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A bit of science July 23, 2008

Posted by Sarah in me.

My paper came out on astro-ph today, pat on back to self.

Many of my colleagues’ eyes glaze over when they even read the title – a common reaction with instrumentation papers. But although it needs a lot more quantitative effort to make it to bring it up to refereed journal level, the work is really quite important for anyone interested in observational astronomy at infrared wavelengths from the ground.

The refractive index of a material is an indicator of how fast light travels through it. When light passes from one medium to another, for example from air to water, the light essentially changes speed. If the light falls onto the water at an angle, the angle will be changed as it passes into the water. This ‘breaking’ of the light is called refraction, and it explains for example why rocks at the bottom of a pool when you reach in to grab one, are in a slightly different place to where they appeared from above the water’s surface.

This is precisely what happens in the Earth’s atmosphere when light from astronomical objects enters it. In its simplest form,  atmospheric refractivity causes astronomical objects to appear closer to the zenith as seen from Earth, as if we were sitting at the bottom of a rock pool looking up at something outside the pool. In addition, variations in temperature,  pressure and composition of the air in the atmsophere, taking place on all scales from the very small (cm) to very large (km) as well as with time, cause tiny fluctuations in the air’s refractive index. So the breaking effect becomes a very complex phenomenon, varying according to the position in the sky of the object and with time, requiring extra optics in the telescope to keep the image stable and centred on the science camera.

At visible wavelengths the refractivity of air varies very strongly with wavelength; much less so in the mid-infrared (> 3 micron). For this reason, refraction effects are usually ignored in the mid-infrared. But in my paper, I talk about this phenomenon at mid-IR wavelengths in the context of the a new generation of ground-based telescopes, the extremely large telescopes (or ELTs, see for example the European ELT or the Thirty Meter Telescope). Because these telescopes will be able to provide far superior image quality to any of today’s telescopes, they will also be much more sensitive to very small distortion effects that may today be considered to be negligible.

I’m studying atmospheric refractivity in the mid-infrared as part of my work on a mid-infrared instrument for the European ELT, for which a design study is currently under way. With the amount of resources invested in this and other similar projects, and the huge scientific potential of such an instrument, we can’t afford to make assumptions about what effects are important and which aren’t based on today’s experiences.



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