{"action":"created","issue":{"url":"https://api.github.com/repos/astropy/astroplan/issues/383","repository_url":"https://api.github.com/repos/astropy/astroplan","labels_url":"https://api.github.com/repos/astropy/astroplan/issues/383/labels{/name}","comments_url":"https://api.github.com/repos/astropy/astroplan/issues/383/comments","events_url":"https://api.github.com/repos/astropy/astroplan/issues/383/events","html_url":"https://github.com/astropy/astroplan/issues/383","id":383609401,"node_id":"MDU6SXNzdWUzODM2MDk0MDE=","number":383,"title":"Axis coordinate problem","user":{"login":"kobe90","id":32132854,"node_id":"MDQ6VXNlcjMyMTMyODU0","avatar_url":"https://avatars3.githubusercontent.com/u/32132854?v=4","gravatar_id":"","url":"https://api.github.com/users/kobe90","html_url":"https://github.com/kobe90","followers_url":"https://api.github.com/users/kobe90/followers","following_url":"https://api.github.com/users/kobe90/following{/other_user}","gists_url":"https://api.github.com/users/kobe90/gists{/gist_id}","starred_url":"https://api.github.com/users/kobe90/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/kobe90/subscriptions","organizations_url":"https://api.github.com/users/kobe90/orgs","repos_url":"https://api.github.com/users/kobe90/repos","events_url":"https://api.github.com/users/kobe90/events{/privacy}","received_events_url":"https://api.github.com/users/kobe90/received_events","type":"User","site_admin":false},"labels":[],"state":"open","locked":false,"assignee":null,"assignees":[],"milestone":null,"comments":8,"created_at":"2018-11-22T16:52:04Z","updated_at":"2018-11-28T16:47:14Z","closed_at":null,"author_association":"NONE","body":"Hello,\r\nI am trying to use the astroplan library to build finding charts.\r\n\r\nI’ve tried to follow this guide\r\nhttps://astroplan.readthedocs.io/en/latest/tutorials/plots.html#plots-sky-charts\r\nin particular, I used the example I found in the very last section ‘Finder Chart/Image’.\r\nIn order to better understand the library, I’ve tried also different coordinates to download the relative archive images.\r\nI’ve tried in particular this:\r\nra, dec = 197.11, 52.23\r\nThe point is that the image I get is somehow strange when considering the x axis (I’ve overplotted some circles for my personal usage):\r\n![figure_1](https://user-images.githubusercontent.com/32132854/48915488-20011800-ee7f-11e8-8d8d-cf4691ab5481.png)\r\nAs you can appreciate, the scale seems to be a little bit wrong, also considering that the download image is supposed to be a square one.\r\nPlease, tell me if I did something wrong.\r\n\r\nThank you in advance.\r\n\r\n\r\n\r\nP.S.:The 2’ scale bar has been drawn using the formula:\r\nbar = AnchoredSizeBar(ax.transData, 2./(60*np.abs(hdu.header['CDELT1'])), \"2'\", 3)\r\nax.add_artist(bar)  \r\n\r\nbeing ax and hdu the same reported in the Finder Chart/Image section of https://astroplan.readthedocs.io/en/latest/tutorials/plots.html#plots-sky-charts . The second argument of AnchoredSizeBar is the length of the bar and hdu.header['CDELT1’] is the pixel scale as specified in the header attached to the image.\r\n"},"comment":{"url":"https://api.github.com/repos/astropy/astroplan/issues/comments/442518134","html_url":"https://github.com/astropy/astroplan/issues/383#issuecomment-442518134","issue_url":"https://api.github.com/repos/astropy/astroplan/issues/383","id":442518134,"node_id":"MDEyOklzc3VlQ29tbWVudDQ0MjUxODEzNA==","user":{"login":"rmathar","id":25105191,"node_id":"MDQ6VXNlcjI1MTA1MTkx","avatar_url":"https://avatars3.githubusercontent.com/u/25105191?v=4","gravatar_id":"","url":"https://api.github.com/users/rmathar","html_url":"https://github.com/rmathar","followers_url":"https://api.github.com/users/rmathar/followers","following_url":"https://api.github.com/users/rmathar/following{/other_user}","gists_url":"https://api.github.com/users/rmathar/gists{/gist_id}","starred_url":"https://api.github.com/users/rmathar/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/rmathar/subscriptions","organizations_url":"https://api.github.com/users/rmathar/orgs","repos_url":"https://api.github.com/users/rmathar/repos","events_url":"https://api.github.com/users/rmathar/events{/privacy}","received_events_url":"https://api.github.com/users/rmathar/received_events","type":"User","site_admin":false},"created_at":"2018-11-28T16:47:14Z","updated_at":"2018-11-28T16:47:14Z","author_association":"NONE","body":"As observed by kobe90, the CDELT1 of a standard WCS coordinate system is the delta in the product of RA*cos(delta), which is the natural unit work with uniform scales on both axes. To get a value that is correct (for a small patch on the sky where delta is almost constant) one must divide through the cosine of delta (which is smaller than 1) to get a bar that is as long as the desired angle. That problem could for example be removed by using CDELT2 and printing that bar vertically, as such a trigonometric scaler is absent on the DEC axis."}}
{"actor":{"display_login":"rmathar"}}