This page contains detailed technical specifications for the SALSA Onsala telescopes. There are three telescopes: Torre, Vale, Brage (in order from left to right as displayed on the webcam). In principle they have identical hardware, although there are differences in cable lengths and connections. The diagrams to the right show the signal path from the sky to a spectrum (left column, top to bottom) and the control path from the webserver to the rotor (right column, bottom to top). The telescopes are located at Onsala Space Observatory, 57°24'N, 11°55'E, approximately 5 m above sea level.
The dish is 2.3 m in diameter. The surface has holes of about 5 mm in diameter, letting air through to reduce wind resistance. The holes do limit the frequency range (too short waves pass through rather than reflect) to less than about 6 GHz, but this is not a problem for the longer waves observed by SALSA. The horn is placed at a focal distance optimised for the 21 cm hydrogen line. Observing significantly off 1420 MHz degrades sensitivity more than the holes do. However, the frequency range is also restricted by filters, see below.
The incoming electromagnetic waves are reflected in the parabolic dish and summed coherently (in-phase for 21 cm wavelength) at the feed horn. Inside the horn is a small (5 cm) metal probe which is the actual antenna, sensitive to a single linear polarisation. The probe has an N-type connector on the outside of the horn, and a short (30 cm) CNT-400 cable connects it to a small box mounted on the horn. This box contains a Low Noise Amplifier (LNA) with a Bias-T that feeds the LNA 12 V DC through the coax. The LNA is custom built by Tommy Henderson at AGO in the US.
After the LNA, the signal travels down the telescope through approximately 10 m of RG-400 cable to a junction box. Here a second Bias-T injects the 12 V supply onto the cable feeding the LNA above. After the junction box, the signal continues via approximately 30 m of CNT-400 coax into the building where the sampler is located. The sampler is a USRP N210 equipped with a DBSRX2 daughter board, which digitizes the waveform and sends samples via 1 Gbps Ethernet to the control computer. The DBSRX2 card covers 0.8–2.3 GHz, but due to strong interference above 1.8 GHz the LNAs include additional filters limiting the usable range to 1350–1600 MHz. The USRP supports up to 25 MHz instantaneous bandwidth; for standard HI observations the default 2.5 MHz is sufficient. Both bandwidth and channel resolution are selectable by the user. The DBSRX2 has a software-controlled gain range of 0–88 dB (73 dB RF gain plus 15 dB baseband gain); the ADC saturates at roughly −10 to 0 dBm at the SMA input depending on gain setting, so the total in-band power should be kept around −20 to −30 dBm for clean linear operation.
The telescope control and booking system is a web application written in Rust, running on a server at Onsala. The web framework is Axum on the Tokio async runtime, with HTMX for dynamic page updates and Tailwind CSS for styling. Observations and bookings are stored in a SQLite database, and authentication is via configurable OAuth2 providers or local accounts. The server communicates with each telescope's USRP sampler via the UHD driver to acquire spectra, and with the MD01 rotor controller to point and track targets. The webserver source code is openly available on GitHub.
The rotor and controller are manufactured by SPID Elektronik. The rotor model is BIG RAS/HR with 0.1 degrees resolution and 1.5 degrees / second rotation speed. The rotor is powered by a MD01 control unit which also reads the current rotor position. The MD01 accepts azimuth/elevation commands via the ROT2PROG protocol over a TCP connection. All coordinate conversions (galactic, equatorial, horizontal) are handled by the SALSA webserver before commanding the rotor.
Signal path ↓
Control path ↑