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Public Universe

Prepare for a night outside

Your observatory

Keep your equipment and observing sites together in this browser by default; an account is not needed.

Browser storage can be cleared by your browser or anyone using this device. Export a backup to keep your records. Saving here or signing in does not upload equipment or named sites. A separate private account backup requires your explicit preview and upload choice.

Your equipment

Use the specifications printed on your equipment. Unknown optional measurements can stay blank. Saving equipment does not predict what you will be able to see.

No equipment saved yet. Add a telescope, binoculars, eyepiece, optical accessory or camera sensor below.

    Add or edit equipment

    A Barlow factor is 1 or greater; a reducer factor is between 0.01 and 1. The actual factor can depend on spacing and the optical setup.

    Your observing sites

    Coordinates are rounded to two decimal places, about a kilometre. Choose Use on a saved site to make it the location used by sky calculations. Those calculations send approximate coordinates to our astronomy API and weather service as explained on the privacy page.

    The night planner can explicitly copy a saved site’s timezone, minimum altitude and horizon mask into its calculation form. Activating a site only selects its approximate location for other sky views. A saved horizon is user-entered data, not surveyed terrain or a guarantee of a clear view.

    You can locate a site from a what3words address as well as from latitude and longitude. A saved site may show an approximate what3words address for its rounded coordinates.

    No observing sites saved yet.

      Add or edit a site

      Optional. Enter three words such as ///filled.count.soap, or a what3words.com link, then choose Locate address. The three words are sent to what3words by our server, which is the only place the API key is used. We show the coordinates and nearest place for confirmation, then fill in latitude and longitude rounded to about a kilometre. A successful lookup is cached so the same address is not requested again, and it is not saved to an account. You can still type coordinates yourself.

      Leave blank for an unknown horizon. Enter 2–72 directions: north is 0°, east 90°, south 180°, west 270°. 360° is the same as 0°. Altitudes may range from −90° to 90°; a depressed horizon can be negative. Coarse masks interpolate linearly between your points, including across north, and can miss obstructions. The independent minimum-altitude preference remains a lower bound. Copy this saved site in the night planner to apply its horizon mask to a calculation.

      Use a timezone such as Europe/London, America/New_York, Australia/Sydney or UTC. Site names and coordinates are private records: include them in a backup only if you are comfortable storing that file.

      Compare your optical setup

      Choose equipment saved above. These geometric estimates help you compare magnification and field width; they do not predict brightness, detail or whether a target is observable. Changes here are temporary and stay in this browser.

      Optical calculation inputs

      Enter an angular size from a source or measurement of your choice. One degree is 60 arcminutes. No planet sizes or catalogue measurements are filled in automatically.

      Effective focal length
      Not available
      Magnification
      Unknown
      Exit pupil
      Unknown
      True field of view
      Unknown
      Central pixel angular width
      Not applicable to visual mode

      Enter an angular diameter and a known field of view to compare their sizes.

      Camera mode uses sensor dimensions and the exact arctangent expression for ideal rectilinear geometry. Central pixel angular width is the angle across one pixel centred on the optical axis, assuming the entered pixel size applies to both axes. It is not resolving power or an exposure recommendation, and no camera is controlled. Unknown pixel size stays unknown.

      Field-stop estimates use the paraxial approximation. Apparent-field estimates are less reliable when eyepiece distortion matters. Optical factors can vary with spacing; vignetting, optical compatibility and the eye’s pupil can further limit the usable field. The circles compare angles only, not an eyepiece image.

      Calculation sources

      Back up or move your workspace

      Export a version-2 JSON file (older version-1 backups remain readable), then import it on another browser or after the domain move. Files are read on this device, never uploaded. Imports replace equipment and named sites only after your confirmation. Each workspace supports up to 100 equipment entries and 100 sites, within 256 KiB.

      Storage recovery and removal

      If a saved workspace cannot be read, download its original data before resetting it. Removing or replacing this workspace keeps the independent observing location in Your settings. Use Forget my location there to remove it too.