[RASMB] interference optics

[John Philo]

Ute, in addition to what Tom said, I think the major part of what you are
calling "jitter" is really integer fringe jumps, not the small (tenths to
hundredths of a fringe) 'breathing' shifts Tom is talking about. The fringe
jumps are perfectly normal and cannot be eliminated. The sample and
reference meniscii block the light at those positions, and thus there are no
fringes there and the fringe position is undefined. In successive scans the
data acquisition algorithms sometimes choose different integers when
crossing a meniscus, which leads to the integer shifts. 

Different analysis programs handle the integer fringe jumps differently. In
SEDFIT both 'jitter' and 'fringe jumps' are lumped together as 'radially
invariant noise' (RIN), and thus the RIN values can differ by more than a
fringe from one scan to the next. Some other programs, such as SEDANAL,
SVEDBERG & DCDT+, separately determine an integer fringe shift and a
sub-integer 'jitter'. 

So the bottom line is yes, for interference data it is often difficult to
discern the boundary motion in the raw data, unless the signal is many
fringes large. 

John

-----Original Message-----
From: rasmb-bounces at list.rasmb.org [mailto:rasmb-bounces at list.rasmb.org] On
Behalf Of Tom Laue
Sent: Tuesday, January 15, 2013 9:53 AM
To: Ute Curth
Cc: rasmb at list.rasmb.org
Subject: Re: [RASMB] interference optics

Hi-
The underlying cause of the jitter is the fact that the source and detector
are referenced mechanically to different spots on the XLI frame, and that
the thermal baseplate is flexible. The bulk of the vertical shift comes from
thermal gradients in the vicinity of the condensing lens (at the bottom of
the chamber). These gradients result in the lens "tipping" relative to the
source, and give rise to both a vertical component and a slight 'rotational'
shift to the fringes (i.e. 
the vertical shift is not quite uniform across the image). The TI/RI noise
will remove most of the effects, with the 'rotational' shift not being quite
as reproducible as the purely vertical 'breathing' shift.
If you shut off the thermo-electric modules (not recommended), the bulk of
the vertical displacement is eliminated. The time constant for the shifts is
on the order of minutes.
Some years back we built a new source and condenser lens mount which
decreased the breathing motion from tenths of a fringe to hundredths of a
fringe. Kristian Schilling has built a similar mount.
Best wishes,
Tom

On 1/15/2013 12:41 PM, Ute Curth wrote:
> Dear all,
>
> I hope you had a good start in 2013 and some of you can possibly give 
> me advice on a problem with our Xl-I machine.The raw data of  our 
> interference optics show (and always showed) such an enormous vertical 
> jitter that it is hard to judge whether the sample sediments at all 
> and where the meniscus is located (see attached file). Whereas, after 
> subtraction of TI and RI noise the data look quite nice, I am 
> wondering what the reason of this jitter is. Has anyone an idea about 
> its possible source and how I can get rid of it?
>
> Thank you in advance
>
> Ute
> --
> PD Dr. rer. nat. Ute Curth
> > Medizinische Hochschule Hannover
> > Strukturanalyse
> > OE 8830
> > Carl-Neuberg-Str. 1
> > 30625 Hannover
> > Germany
> >
> > Tel.: +49-511-5329372
> > Fax : +49-511-5325966
>
>
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--
Department of Biochemistry and Molecular Biology University of New Hampshire
Durham, NH 03824-3544
Phone: 603-862-2459
FAX:   603-862-0031
E-mail: Tom.Laue at unh.edu
www.bitc.unh.edu
www.camis.unh.edu

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