Tropomi measures light travelling through the earth’s atmosphere, and compares it to sunlight. The difference tells scientists exactly how the atmosphere is made up. Light captured by Tropomi undergoes five important steps before scientists can get started.
Step 1: distorting mirrors
Light comes in through the telescope’s aperture. This tiny optical element keeps unwanted diffused light out, just like the lens hood does on a camera. Then, light is reflected through two meticulously distorted mirrors to a split in the instrument. These so-called distorting mirrors turn light into a sharp image.
Step 2: splitting up
A split in Tropomi sends light along two different routes across the instrument. The edges must be incredibly sharp, for even the tiniest irregularity means observations could be less pure. Light with wavelengths equalling 300-500 and 675-775 nanometres may enter the instrument. Light the wavelengths of which represent 270-320 and 2305-2385 nm is reflected to take a different route through Tropomi.
Step 3: immersed grating
Four different beams of light exist, each belongs to a specific wavelength area. Three of them collide with a conventional ‘grating’ unravelling light even more into several wavelengths. It is like a rainbow presenting itself whenever light falls onto a prism. The fourth beam of light (2305-2385 nm) also collides with a grating, yet the latter is immersed into a piece of silicon. With this ‘trick’ the infrared module was performed forty times smaller, while the longest wavelengths were still pulled tightly.
Step 4: taking pictures
A detector captures light in different wavelengths. It is like using a digital camera: with a CCD sensor. But Tropomi’s CCD is the fastest sensor in the world, taking only forty milliseconds to read out all pixels.
Step 5: pack and send off
Tropomi’s detector collects much more data than a satellite connected to earth. A smart system takes the amount of data back to 17.5 Gigabyte per orbit around the earth. These data are forwarded to two earth stations using the antenna of the Sentinel-5 precursor satellite. Data are processed one last time before scientists can use them.
Scientists have been studying the differences between light reflected by the earth’s atmosphere and that coming directly from the sun. So Tropomi should also be able to catch sunlight. To this purpose, a special side entrance has been built. Once every day the telescope briefly switches from terrestrial light to sunlight. The latter follows exactly the same path through Tropomi (all five steps). Hence, the differences between the reference light and terrestrial light are caused merely by the atmosphere, never by the instrument.