{"action":"created","issue":{"url":"https://api.github.com/repos/ratt-ru/bullseye/issues/60","labels_url":"https://api.github.com/repos/ratt-ru/bullseye/issues/60/labels{/name}","comments_url":"https://api.github.com/repos/ratt-ru/bullseye/issues/60/comments","events_url":"https://api.github.com/repos/ratt-ru/bullseye/issues/60/events","html_url":"https://github.com/ratt-ru/bullseye/issues/60","id":89060508,"number":60,"title":"Aliasing","user":{"login":"cyriltasse","id":6523227,"avatar_url":"https://avatars.githubusercontent.com/u/6523227?v=3","gravatar_id":"","url":"https://api.github.com/users/cyriltasse","html_url":"https://github.com/cyriltasse","followers_url":"https://api.github.com/users/cyriltasse/followers","following_url":"https://api.github.com/users/cyriltasse/following{/other_user}","gists_url":"https://api.github.com/users/cyriltasse/gists{/gist_id}","starred_url":"https://api.github.com/users/cyriltasse/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/cyriltasse/subscriptions","organizations_url":"https://api.github.com/users/cyriltasse/orgs","repos_url":"https://api.github.com/users/cyriltasse/repos","events_url":"https://api.github.com/users/cyriltasse/events{/privacy}","received_events_url":"https://api.github.com/users/cyriltasse/received_events","type":"User","site_admin":false},"labels":[],"state":"open","locked":false,"assignee":null,"milestone":null,"comments":7,"created_at":"2015-06-17T16:13:33Z","updated_at":"2015-06-17T21:04:08Z","closed_at":null,"body":"I've noted a simple issue, that is probably absolutely normal, but I did not give any thought about it before. If you make an image with and off-field source, one will see sidelobes across the image, but also some additional noise (plus some fixed ghost sources) that I attribute to alising. I think that is the issue because if you increase the FOV, this noise decrease (we basically decrease the size of the cells in the uv-space). That is the case for my gridder, but also the case for casa. This is somewhat a problem since the smaller the facet, the more we'll have this issue. Do you also observe that with your gridder?\r\n\r\nThe other strange thing is that I think the degridding should not be affected (think of a dataset with only one source in one facet only). So clean still does properly converge even if facets are small.\r\n\r\nI've been thinking about the problem but could not find a proper way to look at it and fully understand it. "},"comment":{"url":"https://api.github.com/repos/ratt-ru/bullseye/issues/comments/112949312","html_url":"https://github.com/ratt-ru/bullseye/issues/60#issuecomment-112949312","issue_url":"https://api.github.com/repos/ratt-ru/bullseye/issues/60","id":112949312,"user":{"login":"bennahugo","id":7934586,"avatar_url":"https://avatars.githubusercontent.com/u/7934586?v=3","gravatar_id":"","url":"https://api.github.com/users/bennahugo","html_url":"https://github.com/bennahugo","followers_url":"https://api.github.com/users/bennahugo/followers","following_url":"https://api.github.com/users/bennahugo/following{/other_user}","gists_url":"https://api.github.com/users/bennahugo/gists{/gist_id}","starred_url":"https://api.github.com/users/bennahugo/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/bennahugo/subscriptions","organizations_url":"https://api.github.com/users/bennahugo/orgs","repos_url":"https://api.github.com/users/bennahugo/repos","events_url":"https://api.github.com/users/bennahugo/events{/privacy}","received_events_url":"https://api.github.com/users/bennahugo/received_events","type":"User","site_admin":false},"created_at":"2015-06-17T21:04:08Z","updated_at":"2015-06-17T21:04:08Z","body":"Upon close inspection I notice there is a very tiny discrepancy between the pixel values. This may very well be a floating point issue. I've also emailed you double precision gridded and FFTed results.\r\n\r\nOne theory:\r\nIf this is the kind of noise you're hunting then I believe we should grid in double precision anyway (perhaps including a reduction step to avoid adding noise-like visibilities to continuously increasing grid cells).\r\n \r\nAnother (or in combination with):\r\nSome of the visibilities fall off the edge of the grid (either completely or some of the convolution filter does). This will have a very real impact on the total energy in the final image. As Oleg suggested pad the grid (total grid resolution twice maximum u and v respectively). However, this means each facet is as large as the original w-projected image (in terms of memory)..."}}