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 2015, a receiver for the 4 mm band was installed, and the 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​
8.1–9.0 GHz​ 80 K​ HEMT amplifier​ Dual​
18–26 GHz​ 30 K​ HEMT amplifier​ Dual​
26–36 GHz​ 50 K​ HEMT amplifier​ Single​
36–50 GHz​ 50 K​ HEMT amplifier​ Dual​
67–87 GHz ("4 mm")​ ​50–60 K​ ​HEMT amplifier​ Dual​
85–116 GHz​ ("3 mm") 45-60 K​ SIS mixer​ Dual​​

Note: 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 GHzbut  there is only sufficient spectrometer capacity to observe single sideband 4 GHz bandwidth, or dual sideband 2.5 GHz if both linear polarisation products are to be recorded. The IF bandwidth for the receivers for < 50 GHz is also 4 GHz ​but only one sideband can here be observed at a time (single-sideband)

Example spectra from the early commissioning observations with the 3 mm and 4 mm receivers can be found below. Weak broad extragalactic lines can be detected down to a line strength of 2 mK or possibly better with the 3 mm recevier, and down to 4 mK or most likely better with the 4 mm receiver (TA*, after baseline subtraction). More information can be found in Belitsky et al. 2015, Astronomy and Astrophysics 580, A29.

Note that the atmospheric transmission decreases towards the low frequency end of the 4 mm band due to several broad atmospheric molecular oxygen lines around 60 GHz.

Science verification observations with 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 three Fast Fourier Transform spectrometers, HRF, SPE and OSA. The spectrometers have the following characteristics:
Spectrometer Total bandwidth​ Resolution ​
HRF*​ 2 x 0.2, ..., 2 x 50 MHz​ ​24, ..., 6000 Hz​
SPE​** 2 x 100, 1 x 1000 MHz 12, 61 kHz​
OSA*** 2 x 4 GHz***, ​2 x 2.5 GHz, 2 x 625 MHz, 2 x 156 MHz 76, 76, 19, 4.8 kHz


Please notify the telescope scientist as soon as possible after proposal submission if the HRF spectrometer is requested; there may be some lead time before it is available. ​

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

***The four modules of the spectrometer can either be combined to cover 4 GHz in both polarisations in the same sideband, or to cover the upper and lower sidebands in one polarisation. An additional mode, 4 x 2.5 GHz simultaneously covering all sidebands and polarisations, is available. Below 50 GHz, the 2 x 4 GHz mode can be used to cover both polarizations of the same sideband for the two dual polarization receivers (18–26 GHz and 36–50 GHz). ​


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

Spectral line data are stored and archived in the FITS format, and the data can be shipped via remote copy such as scp or cloud file sharing of compressed archives (preferred).

VLBI back-end

The VLBI back-end consists of digital base band converters (DBBC). Data is recorded on the Flexbuff system or transported directly to the correlator through the internet (e-VLBI). We have 2 parallel 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.


Page manager Published: Wed 17 Jun 2020.