Re: Laser ranging to moon begs questions

On 31 Oct 2007 11:23:02 -0400, Craig Markwardt
<craigmnet@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:


John C. Polasek <jpolasek@xxxxxxxxxx> writes:
... trimmed ...
That is true. "Averaging over the total experiment time" means
multiplying your (ph/s/pulse) by the pulse width (s/pulse) and by the
pulse rate (pulse/s), to ultimately get returned pulses per second.

Why would I, with a duty cycle of 2e-9, or two parts per billion
would I want to tally all the idle time, which amounts to 500,000,000
empty pulses. The "vigor" of the reception improves with pulses per
second of ON time.

What "you" would want to do is irrelevant. What the experimenter
would want to do

What the experiment would want to do is likewise irrelevant. The
experimenter has no more choice than I do. ...

Actually, the experimenter does have a choice since he is designing
the experiment.
No, he, just like me, has Hobson's choice to the extent I outlined;
He MUST get rid of 1/2 billion idle pulse intervals. Why make capital
out of the obvious?

... Your paper says "10 hz 25
makes photo-electrons per minute" and all the experimenter can do is
live with it. What he would hope to do is stave off all the noise that
must occur in the 500,000,000 non-pulse periods in a system sensitive
enough to identify a one-photon event.

I did not say the experimenter was accepting photons during the
off-pulse-periods, but the total wall clock time (i.e. the duty cycle
of the pulses) is important for the actual feasibility of the
experiment, since it is ~10 wall-clock-minutes of worth returns that
will be averaged. (where *of course* no photons are accepted during
~off-pulse periods)

You have to talk to production control about that.
...
The 3e-8 number is found by taking *your* numbers, and estimating the
number of returns per unit wall clock time. That is relevant because
a typical observation is ~tens of minutes of wall clock time.

The paper for which you kindly produced a .pdf file is loaded with
graphs showing the number of passes, the number of returns and it
details the exquisite difficulties with the moon's libration, etc. but
has no information as to data processing. Elsewhere in the paper it
states that in a large majority of cases, no return is detected. This
makes it especially important if you are going to detect one
photo-electron from the Moon's distance to ascertain a way of shutting
down the noise which very simply exists 500,000,000 times longer than
the possible photon reception.

Yes, that is true. And as I mentioned in my *original* reply, the
relevant question is not why so few ranging photons can determine the
range so precisely, but rather how to remove all the non-ranging
(noise) photons but keep signal photons. The answers were to filter
very stringently in time, wavelength, and position.


One reference was mentioned. I do not have access to library journals.

That is a weak reply for several reasons. (a) More than one reference
was cited; (b) the originally mentioned Shelus paper is freely
available; (c) many municipal public libraries hold scientific
journals; (d) *Science* magazine articles are available for very
modest fee.
I have found that when I become really interested in a paper,
that as member of the unwashed, I have to pay $30 for the paper. The
.pdf cited below for example is not worth $30. (I don't see a source
mentioned-isn't it copyrighted? How would I look it up?)

Why not start with the Science magazine website? I simply went there
and typed "Dickey" into the search box at the top of the screen. Yes,
a price of $30 would have been annoying, but the cost for the Dickey
et al article is far less than $30!

I was not aware of Science magazine or its search feature, and I
looked at it, but even so I am not inclined to pay $10 just to view an
article which may in fact have even less information than the two I
have already read.

Where is your sense of proportion? I already said I was polishing up
the solution to the Dark Matter problem, and I merely posed the lunar
question as a bit of relief. And now you are trying to make me look
totally baffled with this LLR.
After 70 messages nothing good has really happened.
But even if you weren't willing
to spend a few dollars, I would be astonished if no libraries in your
region carried Science magazine.
I am even ambivalent about extending my Popular Mechanics subscription
at the unheard of price of $15.97. I may, and then again, I may not.
My own journal article (google Polasek matrix) is available to me or
anyone for $22. Luckily I have the original journal.
1967 You have far overestimated my interest in the details of this process.
I simply asked a simple question, expecting a simple answer to how you
can determine fractional cm/s with a few pulses per minute and find
myself drawn into a 70 message harangue.
It is beginning to appear that you are not capable of answering a
simple question.

Actually, your claim is incorrect. As I noted above, I *did* answer
your original simple question with a simple answer, namely the
importance of filtering out noise photons by time, space and
wavelength.

Moralizing about the *importance* of filtering is not a sufficient
answer even for a liberal arts scientist.

Then *you* proceeded to make your ad hoc and
unsubstantiated calculations of the number of return photons. If you
are willing to make scientific claims, shouldn't you be willing to
substantiate those claims?

...
If you really care about the subject, why not get off of your petard
and find out some facts about the subject you are speculating upon?
Setting arbitrary boundaries about which journals you will or will not
look at is also rather silly. Scientific experiments and results are
published in scientific journals. If you don't look at scientific
journals, then you are pretty much relegated to uninformed observer
status.
I told you my interest in this topic is perfunctory at best and for
all your volubility, neither of you have come close to answering the
original query of this thread.

As noted above, that is not quite true. What actually happened is
that you changed the simple question by starting to ask about the
total efficiency of the system, which is a complicated question.


And finally, while the discussion has focussed on the low quality
McDonald Observatory results, in fact there are other lunar laser
ranging observatories which detect many more photons such as the OCA
(see Samain ref above), and the Apache Point observtory (APOLLO,
Murphy et al 2007). The results from these observatories definitely
do not "beg the question" as your originally post asserted.

CM


References

Murphy et al. 2007, "APOLLO: Apache Point...", PASP, submitted,
arXiv:0710.0890v1

Shelus Ries Williams Dickey, "A Summary of LLR Activity and Science
Results,"
http://geodaf.mt.asi.it/html_old/news/iwlr/Shelus_et_al_LLR.pdf
Thank you for furnishing this reference which however is devoid of
much technical information except that I glean that the OCA can catch
about 30 times the pulses the McDonald can, and that if the moon were
25% further away, ranging would be impossible, and that they consider
the signal loss proportional to 4th power of distance.

OK, sounds promising. Did you try the Murphy reference? (which has
an extensive discussion of the hardware for APOLLO) Did you try the
Samain reference? (which has an extensive discussion of systematic
errors) Did you try the Dickey reference? (which has a good general
discussion and references to other important papers)

[*] the more important question is whether return photons can be
distinguished from non-return photons.
[*]And that is done slam-dunk with time-gating, the only question
whether it can be done on a picosecond basis.

Again, you are missing the point. In an ideal world, there would be
no noise/background, and a *single* photon returned from a 200 ps
pulse would be enough to provide range information at the ~6 cm level.
Actually 12 cm. (Front in, rear out). And, um, Heisenberg's UP.
The challenge is not in measuring range photon return times to
picosecond accuracy, but in excluding the non-ranging photons.
I think you need quite a bit of both, unless I am missing some subtle
nuance.

CM
John Polasek
.