MultimodalUniverse catalog crossmatch explorer
Astronomers catalogue the sky with many different telescopes. This tool crossmatches two such catalogues — it finds the objects that appear in both, i.e. the same star, galaxy or supernova seen by two different surveys — using the real MultimodalUniverse data on HuggingFace.
What is a catalogue? A survey points a telescope at the sky and records every object it sees — each gets a position and some measurements (brightness, colour, a spectrum, a light curve…). The resulting list is a catalogue: think of it as a census of cosmic objects taken with one particular instrument.
What is crossmatching, and why does it matter? No single telescope sees everything. One survey measures visible light, another X-rays, another infrared. Crossmatching links the same physical object across catalogues by its sky position, so you can combine what each instrument saw.
Example: take a star measured precisely in optical light by Gaia and find it again in the Chandra X-ray catalogue. A star that also shines brightly in X-rays is special — often a binary system where one star is pulling matter onto a neutron star or black hole. Neither catalogue alone tells you that; the match does.
Where are objects on the sky? (RA / Dec) Just as places on Earth have a latitude and longitude, points on the sky have two coordinates: Right Ascension (RA) and Declination (Dec). Every catalogued object carries an (RA, Dec) — that is what we match on.
What is HEALPix? A scheme that divides the whole celestial sphere (the dome of sky around Earth) into equal-area tiles, each with a number. It is a reliable address system for where on the sky something is, independent of any telescope. It lets us index objects by location and download just the relevant patch of sky instead of an entire all-sky catalogue. The coverage map below shows which tiles each catalogue touches.
What is the crossmatch radius (in arcseconds)? Two surveys never report a position perfectly identically, so we count two objects as the same if they fall within a small angular distance. That distance is the radius, measured in arcseconds (1 arcsec = 1/3600 of a degree — the full Moon is about 1800 arcsec across). Larger radius → more matches but more false ones; smaller radius → only the most confident matches. The slider lets you explore this trade-off.
How to use
Pick catalog 1 matched with catalog 2. Catalog 2 and the HEALPix dropdown are trimmed to combinations that are known to produce matches (precomputed by sweeping real crossmatches), so every pixel you can pick returns results. The crossmatch radius slider filters the pixel list live — pixels (and counts) update to reflect matches within the chosen radius. Then Run crossmatch on that pixel — the data is downloaded from HuggingFace and matched for real (the heavy lifting is done by lsdb; hit Show code to see the exact snippet).