by Chris Peterson » Sun Feb 10, 2013 3:36 pm
Ann wrote:By the way, why are most of the streaks so blue? Well, it's because this picture was made using only two filters, 606 nm and invisible infrared at 814 nm. (I believe that the orange filter can also be described as "clear", meaning that it detects all visible light.) The objects detected by the orange or visible filter are shown as blue, and the objects detected by the infrared filter as red. Yellow objects are detected by both the infrared and the orange filter.
Since the cosmic ray rate is statistically uniform, we could even count the number of events collected in each of the two exposures, and use that information to infer the relative exposure times used for each. That is, if there were twice as many blue streaks as yellow, we'd know that image made through the filter mapped to blue was exposed twice as long as the image made through the filter mapped to yellow.
(I don't think that the mapping used here was blue and red, but rather cyan and yellow. It is the white objects that were detected by both filters. Where did you find information on the filters used for the two exposures?)
BTW, this image is also available in its calibrated form, with the cosmic ray hits removed.
[quote="Ann"]By the way, why are most of the streaks so blue? Well, it's because this picture was made using only two filters, [b][size=105][color=#FFBF00]606 nm[/color][/size][/b] and invisible infrared at 814 nm. (I believe that the orange filter can also be described as "clear", meaning that it detects all visible light.) The objects detected by the [b][size=105][color=#FFBF00]orange[/color][/size][/b] or visible filter are shown as [b][size=105][color=#0040FF]blue[/color][/size][/b], and the objects detected by the infrared filter as [b][color=#FF0000]red[/color][/b]. [b][size=115][color=#FFFF00]Yellow[/color][/size][/b] objects are detected by both the infrared and the orange filter.[/quote]
Since the cosmic ray rate is statistically uniform, we could even count the number of events collected in each of the two exposures, and use that information to infer the relative exposure times used for each. That is, if there were twice as many blue streaks as yellow, we'd know that image made through the filter mapped to blue was exposed twice as long as the image made through the filter mapped to yellow.
(I don't think that the mapping used here was blue and red, but rather cyan and yellow. It is the white objects that were detected by both filters. Where did you find information on the filters used for the two exposures?)
[float=left][attachment=0]small_asteroidI_before_after.jpg[/attachment][/float]BTW, this image is also available in its calibrated form, with the cosmic ray hits removed.