AstroSat has two tools: the Trail Forecaster, which predicts satellites crossing the patch of sky you're imaging, and Meridian, which converts between coordinate systems. Here's how to drive them.
The forecaster, in three steps
Tell it where and when you're shooting and what you're shooting with, and it works out which satellites will pass through your frame during the session.
- Set your site (latitude and longitude) and the date and time window you plan to image.
- Pick your target — start typing a catalogue name (for example "M31" or "NGC 7000") and choose from the list, or enter its coordinates directly.
- Enter your optics and camera — focal length and the sensor's pixel size and dimensions — which together set the field of view. Then run the forecast.
Reading the results
The field-of-view plot shows your framed sky and draws the path of each satellite that crosses it, so you can see where on the sensor a trail would land. The night timeline below stacks the crossings half-hour by half-hour, so the busy stretches just after dusk and before dawn stand out from the quieter middle of the night.
A satellite only leaves a trail when it's lit by the Sun against a dark sky. Crossings that happen while the satellite is in the Earth's shadow leave nothing on your frame, so they're shown dimmed. The timeline also marks your cleanest stretch — the longest run with the least contamination — which is a good place to start a sequence of subs.
The converter
Meridian converts between equatorial coordinates (right ascension and declination, RA/Dec) and horizontal coordinates (altitude and azimuth, Alt/Az) for a given site and moment, and shows where the target sits on an all-sky view. Enter one pair and your site and time, and it returns the other.
Accounts, sync and sharing
You don't need an account to use either tool. If you create one, your gear, saved sites and saved forecasts sync across your devices. The "Copy share link" button makes a short link that reopens a forecast with all its inputs — handy for posting a plan to a forum or sending it to an imaging partner. Share links are public to anyone who has them, and they include the site coordinates, so don't share a location you'd rather keep private.
Glossary
- Field of view (FOV) — the patch of sky your camera and optics actually capture, set by focal length and sensor size.
- RA / Dec — right ascension and declination, the "equatorial" coordinates fixed to the stars, the way catalogues list a target.
- Alt / Az — altitude and azimuth, the "horizontal" coordinates relative to your horizon right now; they change through the night as the sky turns.
- Apparent magnitude — how bright something looks from the ground; smaller (and negative) numbers are brighter.
- Sub — a single exposure ("sub-exposure") in a sequence that's later stacked into one image.
- In shadow / sunlit — whether the satellite is lit by the Sun. A satellite in the Earth's shadow is dark and leaves no trail.
- Terminator — the day/night line; near it, satellites slip in and out of sunlight.
- SGP4 — the standard model used to propagate a satellite's orbit forward from its published elements.
- Element set — the periodically published description of an orbit (a TLE or its OMM/JSON form) that SGP4 takes as input.
More questions
Anything missing here? Email hello@astrosat.org.