XPost: sci.physics, sci.electronics.design   
   From: jkirwan@easystreet.com   
      
   On 21 Nov 2004 10:11:58 -0800, dave.harper@gmail.com (David Harper) wrote:   
      
   >   
   >Does anyone else have knowledge of other techniques involved in sun   
   >sensors?  Are any photocells or other light sensors better (or more   
   >precise) than others?   
   >   
      
   I'm not sure I understand fully your need, but it did stimulate a thought.  I'd   
   consider the idea of an optical system designed to image the entire sky, but   
   with a very poor focus so that the sun is spread across 30% or so of the image   
   plane.  (I'm kind of thinking a defocused fish-eye lens.)  A quad-detector   
   placed at this plane will provide four separate signals, the sum of which   
   provides your reference level.  Orienting the quad detector optimally against   
   the ecliptic for the largest sensitivity, as the sun crosses over, may take   
   some   
   thought or experiment or both, but I imagine that this can be arranged into a   
   small self-contained system.  Whether or not it provides the precision or   
   repeatability you need... is another story.   
      
   >Also, are there any wavelength filters I should   
   >consider?   
      
   I'd tend to want to use a narrow part of the (sub)visible band with Si   
   detectors, perhaps in the 800nm or 880nm (far enough above the bandgap that   
   there is little temperature dependence of the wavelength response function)   
   only   
   because they are very good (good NF, stable, cheap, widely available, high   
   impedance and low leakage against offset voltage, etc.)  But I really have no   
   experience here, at all.   
      
   I'm curious about the answers, so I'll read along with you.   
      
   Jon   
      
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