1. Creating an account
To use SALSA you need to log in. Click Login in the top navigation bar and follow the instructions to authenticate. SALSA supports several login providers — choose whichever you already have an account with. Once logged in your name will appear in the top navigation bar. Clicking it reveals account settings and a log out link.
2. Booking a telescope
Go to Bookings in the navigation bar. You will see a weekly calendar showing time slots for a selected telescope.
Slot colours indicate availability:
- Green — free, click to book
- Blue — your existing booking, click to cancel
- Grey — booked by another user
- Faded — past slot
- Yellow — telescope in maintenance, not available
Click a green slot to book it. Bookings are one hour long. You can have up to a limited number of upcoming bookings at a time. To cancel a booking, click your blue slot and confirm the cancellation.
The page shows the current UTC time so you can plan accordingly. All times in the calendar are in UTC.
Your bookings are shown at the top of the bookings page, under Upcoming bookings. Each booking shows an Observe button — it is grayed out until your slot starts, then becomes active. Click it to go to the telescope control interface.
3. Making an observation
The observation page is only accessible during your booked time slot. It shows the current telescope status, pointing controls, receiver settings, and a live spectrum display.
A countdown timer at the top of the page shows how much time remains in your booking. You will receive a warning five minutes before your slot ends.
If the wind speed at the observatory exceeds the safe limit, a strong wind warning will appear at the top of the page. You are advised to stow the telescope and return when conditions improve. You may still observe, but do so at your own risk.
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.
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.
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.
- 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, 10.0, 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. Raw mode records without switching.
- RFI filter — A median filter to reduce radio frequency interference. Enable if you see narrow spikes in your spectrum.
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.
Click End to end the integration. The observation is automatically saved to your observation archive.
For HI observations, you can toggle the horizontal axis between frequency (MHz) and VLSR (km/s) using the button below the chart.
4. Observation archive
Go to Observation archive in the navigation bar to view all your saved observations. Each entry shows the start time, telescope, coordinate system, target position, and integration time. Click an observation to load its spectrum.
You can zoom into a region of the spectrum by drawing a box with the mouse. Double-click to reset the zoom.
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
Select a frequency range on the spectrum that contains only noise (no signal), then choose a polynomial order (linear or quadratic) and click Fit baseline. The fitted baseline is subtracted from the 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.
