Subject: Re: Optical siesmometer
From: Charles R Patton charles.r.patton@........
Date: Wed, 16 Jan 2013 18:39:14 -0800
On 1/16/2013 9:55 AM, Charles R Patton wrote:
> On 1/16/2013 3:14 AM, chrisatupw@....... wrote:
>> From: Larry Cochrane
>> To: psnlist
>> Sent: Wed, 16 Jan 2013 3:21
>> Subject: Fwd: Optical siesmometer
>>
>> Subject:Optical seismometer
>> Date:Tue,15 Jan 2013 15:33:51 -0600
>>
>> Larry,
>> Ithought you might find this interesting:
>> http://www.ctbto.org/fileadmin/user_upload/SandT_2011/presentations/T3-O5%20J_Berger%20Optical%20Seismometer.pdf
>> -Charlie
>>
>> Hi Charlie,
>> It looks as if Mark & Co have done quite a bit more development work.
>> But amateurs would likely to have difficulty measuring optical fringes to
>> 1/2 ppm / Root Hz and Michaelson Interferometers are not cheap.
>> Amateurs can get about 10 nano metres resolution over 10 Hz using large area
>> photocells and a stabilised light source, but this is likely to be adequate -
>> unless you can 'lay your hands' on a couple of redundant Streckheisens !
>> Regards,
>> Chris Chapman
>
> OK, how about this for a thought experiment?
>
> Take a standard $10 or so USBweb cam -- definitely a cheaper one, no
> autofocus, but rather one where you can unscrew the lens easily.It
> will have a 640 x 480 or better resolution at a 60 Hz sample rate.Use
> it in an optical lever arrangement on the seismometer and project a
> laser beam spot on the face of the sensor.So how sensitive could it be?
>
> Let's assume a 20" pendulum and a 20" optical lever length.We're
> interested in duplicating the interferometer capability in the
> Zumberge/Berger paper -- about:
>
> 3e-7 * 1e-6 = 3e-13m=1.18e-12"(see pg 8).
>
> Although I have a problem with this number.They describe a 16 bit
> conversion so the number can't be much better than:
>
> 3e-7 / 65536 = 4.6e-11m=1. 8e-9"
>
> So, assuming the typical 1/3" sensor in the webcam.Therefore
>
> 0.33" / 640 = 5.21e-4"640 pixel spacing
>
> 5.21e-4" / 256= 2.03e-6"due to interpolation from the 8-bit analog
> digitization
>
> As the optical lever length is assumed equal to the pendulum length,
> then for small movements, the projected laser dot displacement will
> equal the pendulum movement.
>
> So the optical sensor is still a factor of 1000 away from the
> interferometer.Averaging the sample rate from 60 Hz/16 down to approx
> 4Hz, could add another 4x resolution improvement or about:
>
> 2.03e-6" / 4 = 0.5e-6"
>
> Not really close enough.No joy there.
>
> I can't think of another major improvement to the resolution except:
>
> 1) Maybe project the beam through a cylindrical lens that would
> increase the deviation, but also spread the beam so probably a wash.
>
> 2) An optical lever distance of 1000 x 20" = 2e4"= 1667'.I don't think
> so, unless we did it with a set of parallel mirrors spaced perhaps 2'
> apart and where we allow the laser beam to enter at an almost
> perpendicular angle to bounce back and forth 800 times before
> exiting.Mirror loss per bounce of 1% would attenuate the beam by
> 8.That shouldn't be a problem -- just the quality of the mirrors would
> be tough.
>
> Any other ideas?
>
> Regards,
>
> Charles R. Patton
Part 2 with further thoughts
A further thought experiment.I started considering the concept of Ronchi
rulings and cast about my mental closet for fine Ronchi rulings.What
came to mind is diffractions gratings, and of practical, widely
available gratings -- CD's and DVD's.The optical track pitch is 1.5 to
1.6um on CD's.So calculating the fringe movement times the resolution of
the webcam previously calculated gives:
1.5um = 59e-6" * 2.03e-6 = 1.2e-10"
this is 10x better than the system outlined in the paper.
If you want to see the effect, look in a container of blank CD's.Often
there is a fully transparent disk at the bottom.In some brands, this
disk has the track imprint of equal to that of an aluminized disk.In my
case, a PNY disk was good.Not all disks are imprinted.Look through the
disk and if you don't see a lot of colored rings looking at a light then
it won't be satisfactory.
