On 11/10/2017 1:38 PM, Robert Sweet wrote:
> This has been a fascinating thread. Thanks.
>
> I will dip my oar in the water. Here are a couple of snippets.
>
>> Jacob: It was good of proto-crystallographers to invent diffraction as
>> a way to apply Fourier Series.
>
> and
>
>> Ethan: So here's the brain-teaser: Why does Nature use Fourier
>> transforms rather than Wavelet transforms? Or does she?
>
> Probably Jacob was joking, but I believe we should say that physicists
> (and Ms. Nature) employ the Fourier transform/synthesis because this
> models pretty precisely the way that we believe light rays/waves of all
> energies interfere with one another.
>
> Warm regards, Bob
My belief is that the fact that our spot intensities represent the
amplitude (squared) of a series of Sin waves is the result of the hard
work of people like Bob who give us monochromatic illumination.
"Monochromatic" simply means it is a pure Sin wave. If Bob could get
that shiny new ring of his to produce an electromagnetic square wave his
users would still get diffraction patterns with spots but they would
have to come up with programs that would perform Fourier summations of
square waves to calculate electron density. Our instrument is an analog
computer for calculating the Sin wave Fourier transform of the electron
density of our crystal because we designed it to do exactly that.
Dale Tronrud
>
>
> On Fri, 10 Nov 2017, Keller, Jacob wrote:
>
>>>> My understanding is that EM people will routinely switch to
>>>> diffraction mode when they want accurate measurements. You lose the
>>>> phase information but, since EM lenses tend to have imperfections,
>>>> you get better measurements of the intensities.
>>
>> Only to my knowledge in the case of crystalline samples like 2D crystals.
>>
>>>> Of course the loss of phases is a serious problem when you don't
>>>> have a model of the object as precise as our atomic models.
>>
>> From where does this precision arise, I wonder? I guess priors for
>> atom-based models are pretty invariant. On the other hand, who says
>> that such priors, albeit of many more varieties, don't exist for
>> larger biological samples, such as zebrafish brains and drosophila
>> embryos/larvae? Anyway, right now, the state of the art of modelling
>> in these fluorescence data sets is hand-drawing circles around things
>> that look interesting, hoping the sample does not shift too much, or
>> perhaps using some tracking. But it could be so much better!
>>
>> JPK
>>
>>
>
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