Swedish-ESO PI receiver for APEX

SEPIA Instrument on APEX

It is easy to find water on the surface of the Earth, but it is extremely difficult to discover it in the rest of the Universe. The main problem is that the Earth's atmosphere absorbs a lot of the very faint traces of water, which may have traveled for thousands of light-years to get here. But help is at hand.

Joining an established suite of instruments attached to the ESO-operated Atacama Pathfinder Experiment (APEX) telescope, the Swedish-ESO PI receiver for APEX, or SEPIA, is now spearheading the search for water in the Universe.

"SEPIA will be able to study the emission from water in the Universe. Water is not just a molecule which is well known on Earth, but also an important tracer of star-forming gas." says Carlos De Breuck, instrument scientist behind this state-of-the-art device.

How does SEPIA detect water? "SEPIA is a spectrograph, meaning that it decomposes the light according to wavelength," says De Breuck. That means light is split into its different colours (or wavelengths), in the same way that rain droplets disperse the light to form a rainbow. This allows astronomers to analyse it, and detect the faint traces of water in the light collected by APEX. "Thanks to the high and dry location of APEX on the Chajnantor Plateau, it is now possible to routinely perform these observations with SEPIA".

In addition to water, SEPIA will also trace a wide variety of other molecular transitions that have never been studied before. "Some of the brightest molecules that SEPIA can observe are the HCN, HNC and HCO+ molecules in the 2-1 transition. These molecules have been observed with other telescopes (e.g. the IRAM 30m) in the 1-0 transition, but never in this 2-1 transition. With this information, we can learn more about how the dense gas forms baby stars".

These advances are due to the detectors of SEPIA, called Band 5 receivers, which are based on technology developed for the Atacama Large Millimetre/submillimetre Array (ALMA). SEPIA’s detectors are cooled down to extremely low temperatures — barely 4 degrees above absolute zero — to work effectively.

The detectors inside SEPIA make up one of six prototype receivers that have been built for ALMA, and were partially funded by an FP6 project from the European Union. But ALMA will need to have all 66 receivers ready before using them for science observations.

"Now we can do the preparatory work with APEX already two years earlier. As such, APEX will find the best targets to study in greater detail with ALMA in a few years. So it will not only benefit APEX itself, but also make the use of these band 5 receivers at ALMA more efficiently, as APEX will have discovered the best targets to follow-up. That said, the APEX results that are coming out from the first SEPIA observations are very nice science results in their own right," says Carlos De Breuck.

Sepia is also a colour with a close connection to water. The reddish-brown shade, characteristic of pigment collected from the cuttlefish of genus Sepia (found in the waters of both Sweden and Chile), has been used in ink since ancient times and sepia toning is a well-known way of giving photographic prints a longer life.

The first of the new APEX facility receivers was successfully installed at the telescope in August 2018 adding Band 9 (at 660 GHz) to the already operating Band 5 (at 180 GHz). The new Band 9 facility receiver for SEPIA dubbed SEPIA660 records both sidebands simultaneously, with very good sideband rejection at the level of >15 dB at all frequencies (>20 dB in most cases). This new feature duplicates the frequency coverage per tuning, adds flexibility to the design of spectral setups covering multiple lines, and reduces the noise level compared to the old double sideband receiver that was used from 2016 to 2017. An additional improvement is the extension of the frequency range, which now allows to reach down to on-sky frequencies of 581 GHz, and up to 726 GHz. This range extends well beyond the previous one (600–722 GHz) and also makes this joint development by the Group of Advanced Receiver Development (GARD, Gothenburg, Sweden) from the Onsala Space Observatory (OSO, Sweden), the Netherlands Research School for Astronomy (NOVA, The Netherlands) and ESO interesting as a potential future upgrade to the ALMA Observatory.


The authoritative technical specifications as offered for astronomical observations are available from the Science Operation page.

Location: Chajnantor Plateau
Telescope: APEX
Focus: Nasmyth
Type: Single pixel heterodyne spectrograph
Wavelength coverage:

1420-1900 μm (Band 5)

406-518 μm (Band 9)

Spatial resolution: n/a, 35 arcsec beam
Spectral resolution: 0.065 km/s at 177 GHz
First light date: February 2015
Images taken with the instrument: Link
Images of the instrument: Link
Press Releases with the instrument: Link
Data papers:

(Includes ESO and non-ESO papers)

ESO data citation policy

Science goals: Water molecules, star-forming gas.


Max Planck Institute for Radio Astronomy (MPIfR), Germany

Onsala Space Observatory (OSO), Sweden