Firstly, there are multiple feeds in the network that you can post to, covering many topics - from the general Astronomy feed to specific topics like Exoplanets. Here's a list of all of them:
Finally, if you ever have any problems, you can get in touch with the moderation account @moderation.astronomy.blue. Also, @emily.space is the admin of the feeds, and can help with technical issues.
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Here's all the information you need to know:
IMPORTANT: It can take up to 2 minutes for the server to refresh before you can post to the feed.