Astrometry.net allows you to access parts of the service through a JSON API. Your API key is required to do so, but allows the service to track your usage as if you were using the service through the website interface.
To access the Astrometry.net API, first create a session by sending a JSON request with your API key as the argument 'apikey' to {% url 'astrometry.net.api.api_login' %}. The API will return a session key, which must be included in subsequent requests sent to the API.