Red and green makes orange (solar imaging again!

Posted: June 20, 2012 in Imaging, Sun

Another image of the Sun taken with the PST yesterday afternoon:

My best yet!  This was made possible by an interesting discovery about my Nikon camera.  The pictures I’ve been taking prior to this have been careful not to saturate the chip – typically using exposures of about 1/100s at ISO200.

Yesterday I tried to do some longer exposures to try and extract more signal from those prominences around the limb at the cost of overexposing the disk.  The PST only allows a specific colour of red light through to the camera so I was expecting to see a washed out red disk with more obvious prominences.  I began taking pictures with exposure times ranging from 1/20 to 1/5 of a second.

What I actually recorded was a glowing orange ball similar to the final image above!  How can you get orange when the only colour available is red?

The camera chip is an array of red, green and blue sensors.  At fast exposures those photons of red light were activating the red sensors on the chip.  At slower shutter speeds the red sensors become saturated and the signal leaked into the green sensors.  A lot of red plus a dash of green equals orange in this situation.

Splitting the raw image into its RGB components gives this:

 The “B” component was a black image – nothing had leaked into that channel.  The green channel contained a nicely exposed image of the disk, with all the detail around the edge contained in the red channel.  Presumably with an even longer exposure the signal may have leaked into the blue channel as well!  From what I can tell, the green channel contains a higher quality image than the equivalent red channel – it certainly seemed to stand up to the contrast enhancement and sharpening process that followed.

  1. trev says:

    Remarkable Adrian and a great piece of detective work. I wonder whether the NIKON people are aware of this and whether anyone else has looked at the issue
    I was on the beach yesterday with the grandkids but I’d looked at the wonderful run in a clear blue sky and I just knew you’d be delightedly using your scope.

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