[Laser] Astronomy frame stacking software

[glennt at charter.net]

Hi all.

The trick with a CCD detector for laser comms is to be able to control the integration time of the CCD detector and to be able to predict with some accuracy what the dark current noise of the detector will be for the selected integration time. No need for fancy frame adding, just let the CCD itself do the integration, subtract off the dark current and you have your integrated signal. With a 1 second integration time and appropriate synchronization, you have a 1 BPS comm channel. Weaker signals? Use a longer integration time and get a correspondingly lower bit rate channel.

No sound card, but you do have to build up a CCD detector, including its control circuitry - not a trivial task.

You only need one CCD detector that way. These days, CCDs  are usually in focal plane arrays. That can be used for pointing the receiver array if the receiver optics create a real image of the transmitter. The computer then needs only to locate the pixel that is under the image of the transmitter. Again, non-trivial control circuitry for the CCD and some moderately complicated software to use it.

73 de Glenn wb6w

---- James Whitfield <n5gui at cox.net> wrote: 
> If the intent is to communicate with weak optical signals, perhaps we should
> consider the tools used by astronomers, such as CCD imaging devices and
> image stacking software.  It requires some re-thinking and throwing out the
> idea of using a sound card or frequency spectrum software.  These are used
> to detect and integrate photons from weak sources.
> 
> The image stacking software sifts through hundreds of frames to improve the
> end result image.  The communication task would be to separate the frames
> into a time sequence of bright and dark.  The sequence might represent the
> transmitted tone of a modulated signal, but the time scale would be many
> times slower than can be processed by a sound card.
> 
> Suppose that you have a message that lasts about three minutes if sent as a
> 1 baud BPSK signal with a carrier tone of 1 Hz.  If the CCD is sampling 10
> frames per second, you get about 1800 frames.  With the modified image
> stacking software, you look for clusters of pixels that fit the pattern of 5
> or 10 frames of increased brightness followed by 5 or 10 frames without.
> Then you rescan the images to find the synchronization, which is basicly
> choosing the grouping that minimizes the sets of 10 frames that show more
> than 6 bright.  Then group the images ten at a time, if the first 5 are
> brighter than the rest, it is a one, if the last five are brighter it is a
> zero.
> 
> 
> James
> N5GUI
> 
> 
> 
> 
> 
> 
> 
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