Travis:

Yes, it has pixels. The main design problem is to get aCCD
array with LOTS of pixels. This will directly determine the
dynamic range of the instrument in Db (decibels related to some
given mechanical displacement.) So to have 30db dynamic range,
 you would need to have 1000 pixels in the CCD array.  This is not
very good, even for a crude seismo.  So to get 40db. dynamic range,
you need 10,000 pixels. This is about the limit for affordable CCD
chips.  One way around this is to use a large array  for the low-level
displacement and a second CCD for the larger excursions. Yes, you
get "steps" but this is true of all digitizing devices including an A/D
converter chip. A 12-bit A/D translates the incoming analog waveform
into 4096 little stairsteps.  The catch-22 in seismic signal  processing is to

have enough bits to accurately sample very low amplitude signals, no
matter how what kind of seismometer generated them. The only tricky
part to all this is to construct a stable optical lever.

Dave Latsch

Travis Farmer wrote:

> But doesn't a CCD have pixels?
> i would think this would create kind of a stepping data output.
> Though i have never actualy seen a CCD array so i may be compleatly wrong.
> By chance have you got a schematic i can study?
>
> ~Travis
>
> >
> >Hi Chris,
> >
> >You are almost there-forget the photocells and use a CCD
> >linear array
> >and a laser beam reflected off the vertical pendulum. The
> >CCD array
> >gives you a clean, robust output that is a direct analog of
> >the POSITION
> >of the seismic mass. Hence it is not frequency-response
> >limited. It is
> >truly an absolute position sensor, rather than the classic
> >velocity
> >sensor with all its' attendant problems. I am cobbling up a
> >system
> >using the guts from an old HP Laser-Jet printer.  Another
> >similar
> >method is to monitor the relected laser beam with a simple
> >optical
> >interferometer and count the resulting fringe patterns. This
> >is done in the
> >USGS gravity observatory instrument here in Boulder. Now you
> >can
> >measure displacements down to to wavelength of whatever
> >color of
> >light your laser is!!
> >
> >Best wishes-
> >Dave Latsch
> >
> >
> >
> >ChrisAtUpw@....... wrote:
> >
> > > In a message dated 25/05/01, travis5765@...........
> > > writes:
> > >
> > >
> > >> Although I have not tried it, it seems like it should
> > >> work. You would have
> > >> to make sure NO excess light fell onto the sensors
> > >> though.
> > >> By the way, I missed the page in scanning that told that
> > >> the output needs
> > >> only minimal amplification though it varies with the
> > >> rated output of the
> > >> solar panels and the consistency of the light output.
> > >> My idea for a modification would be to use a laser
> > >> pointer as a light
> > >> source. make sure to have a well regulated power supply
> > >> for it.
> > >
> > >       The larger area silicon photocells as opposed to
> > > 'solar cells' tend to
> > > be better matched as to sensitivity, change with
> > > temperature and reverse
> > > leakage. Solid state Lasers are very noisy indeed. LED's
> > > are also noisy and
> > > their output is quite temperature dependant. A tungsten
> > > bulb driven from a
> > > voltage stabilised source at about 3/4 it's rated voltage
> > > will give good
> > > performance. You also benefit from the Infra Red
> > > sensitivity of the Si cell.
> > > You can get good performance by butting two cells end to
> > > end and using a slit
> > > which exposes about half each cell.
> > >
> > >       Regards,
> > >
> > >       Chris Chapman
>
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