The idea of RAID (redundant array of inexpensive disks) must seem
pretty silly to the Brits- disks may be inexpensive but they're
not free- why waste money on a redundant system?
Ethan Merritt wrote:
On Tuesday, May 14, 2013 01:58:06 pm Colin Nave wrote:
The use of the term redundancy (real or otherwise!) in crystallography
is potentially misleading as the normal usages means superfluous/ surplus
to requirements.
That may be true in the UK, but on this side of the pond "redundancy"
normally refers to ensuring a safety margin by having more of whatever
than is strictly needed for functionality, so that even if some of the
whatsits fail you have enough remaining to go on with. The use of
the term in crystallography is perfectly normal to American ears.
Here's a definition from Wikipedia
"redundancy is the duplication of critical components or functions
of a system with the intention of increasing reliability of the
system..."
just another tidbit of cross-pond difference in language.
Ethan
The closest usage I can find from elsewhere is in information theory where it
is applied for purposes of error detection when communicating over a noisy
channel. Seems similar to the crystallographic use.
The more relevant point is what sort of errors would be mitigated by having
different paths through the crystal. The obvious ones are absorption errors and
errors in detector calibration. Inverse beam methods can mitigate these by
ensuring the systematic errors are similar for the reflections being compared.
However, my interpretation of the Acta D59 paper is that it is accepted that
systematic errors are present and, by making multiple measurements under
different conditions, the effect of these systematic errors will be minimised.
Can anyone suggest other sources of error which would be mitigated by having
different paths through the crystal. I don't think radiation damage (mentioned
by several people) is one.
Colin
From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Frank von
Delft
Sent: 14 May 2013 14:23
To: ccp4bb
Subject: [ccp4bb] Fwd: Re: [ccp4bb] reference for "true multiplicity"?
George points out that the quote I referred to did not make it to the BB --
here we go, read below and learn, it is a most succinct summary.
phx
-------- Original Message --------
Subject:
Re: [ccp4bb] reference for "true multiplicity"?
Date:
Tue, 14 May 2013 09:25:22 +0100
From:
Frank von Delft
<frank.vonde...@sgc.ox.ac.uk><mailto:%3cfrank.vonde...@sgc.ox.ac.uk%3e>
To:
George Sheldrick
<gshe...@shelx.uni-ac.gwdg.de><mailto:gshe...@shelx.uni-ac.gwdg.de>
Thanks! It's the Acta D59 p688 I was thinking of - start of discussion:
"The results presented here show that it is possible to solve
protein structures using the anomalous scattering from native
S atoms measured on a laboratory instrument in a careful but
relatively routine manner, provided that a sufficiently high
real redundancy is obtained (ranging from 16 to 44 in these
experiments). Real redundancy implies measurement of
equivalent or identical re�ections with different paths through
the crystal, not just repeated measurements; this is expedited
by high crystal symmetry and by the use of a three-circle (or )
goniometer."
Wise words...
phx
On 14/05/2013 08:06, George Sheldrick wrote:
Dear Frank,
We did extensive testing of this approach at the beginning of this millenium -
see
Acta Cryst. D59 (2003) 393 and 688 - but never claimed that it was our idea.
Best wishes,
George
On 05/14/2013 06:50 AM, Frank von Delft wrote:
Hi, I'm meant to know this but I'm blanking, so I'll crowdsource instead:
Anybody know a (the) reference where it was showed that the best SAD data is
obtained by collecting multiple revolutions at different crystal offsets (kappa
settings)? It's axiomatic now (I hope!), but I remember seeing someone
actually show this. I thought Sheldrick early tweens, but PubMed is not being
useful.
(Oh dear, this will unleash references from the 60s, won't it.)
phx
