{"action":"created","issue":{"url":"https://api.github.com/repos/spacetelescope/hstcal/issues/347","repository_url":"https://api.github.com/repos/spacetelescope/hstcal","labels_url":"https://api.github.com/repos/spacetelescope/hstcal/issues/347/labels{/name}","comments_url":"https://api.github.com/repos/spacetelescope/hstcal/issues/347/comments","events_url":"https://api.github.com/repos/spacetelescope/hstcal/issues/347/events","html_url":"https://github.com/spacetelescope/hstcal/issues/347","id":354912519,"node_id":"MDU6SXNzdWUzNTQ5MTI1MTk=","number":347,"title":"Do not add extra error to ERR extensions in ACS CTE forward model","user":{"login":"jryon","id":7647126,"node_id":"MDQ6VXNlcjc2NDcxMjY=","avatar_url":"https://avatars1.githubusercontent.com/u/7647126?v=4","gravatar_id":"","url":"https://api.github.com/users/jryon","html_url":"https://github.com/jryon","followers_url":"https://api.github.com/users/jryon/followers","following_url":"https://api.github.com/users/jryon/following{/other_user}","gists_url":"https://api.github.com/users/jryon/gists{/gist_id}","starred_url":"https://api.github.com/users/jryon/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/jryon/subscriptions","organizations_url":"https://api.github.com/users/jryon/orgs","repos_url":"https://api.github.com/users/jryon/repos","events_url":"https://api.github.com/users/jryon/events{/privacy}","received_events_url":"https://api.github.com/users/jryon/received_events","type":"User","site_admin":false},"labels":[],"state":"open","locked":false,"assignee":null,"assignees":[],"milestone":null,"comments":10,"created_at":"2018-08-28T21:33:19Z","updated_at":"2018-08-29T01:39:37Z","closed_at":null,"author_association":"NONE","body":"According to @jamienoss in [#330](https://github.com/spacetelescope/hstcal/pull/330#issuecomment-410352332),\r\n\r\n> The CTE correction simply adds 10% of the correction for the error induced, what this should be for the forward model I have no idea.\r\n\r\nIdeally, the ERR extensions should be the same before and after forward modeling, so no extra error should be added by the CTE forward model. This is because the input data into the forward model may vary in terms of how many calibration steps have been performed, and so it may not be accurate to add more noise. We'd like to simply simulate the effects of readout on the SCI extensions, and leave any ERR extension updates to the user."},"comment":{"url":"https://api.github.com/repos/spacetelescope/hstcal/issues/comments/416794487","html_url":"https://github.com/spacetelescope/hstcal/issues/347#issuecomment-416794487","issue_url":"https://api.github.com/repos/spacetelescope/hstcal/issues/347","id":416794487,"node_id":"MDEyOklzc3VlQ29tbWVudDQxNjc5NDQ4Nw==","user":{"login":"hcferguson","id":2261329,"node_id":"MDQ6VXNlcjIyNjEzMjk=","avatar_url":"https://avatars0.githubusercontent.com/u/2261329?v=4","gravatar_id":"","url":"https://api.github.com/users/hcferguson","html_url":"https://github.com/hcferguson","followers_url":"https://api.github.com/users/hcferguson/followers","following_url":"https://api.github.com/users/hcferguson/following{/other_user}","gists_url":"https://api.github.com/users/hcferguson/gists{/gist_id}","starred_url":"https://api.github.com/users/hcferguson/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/hcferguson/subscriptions","organizations_url":"https://api.github.com/users/hcferguson/orgs","repos_url":"https://api.github.com/users/hcferguson/repos","events_url":"https://api.github.com/users/hcferguson/events{/privacy}","received_events_url":"https://api.github.com/users/hcferguson/received_events","type":"User","site_admin":false},"created_at":"2018-08-29T01:39:37Z","updated_at":"2018-08-29T01:39:37Z","author_association":"NONE","body":"Yes, by and large the ERR arrays in HST data are not accurate and careful astronomers have to propagate uncertainties on their own. For imaging it does the least harm, though, to carry just the uncertainties of the background at the sky level computed from the CCD noise model (i.e. taking into account read noise, but not Poisson noise due to the sources themselves). This is what astrodrizzle does, I'm pretty sure.\r\n\r\nI agree that adding the 10% is cumbersome/impractical after the fact. I just don't think adding 10% to the ERR array helps. \r\n\r\nIn the case of the backward correction, I think probably people could compare their photometry on data before and after the correction, take the ratio as the size of the correction and 10% of that as the part of the uncertainty due to CTE.\r\n\r\nI suppose one could do the same for the forward modeling. But there are two possible ways to think of the purpose of forward modeling: (1)  \"if the CTE model is perfect, what does it do to my photometry?\" or (2) \"If the CTE model is imperfect to the level thought (10%) how does this propagate into my photometric uncertainties?\" For (1) you don't need to propagate errors. \r\n\r\nFor (2), you may be okay as long as you are doing just one dither, but otherwise are probably going to get it wrong.  If I were doing this type of analysis I might be inclined to take the brute-force approach and draw the CTE correction from a probability distribution -- always using the same value the same for all my dithers on each realization...and then run this a hundred times, redoing the photometry with whatever technique I used on the real data. Cumbersome and there are probably shortcuts. "}}
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