Hi Chris,

I agree with Randall that the Ronchi grating approach may be worth looking 
into.  In the simplest setup you would need two gratings, one in front of a 
diffuse light source and one in front of the detector pair, which detector 
would not need extremely high spatial resolution.  A mirror (possibly 
spherical concave) would be mounted on the seismo boom and the detectors 
would register small changes in mirror tilt.  This setup has the effect of 
adding a second optical lever to what you'd get with a simple light beam 
reflecting off a flat, moving mirror, and it avoids most of the 
difficulties encountered with having to use a collimated light source.  I 
agree that the intensity would be fairly low, but since resolution is 
likely not a big issue, you might be able to use fairly sensitive (=larger) 
photo detectors.

What now needs to be done is to develop a formulation relating grating 
frequency, mirror characteristics and detector distance to image motion at 
the photo detectors vs mirror tilt, that is, to tilt sensitivity.

Source for high-quality gratings priced from $35 and (way) up.
    http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=2343&srtitem_price=D#Products

Good resource on Ronchi technique, though aimed at amateur telescope mirror 
testing.
    http://www.users.bigpond.com/PJIFL/ronchi_index.html

An image which can be printed by a high-resolution printer onto 
overhead-projector film to make a 2"x2" 100 lpi Ronchi screen.
Some claim that gratings made in this way are not very good while others 
say they work fine.  It probably depends on the printer.
     http://stellafane.org/atm/atm_foucault_tester/images/atm_ronchi_screen.gif

At 11:43 AM 2/26/2008 -0500, you wrote:

>Hi Randall,
>
>        Ronchi gratings have certainly been mentioned in past psn Emails. 
> There are ate least two problems with this approach. Firstly it requires 
> the production of fairly expensive gratings. Secondly the line 
> intensities are likely to be very low.
>
>        I believe that Zumberge was using this type of sensor. 
> http://gravity.ucsd.edu/research/SIFO/

I think he was using a rather fancy laser interferometer--optical bench 
stuff.  Ronchi screens are more along the line of plywood and nails.  They 
use a diffuse light sourch which doesn't even have to be 
monochromatic.  And they are frequently made by amateurs.

Regards,
Brett
  


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