Cut the disk in half.We want equal pitches on the two pieces so it's
best if they come from the same disk.Shine a laser pointer through the
1/2 disk piece and note the direct spot on the wall, but notice that
there is also a diffraction image off to the side.Add the second 1/2
piece in contact and adjust until the two diffraction images
overlap.Carefully adjust in that area and you should see strong
alternate light and dark bands form along the length of the diffraction
image. There is a good contact side and a bad side -- the good way will
be with the imprinted sides in contact -- the same sides that you would
normally write on.I did it with a business card spacer (about 0.01") and
it still had strong bands.So here is a $10 sensor with 1.2e-10"
sensitivity comparable to the expensive optical system in the paper..
Enjoy.
Charles R. Patton
It looks as if Mark & Co have done quite a bit more development work.
But amateurs would likely to have difficulty measuring optical fringes to
1/2 ppm / Root Hz and Michaelson Interferometers are not cheap.
Amateurs can get about 10 nano metres resolution over 10 Hz using large area
photocells and a stabilised light source, but this is likely to be adequate -
unless you can 'lay your hands' on a couple of redundant Streckheisens !
Regards,
Chris Chapman
OK, how about this for a thought experiment?
Take a standard $10 or so USBweb cam -- definitely a
cheaper one, no autofocus, but rather one where you can
unscrew the lens easily.It
will have a 640 x 480 or better resolution at a 60 Hz sample
rate.Use it in an
optical lever arrangement on the seismometer and project a
laser beam spot on the face of the sensor.So how sensitive could
it be?
Let's assume a 20" pendulum and a 20" optical
lever length.We're
interested in duplicating the interferometer capability in the
Zumberge/Berger paper -- about:
3e-7 * 1e-6 = 3e-13m=1.18e-12"(see pg 8).
Although I have a problem with this number.They describe a 16 bit
conversion so the number can't be much better than:
3e-7 / 65536 = 4.6e-11m=1. 8e-9"
So, assuming the typical 1/3" sensor in the
webcam.Therefore
0.33" / 640 = 5.21e-4"640 pixel spacing
5.21e-4" / 256= 2.03e-6"due
to interpolation from the 8-bit analog digitization
As the optical lever length is assumed equal
to the pendulum length, then for small movements, the
projected laser dot displacement will equal the pendulum
movement.
So the optical sensor is still a factor of
1000 away from the interferometer.Averaging the sample rate from 60 Hz/16 down
to approx 4Hz, could add another 4x resolution improvement or
about:
2.03e-6" / 4 = 0.5e-6"
Not really close enough.No joy there.
I can't think of another major improvement to
the resolution except:
1) Maybe project the beam through a
cylindrical lens that would increase the deviation, but also
spread the beam so probably a wash.
2) An optical lever distance of 1000 x 20" =
2e4"= 1667'.I don't
think so, unless we did it with a set of parallel mirrors
spaced perhaps 2' apart and where we allow the laser beam to
enter at an almost perpendicular angle to bounce back and
forth 800 times before exiting.Mirror loss per bounce of 1% would attenuate the beam
by 8.That shouldn't
be a problem -- just the quality of the mirrors would be
tough.
Any other ideas?
Regards,
Charles
R.
Patton
Part 2 with further thoughts
A
further thought experiment.I
started
considering the concept of Ronchi rulings and cast about my
mental closet for
fine Ronchi rulings.What
came to mind
is diffractions gratings, and of practical, widely available
gratings -- CD's
and DVD's.The optical
track pitch is
1.5 to 1.6um on CD's.So
calculating
the fringe movement times the resolution of the webcam
previously calculated
gives:
1.5um
= 59e-6" * 2.03e-6 = 1.2e-10"
this
is 10x better than the system outlined in the paper.
If
you want to see the effect, look in a container of blank CD's.Often there is a fully
transparent disk at
the bottom.In some
brands, this disk
has the track imprint of equal to that of an aluminized disk.In my case, a PNY disk was
good.Not all disks are
imprinted.Look through
the disk and if you don't see a
lot of colored rings looking at a light then it won't be
satisfactory.
Cut
the disk in half.We
want equal pitches
on the two pieces so it's best if they come from the same disk.Shine a laser pointer
through the 1/2 disk
piece and note the direct spot on the wall, but notice that
there is also a
diffraction image off to the side.Add
the second 1/2 piece in contact and adjust until the two
diffraction images
overlap.Carefully
adjust in that area
and you should see strong alternate light and dark bands form
along the length
of the diffraction image. There is a good contact side and a bad
side -- the
good way will be with the imprinted sides in contact -- the same
sides that you
would normally write on.I
did it with
a business card spacer (about 0.01") and it still had strong
bands.So here is a $10
sensor with 1.2e-10"
sensitivity comparable to the expensive optical system in the
paper..