3.1 Pointing the telescope

Choose a coordinate system from the dropdown, then enter the target coordinates and click Track. The telescope will slew to the target position. The Calculated and Current azimuth and elevation angles are displayed below the input coordinates. Once you select a target position, the Calculated row will update to show the calculated horizontal (az/el) coordinates needed to reach your desired position. If this is too low, you cannot track the desired position. A negative elevation angle means that your target is below the horizon. Some targets are only visible during some time ranges. The calculated coordinates update once per second, due to the Earth movement through space.

Available coordinate systems:

• Galactic — Galactic longitude (l) and latitude (b) in degrees. Recommended for HI observations of the Milky Way.
• Equatorial (J2000) — Right Ascension and Declination in degrees, in the J2000 (ICRS) frame.
• Horizontal — Azimuth and elevation in degrees.
• Sun — Automatically tracks the Sun. The SALSA software automatically calculates the current position of the Sun.
• GNSS satellite — Select a visible satellite from the dropdown. The SALSA software updates the satellite orbits automatically once per day.
• Stow — Parks the telescope in its resting position.
• Service — Moves the telescope to a fixed maintenance position. Only shown on telescopes where one is configured.

In the Advanced section you can also apply small azimuth and elevation offsets (in degrees) to fine-tune the pointing, for example when measuring the beam pattern. These will be added to the calculation after your input coordinates have been converted to a horizontal position.

The telescope will not move below its minimum elevation limit. If your target is too low, an error will be shown.

The same limits apply while you observe, not just when you start. A target that is high enough now will eventually set, and once it drops below the minimum elevation the telescope can no longer follow it. If an integration is running when that happens, it is stopped and the data collected so far is saved, so you keep what was measured on target rather than a spectrum polluted by the telescope drifting away. For a long integration, check that your target stays up for the whole run.

A second, less obvious interruption comes from azimuth. The telescope moves between 0° and 360° in azimuth and cannot pass through that boundary — so a target whose azimuth crosses due north (0°/360°) as it moves across the sky forces the telescope to unwind almost a full circle the other way before it can carry on. This affects targets that pass north of the zenith, which at Onsala means anything far enough north to stay above the horizon all day. The result is a long slew, during which the telescope is off target; a running integration is stopped and saved just as it is for a setting target. If you are planning a long run on a northern target, it is worth checking whether it will cross due north partway through.

3.2 Receiver settings

The receiver settings control the frequency, bandwidth, and signal processing. The default values are optimised for 21 cm hydrogen line observations and are a good starting point for most experiments. If you select "The sun" or "GNSS satellites" as targets, the receiver settings will automatically update to sensible default values. After selecting the target, you may also change the settings if you want.

• Integration time — Choose how the integration ends. Interactive end (default) lets you click Begin and End manually. Fixed auto-stops after a set number of seconds; useful when you want to walk away from the telescope while it records, or when you want a precise duration for comparable observations.
• Center frequency — The frequency tuned to by the receiver, in MHz. The hydrogen line rest frequency is 1420.4 MHz.
• Bandwidth — The total frequency range recorded: 1.0, 2.5, 5.0, 12.5, or 25.0 MHz. Wider bandwidth covers more velocity range; narrower gives better resolution for a given number of channels.
• Channels — Number of spectral channels: 64 to 8192. More channels give finer frequency resolution. The resulting resolution in MHz is shown automatically.
• Observation mode — Frequency-switched mode alternates between the target frequency and a reference frequency to subtract background and standing waves; the spectrum is calibrated against the system temperature and the y-axis is in antenna temperature (K). Use this mode for the HI line. Raw mode records without switching, so no calibration is applied; the y-axis is the received power as a fraction of the digitiser's full-scale range (dimensionless), and only relative differences between observations are meaningful.
• RFI filter — A sliding-window robust statistical filter that removes narrow radio-frequency interference spikes from each cycle's spectrum. The site has some local RFI (most noticeably on vale), so the filter is enabled by default and should normally be left on. It builds a baseline tracking the bandpass shape and any broad real signal (such as the HI line), then flags channels deviating more than a few MAD-σ above that baseline and replaces them with the baseline value — narrow spikes go away, broad emission is preserved. Disable only if you specifically want to inspect raw, unfiltered data.

3.3 Integrating

Once the telescope is on target and the receiver is configured, click Begin to begin integrating. The live spectrum updates continuously as data accumulates. The elapsed integration time and average power level are shown below the spectrum. The longer you integrate, the weaker the noise becomes since the random fluctuations cancel each other. For HI observations, about 30 seconds is usually enough to get reasonable results, but you can try different values to see what gives good results for your specific target.

How the integration ends depends on the Integration time setting in the advanced receiver settings. In Interactive end mode (default), click End when you're satisfied with the spectrum. In Fixed mode, the integration stops automatically after the configured duration; the timer shows elapsed / total seconds while it's running. Either way, the observation is saved to your observation archive when it stops.

Two things can end an integration early regardless of that setting: the antenna losing track of the target (for example a target low on the horizon forcing a large cable-unwrap slew), or your booking slot ending. In both cases the data collected up to that point is saved to your observation archive — an integration cut short is simply shorter, not lost. After a slew you can start a fresh integration once the telescope is back on target.

For galactic-coordinate observations, you can toggle the horizontal axis between frequency (MHz) and velocity (km/s) using the button below the chart. The velocity axis is in the Local Standard of Rest frame — SALSA applies the LSR correction automatically using the 21 cm hydrogen-line rest frequency, so values are directly comparable with published HI data. See HI experiment §1.5 for the details.

4.1 Exporting data

Three export formats are available:

• PNG — An image of the corrected spectrum as displayed.
• CSV — Frequency and amplitude columns for use in a spreadsheet or analysis tool such as LibreOffice Calc or Matlab.
• FITS — The raw uncorrected spectrum in the standard astronomical FITS format, suitable for use with tools such as Astropy.

4.2 Spectral analysis tools

The archive page includes built-in tools for basic spectral analysis. Note that analysis results are not saved — take notes or export before leaving the page.

Baseline correction

Pick one or more frequency ranges that contain only noise (no signal), choose a polynomial order (linear or quadratic), then click Fit. A dashed orange curve shows the fitted polynomial. If it looks right, click Subtract to apply it; the spectrum flattens and the dashed curve disappears. If it doesn't, adjust the picks or the order and Fit again. Clear wipes the picked ranges, the pending fit, and undoes any previously applied subtraction — restoring the original spectrum.

Gaussian fitting

After baseline correction, click on a peak in the spectrum to select it, then click Fit Gaussian. The fitted peak parameters (centre frequency, amplitude, width) are displayed.

Getting the fitter to converge on the right peaks can be tricky. For best results, click as close as possible to the very centre of each peak — both horizontally and vertically — during the peak-picking phase. See the example below.

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