Are astronomical events the only way to obtain starcharts?

According to the wiki: yes.

starchart - found by observing rare astronomical events after researching Astronomy tech

According to the code (of this version (0.9.6.9): yes. There is no other mention of starcharts, only in the workshop (as required researching resource) and the astronomical events. Note that the observatories can auto-observe these events, and therefor you can end up with a lot of charts if you let the game running overnight.

New information (sourced fromwiki page)

Starcharts are produced by:

Get more by building Observatories, Research Vessels, Space Beacons, launching satellites or from kitten Scholars with the Astrophysicists upgrade.

• Observatories simultaneously boost the chance of events and give a small chance to automatically observe them
• Research Vessels have a 0.05/sec production of star charts. Does not appear to be upgradeable
• Space Beacons have 0.125/sec production of star charts. Also does not appear to be upgradeable
• Satellites have a .005/sec production of star charts. Can be upgraded by 30% with Hubble Space Telescope
• Scholar kittens can get the Astrophysicist upgrade for .0005/sec production

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