Hello Everyone: When photographing the dark sky through thin clouds, even very thin Cirrus, you get starblobs insteads of points. Often the blobs are bluish. I do not understand the science behind this and wonder whether any of you out there can speculate? Clouds consist of ice crystals or water droplets, often mixed, and usually of modal size around 10 microns, large in comparison to wavelength of visible light. Mie theory is appropriate, but Mie scattering predicts something akin to Fraunfoer scattering: that you will get light scattered in a cone around an angle theta (in radians) of 2 times pi times wavelegnth divided by radius of scattering object. This would give star blobs of 30 degrees or so, instead of the observed blobs of a few minutes of arc. For example, you often see the moon sourrounded by corona of a few degrees (not the 22 or 45 degree haloes, which are caused by refraction through the ice crystals). I am mystified by why thin clouds give small blobs of light. Glenn Shaw
|