[Laser] Lunar downlink

[Glenn Thomas]

I gather that there were quite a few projects that attempted to use 
the Apollo retroreflectors. The one I had in mind was reported in the 
San Jose Mockery Snooze as using the 150" telescope at Lick 
Observatory, strangely enough located near San Jose. The details were 
rather lacking as the article author would have had to explain what a 
photon is...

I'm not sure that the tale of the observatory location being off by 
100m is credible. If the beam is already 2 km on the moon and 18 km 
on the return, a translational error of 0.1 km seems insignificant. 
Likewise a return signal measured in single photons seems 
questionable when you consider the effect of noise due to thermal 
radiation from the atmosphere and even the telescope itself. Perhaps 
someone with a better grounding in physical optics than I can 
comment. Not having worked through the math, I might believe 100's or 
1000's of photons!

73 de Glenn wb6w

At 04:30 PM 2/21/2006, you wrote:
>I do not remember the number of joules per pulse used by the laser lunar
>range experiment, or how that translates to watts in the very short 
>pulse.   The
>mirror used at the obseratory in west Texas (my native state) started at one
>meter ( about 40 inches ) and later used a half meter.  As I recall 
>the  pulse
>expanded to about two kilometers at the Moon on a target less than a half
>square meter area and reflected back to a beam about 18 kilometes 
>diameter, into
>  the same telescope.  Most of the return pulses counted as single photons,
>but there were plenty of repeated pulses to get a statistical measurement of
>the  flight time that translates to less than two inch error.  Several
>reflectors there.  I beleive the experiment is still going.
>
>I think you may be thinking of the story that the first attempt, to  find the
>reflector left by Apollo XI was agravatingly un-successful.  That  is, until
>someone realized that they had been using the latitude and longitude  of the
>observatory office which was some 100 meters from the telescope.   When that
>correction was made to the calculations for pointing the telescope,  the next
>pulse was detected.  I do not remember which observatory 
>or  telescope was being
>used, but it did seem that bigger than one meter.  It  may have used a
>brighter laser than the Texas instrument.
>
>James
>N5GUI


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