turn mounted vertically on a rotating bearing held off the side of the NTT telescope. Keeping the instrument vertical removes the risk of any flexure as the telescope tracks the target across the sky.


ExoMOS is begin developed by a European consortium led by David Sing at Exeter (UK). The main instrumentation institutes in the consortium are Oxford, UKATC, MPIA-Heidelberg, ETH-Zurich, Amsterdam, and Vienna.

Current Status

ExoMOS is currently being proposed to ESO for use on the NTT from 2019 onwards. A decision on this should be made by the middle of 2015. If successful, and dependent on securing funding, we hope Oxford will build the spectrograph module for ExoMOS.