PSN-L Email List Message
Subject: Re: wonderful invention of R. V. Jones
From: Brett Nordgren Brett3mr@.............
Date: Wed, 27 Feb 2008 09:31:47 -0500
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)
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.
Good resource on Ronchi technique, though aimed at amateur telescope mirror
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.
At 11:43 AM 2/26/2008 -0500, you wrote:
> 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.
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.
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