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Re: [GPSL] Limb Sounding

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  • L. Paul Verhage
    I ll look into RAW format. That may be tghe way to go. I have a 2 GB SD card, so I still should be able to store a lot of photos. Paul ... -- Onwards and
    Message 1 of 3 , Oct 31, 2009
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      I'll look into RAW format.  That may be tghe way to go.  I have a 2 GB SD card, so I still should be able to store a lot of photos.

      On Sat, Oct 31, 2009 at 12:22 PM, Mark Conner <mconner1@...> wrote:

      One issue to keep in mind is the exposure compensation the camera performs to adjust the "raw" camera sensor values to RGB.  I'm not too familiar with the transformations involved, but a bad choice by the camera could cause misleading values in the RGB result.
      It's possible using images saved in RAW format might yield better results, but I'd have to explore that some more.  The values might be 12 to 16 bits in depth for each channel.  I know the Canon A560 will save in RAW format (at a considerable penalty in file size increase).  Picasa now offers (limited) RAW file manipulation.  If you know the exposure conditions (aperture and shutter) you may be able to back up from a RAW value to some absolute brightness number in each channel.
      73 de Mark N9XTN
      On Sat, Oct 31, 2009 at 11:21, L. Paul Verhage <nearsys@...> wrote:
      [Attachment(s) from L. Paul Verhage included below]

      I'm trying an experiment in limb sounding from near space.  This first iteration has acquainted me to some of the problems involved, but will also help me develop solutions.  I'm attaching two files, one is an image of the horizon as strips from every 10,000 feet.  The second is a chart based on data derived from sampling the colors of the images at 0.5 inch points above the horizon.  
      The first problem to solve is verifying the camera is pointed the same direction everytime.  I've developed a sun sensor that I'll fly on future missions so that the flight computer will only record images when the sun is located in the opposite quadrant.  The second part of the solution is to fly a sun compass.  The shadow it creates in the images will further refine the sun's position and let me weed out more pictures.  The flight computer will be programmed to snap several pictures every 5,000 feet when the sun is located in the proper quadrant.  This should give me a large selection of images to choose among.    
      I need to calibrate angles and fields of view with my camera.  Right now I can measure the same elevation above the horizon, based on pixel hieght.  I need to convert this into angles so the results are more meaningful.  
      Third, I'll rotate the camera 90 degrees so more of the image is above the horizon and I can get higher elevation data.
      Fourth, I need to understand how colors are converted into values inside an image.  I'm assuming things are nearly linear, so I take the values for RED, GREEN, and BLUE (in 8-bit resolution their maximum values are 255), add them together, and then divide each color by the sum of all three colors.  The computed value is then divided by 765 (the sum of 255 + 255 + 255), or the maximum value the colors are allowed to have.  Perhaps this arithmetic average is not as good as a geometric average.  If the colors are considered orthogonal to each other, then it would seem the geometric mean would be more appropriate.
      This is what I like about near space exploration.  Everytime you look into an experiment, it opens a can of worms of problems to solve.  This activity certainly isn't a closed-end experiment like high school chemistry class.  
      Onwards and Upwards,

      Attachment(s) from L. Paul Verhage

      2 of 2 Photo(s)

      Onwards and Upwards,
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