{"action":"closed","issue":{"url":"https://api.github.com/repos/Starlink/palpy/issues/9","repository_url":"https://api.github.com/repos/Starlink/palpy","labels_url":"https://api.github.com/repos/Starlink/palpy/issues/9/labels{/name}","comments_url":"https://api.github.com/repos/Starlink/palpy/issues/9/comments","events_url":"https://api.github.com/repos/Starlink/palpy/issues/9/events","html_url":"https://github.com/Starlink/palpy/issues/9","id":189267959,"number":9,"title":"relationship between mapqk and ampqk","user":{"login":"danielsf","id":1682854,"avatar_url":"https://avatars.githubusercontent.com/u/1682854?v=3","gravatar_id":"","url":"https://api.github.com/users/danielsf","html_url":"https://github.com/danielsf","followers_url":"https://api.github.com/users/danielsf/followers","following_url":"https://api.github.com/users/danielsf/following{/other_user}","gists_url":"https://api.github.com/users/danielsf/gists{/gist_id}","starred_url":"https://api.github.com/users/danielsf/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/danielsf/subscriptions","organizations_url":"https://api.github.com/users/danielsf/orgs","repos_url":"https://api.github.com/users/danielsf/repos","events_url":"https://api.github.com/users/danielsf/events{/privacy}","received_events_url":"https://api.github.com/users/danielsf/received_events","type":"User","site_admin":false},"labels":[],"state":"closed","locked":false,"assignee":null,"assignees":[],"milestone":null,"comments":6,"created_at":"2016-11-15T00:42:16Z","updated_at":"2016-11-21T23:17:15Z","closed_at":"2016-11-21T23:17:15Z","body":"Reading the documentation for mapqk and ampqk, I get the sense that they are supposed to invert each other.  Mapqk transforms the mean position of a star into the apparent geocentric position.  Ampqk does the opposite.  However, Mapqk accepts parallax as one of its arguments.  Ampqk does not.  Indeed, in using the functions, I have noticed that they do invert each other to sub-milliarcsecond precision when parallax is zero.  When parallax is not zero, running the roundtrip through mapqk and ampqk can result in a residual on the order of a few 0.1 arcseconds.  Is there some reason these functions are not exact inverses of each other?  I understand that subtracting out the effects of proper motion and radial velocity might not make much sense, but I would have assumed that parallax was something that we would want to be able to account for in converting from geocentric apparent to mean position.\r\n\r\nThanks."}}
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