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Data Rate Reduction By Setting A High Threshold

9505-001

 

Background noise count  reduction. (16501 bytes)

 

This graph shows how initial observations at The COSETI Observatory will be with a desensitized photon-counter, in order to reduced the noise background data-rate problem. This is an unorthodox (unconventional) approach to the problem.

For a photomultiplier optical front-end, the high voltage will be backed off to reduced saturation effects during received pulses, and the discriminator threshold will be increased. For this technique to work, it is initially assumed that the beacon pulses to be detected will consist of a large burst of photons - not just one or two photons, i.e., the laser pulses will be relatively powerful. See illustration 9512-001 for details. Thus, the photon-counter head will initially be employed as a lower sensitivity direct detection laser pulse receiver.

As more sophisticated signal processing and extensive data storage facilities become available, the sensitivity of the laser beacon pulse receiver will be improved by increasing the photodetector gain and lowering the discriminator threshold, thus allowing more stellar and sky background noise photons to be counted. Eventually, the optical front-end receiver will be operated in the true photon-counting mode, i.e., where every photon detected produces an electrical output (TTL) pulse from the photon-counter discriminator. All this can be accomplished without changing any of the front-end hardware.

Today's Microwave SETI experiments are 14 orders of magnitude more sensitive than Project Ozma. So says Seth Shostak in the Second Quarter 1996, Vol. 5, No. 2 issue of SETI News. Similarly, the initial OSETI activities will be far less sensitive than ones to come later. If it was alright for Microwave SETI researchers to use, what today would be termed "crude" equipment, then it is alright for Optical SETI researchers today. One has to learn to walk before one can run!

 

Other related materials:

9308-001

9308-002

45th Astronautical Congress Paper

 

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