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"LOOK-NOW"---3 Days of Darkness Approach! NASA Stereo Behind HI1: 12/21/2012 it All Goes Dark!
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[quote:Anonymous Coward 30420838:MV8yMDg4MzcxXzM1MTY5ODc0XzQ2NTkzQTQ0] [quote:Dr. Astro:MV8yMDg4MzcxXzM1MTY4MzM4XzlGNkEzQkVG] [quote:Anonymous Coward 30420838:MV8yMDg4MzcxXzM1MTY3NTY4X0JFRjNBMzRF] [quote:Dr. Astro:MV8yMDg4MzcxXzM1MTY3MjM1X0I4N0EzRDMz] [quote:Anonymous Coward 1003086:MV8yMDg4MzcxXzM1MTY3MDc1X0RGOTIyQzU5] [quote:Dr. Astro:MV8yMDg4MzcxXzM1MTY3MDI2XzNCRUM4OUE2] [quote:biscuits and gravy:MV8yMDg4MzcxXzJCRjk0QTRE] [b]3 Days of Darkness Approach! NASA Stereo Behind HI1[/b] http://stereo-ssc.nascom.nasa.gov/cgi-bin/images Behind HI1 > 512x512 > 20121217 - 20121220 > Slideshow > Search [/quote] What about it? Here's the latest image (at its native resolution): :behind122012: Pardon the glare from the sun on the left, I haven't done anything to subtract it from the raw image. That's precisely what they did to the jpg version of the beacon image you see on the website (and not that well imho), but there's not literally a dark region there. They process those versions quite heavily, I strongly recommend downloading the raw FITS images from the FTP before you draw conclusions like "3 days of darkness approach." [/quote] sooo.....this mean exactly what astro? [/quote] Processing artifact from the background subtraction they're using. They usually average a previous day's images together to create a calibration image which is subtracted from the latest image so that they can elminate the glare from the sun and see the tenuous heliosphere. They only update the calibration image every so often, and as time passes it may become less and less appropriate compared to the actual glare that's present and how the CCD is responding to the light. [b]In other words, the calibration image is currently subtracting too much[/b]. There isn't really a "dark" region there, it's actually quite bright and if you don't compensate for it and try to just adjust the levels of the raw unprocessed beacon image to match the JPGs present online you end up blowing out the glare which is what you see with the large white region on the left side of the image: :behind122012: It seems like the calibration image they're using for the non-BEACON images is much better right now (high quality low compression versions of the same images), which is why if you go back a few days the problem vanishes instantly. Once you step forward one image into the BEACON images the problem with the calibration frame appears. In a few days when you check back at the stereo site, the beacon JPGs will be replaced with full resolution JPGs from the non-BEACON versions, at which time the "black region" will probably either be greatly reduced or gone altogether. Standard non-BEACON images have to be processed from scratch so just because there's a problem with the BEACON processed version does not always mean there will be a problem with the full res version a few days later. [/quote] You are literally saying that the glare from the sun has subsided beyond established non-BEACON glare-reducing image processing parameters resulting in the recent darkened STEREO images. Presumably, glare is a direct function of sun luminosity, which would mean that the sun has darkened beyond established image correcting parameters. This begs the question: is this level of darkening unusual? If so, to what quantitative degree? [/quote] The amount of glare is subject to a number of variables including the brightness of the sun (which varies with the presence or absence of sunspots), the spacecraft attitude, and the response of the CCD to light. So no, it's not a direct function of solar luminosity, that is only one component. Yes, solar irradiance is variable, but the amount of variation is quite small in amplitude, about .1~.3% : http://blogs.edf.org/climate411/wp-content/files/2007/07/SolarOutput.png When processing these images, adjusting the strength of the background subtraction by even that very small percentage can have a large impact on the result - the processing includes a lot of aggressive contrast enhancement so even tiny differences are exaggerated. Having done a lot of STEREO HI image processing myself, I know for a fact it's quite normal for the amount of glare to change over time which can result in artifically bright or dark regions in the image. In fact, it's why I started developing my own processing method which doesn't involve subtracting previous images but rather involves a dynamic background subtraction using only the current image: [youtube]http://www.youtube.com/watch?v=tVDoN4bpODk[/youtube] [/quote] Thanks again for taking the time to answer these questions in a civil and rational manner. [/quote]
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3 Days of Darkness Approach! NASA Stereo Behind HI1
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link to stereo-ssc.nascom.nasa.gov
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Behind HI1 > 512x512 > 20121217 - 20121220 > Slideshow > Search
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