[RASMB] measuring cell positions

[John Philo]

Borries,

Nothing tricky with regard to software is needed. I was merely referring to
adjusting the 'delay' setting in the normal 'laser setup' window such that
either only the sample side or only the reference side is illuminated. Since
the center-to-center spacing of the channels is ~6 mm, moving the delay
(which is in degrees of rotation) > +/- arctan(3 mm/65 mm) = 2.6 degrees
from the normal setting should do that (or just adjust in each direction
until the fringes completely disappear).

John

-----Original Message-----
From: rasmb-admin at rasmb-email.bbri.org
[mailto:rasmb-admin at rasmb-email.bbri.org]On Behalf Of Borries Demeler
Sent: Monday, November 19, 2001 4:12 AM
To: jphilo at mailway.com
Cc: Rasmb
Subject: Re: [RASMB] measuring cell positions


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> Regarding the comment that the interference optics appears to give the
> average position for the sample and reference sides, it seems to me the
> physics says the fringes can only exist where the light goes through both
> sides, so their disappearance should measure the position of whichever
side
> is closest to the center of the channel rather than the average of the
two.
> Further, if the averaging idea was correct then the radial calibration
> procedure for interference would not work, since the counterbalance is
> designed to put the edge at 5.85 and 6.15 cm on only one side.

I used the word "average" in quotes, because of course it isn't a true
average, it just looks that way if you compare the scans in the plot
image I sent. I suspect the FFT causes some sort of interpolation
that results in the plot image.

> However, in theory it should be possible to image the two sides separately
> by deliberately moving the delay setting (laser timing) so that only one
> side or the other is actually illuminated. You won't see any fringes under
> such conditions, of course, but nonetheless the superimposed image of the
> cell should tell you the position of that channel (and probably with more
> precision than the absorbance optics).

Is there a way to accomplish what John suggests here? It would be nice
if this could indeed be done, because the IF optics have about twice the
radial resolution as the UV optics. Is there a way to change the laser
timing with Beckman's software?

-Borries
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