Technical description (20 m telescope)

​The Onsala 20 m diameter millimetre wave telescope is of Schmidt-Cassegrain type. It is protected by a radome, and is equipped with receivers for frequencies up to 116 GHz. It is used for single-dish observations of astronomical objects, and for astronomical and geodetic VLBI.

In 2014, the telescope was upgraded with a new radome, a new 3 mm receiver, and a new spectrometer. In 2014, a receiver for the 4 mm band was installed, and the new spectrometer was extended to cover 2 x 4 GHz.


Latitude: 57°23' 45.0046" N
Longitude: 11° 55' 34.8719" E
Elevation: 22.758 meters


The telescope is equipped with the following receivers:
Frequency range​ Receiver temperature​ Receiver type​ Polarization​
2.2–2.4 GHz​ 60 K HEMT amplifier​ Single​
8.2–8.4 GHz​ 80 K​ HEMT amplifier​ Dual​
18.0–26.0 GHz​ 30 K​ HEMT amplifier​ Dual​
26.0–36.0 GHz​ 50 K​ HEMT amplifier​ Single​
36.0–49.8 GHz​ 50 K​ HEMT amplifier​ Dual​
67–87 GHz ("4 mm")​ ​50–60 K​ ​HEMT amplifier​ Dual​
85–116 GHz​ ("3 mm") 50-60 K​ SIS mixer​ Dual​​

Note: The 3 mm receiver was installed in March 2014, and the 4 mm receiver in October 2015. Both the 3 mm and the 4 mm receivers are dual polarisation sideband separating with USB and LSB having centres presently 12 GHz apart. Each sideband has IF bandwidth 4 GHz. Example spectra from the early commissioning observations can be found below. More information can be found in Belitsky et al. 2015, Astronomy and Astrophysics 580, A29.

Science verification observations witht the 4 mm receiver were performed during the first months of 2016. The following projects were observed:

W. Vlemmings et al.: A new circumstellar water maser at 67.8 GHz
E. De Beck et al.: The molecular content of supergiant outflows: a legacy of the history and future of massive stars
T. Danilovich et al.: The sulphur chemistry in circumstellar envelopes of oxygen-rich AGB stars
W. Geppert et al.: Search for water clusters in massive star-forming regions
P. Bjerkeli et al.: An unbiased 4mm spectral survey of the Class 0 protostar NGC 1333 IRAS 2A
W. Geppert et al.: Mapping of formaldehyde in star-forming regions
E. Wirström et al.: Deuteration in Barnard 5
J. Black et al.: Atmospheric emission lines for science verification at 4 mm wavelength
M. Hajigholi et al.: DCO+/DCN in the outer and inner galaxy

Spectral line back-ends and observing modes

The back-ends for spectral line observations consist of one hybrid digital autocorrelation spectrometers (ACS) and two Fast Fourier Transform spectrometers (FFTS). The spectrometers have the following characteristics:
​Spectrometer Total bandwidth​ Resolution ​
High resolution ACS*​ 0.05, ..., 6.4, 12.8 MHz​ ​0.03, ..., 4, 8 kHz​
FFTS​** 2 x 100, 1 x 1000 MHz 12, 61 kHz​
New FFTS*** 2 x 4 GHz***, ​2 x 2.5 GHz, 2 x 625 MHz, 2 x 156 MHz 76, 76, 19, 4.8 kHz


*The high resolution ACS is not always available. Please contact the telescope scientist for more information.

**In dual polarization and low resolution mode, the bandwidth is 500 MHz per polarization.

***The two parts of the spectrometer can be connected either to two polarisations of the same sideband, or to the upper and lower sidebands. An additional new mode, 4 x 2.5 GHz simultaneously covering all sidebands and polarisations, may become available during the next observing season (winter 2016/2017). Below 50 GHz, the 2 x 4 GHz mode is not available at present. An example spectrum from the 2 x 4 GHz mode can be found below.


Observing modes: Position switching, frequency switching, and (above 67 GHz) beam switching (11 arcmin).

Spectral line data are stored and archived in the FITS format on compact disks.

VLBI back-end

The VLBI back-end consists of digital base band converters (DBBC). Data is recorded on the Mark 5 system or transported directly to the correlator through the internet (e-VLBI). We have 2 parallell VLBI systems and can observe with the 20 m and 25 m telescope in VLBI-mode simultaneously.

Example spectrum from the 2 x 4 GHz FFT spectrometer

The new FFTS has a maximum bandwidth of 2 x 4 GHz. In the spectrum below (observed with the 3 mm receiver), the two polarizations have been combined into one 4 GHz spectrum.

Observations of the galaxy M82 with the 3 mm receiver and the new FFTS with a bandwidth of 2 x 4 GHz. In the spectrum, the two polarizations were combined into one 4 GHz wide spectrum. No baseline was subtracted. A pdf-file with better resolution: m82spec160129.pdf.

Example spectra from the 3 mm receiver

A new, dual polarization, receiver for the 85–116 GHz ("3 mm") range was installed in March 2014. Example spectra from the early commissioning observatations can be found below (and see also the spectrum above). No baseline has been subtracted (except for a constant in the cases of M82 and R Leo).

Early commissioning observations of DR21(OH) at 86.2 GHz, position switching, single polarization. The unmarked line at edge is a ghost image of a line just outside the edge (FFTS artefact). A pdf-file with better resolution: dr21oh140318.pdf .

Early commissioning observations of CS in M82 at 98 GHz, dual beam switching. Two polarizations are shown. Note the flat baseline and 1 mK noise per polarization (at 10 km/s resolution) after 8.8 h observation (on source). A pdf-file with better resolution: m82cs140321.pdf.

Early commissioning observations of R Leo at 86.2 GHz, position switching. Two polarizations are shown. The intensity scale is preliminary.

Example spectra from the 4 mm receiver

A new, dual polarization, receiver for the 67–87 GHz ("4 mm") range was installed in October 2015. Example spectra from the early commissioning observatations can be found below.

Spectrum showing observations of HDO (and some other molecules as well) in Orion KL. A pdf-file with better resolution: orionspec151021.pdf.

Spectrum showing observations of two spectral lines in the galaxy IC 342. A pdf-file with better resolution: ic342spec151020.pdf.

Spectrum showing several interesting spectral lines in DR21(OH). Note that the noise increases towards the lower end of the spectrum, due to several broad atmospheric molecular oxygen lines around 60 GHz. A pdf-file with better resolution: dr21oh_67_71.pdf.


Published: Fri 30 Aug 2013. Modified: Tue 12 Apr 2016