Spots don't fall off with the inverse square law. It is a very easy
experiment to do. Just take exposures at several distances and scale
the data together, noting the correction for air absorption.
A good reference for the underlying theory is Chapter 6 of M. M.
Woolfson's book (1997). But
Hi,
There's no real conflict at all here, and I am surprised at the amount of
time spent on this subject :)
I hope that people *do* mention which units they refer to and that they
*don't* name new units without reasonable justification. If I encounter a
situation where a number that is relevant t
That could depend where the beam is focused- if focused on the crystal
then it diverges from that point, like the bulk of the scattered x-rays
that give rise to background. If focused on the detector, it could actually
be convergent over that distance while the scattering is divergent.
Also on th
I mean I'm not assuming an ideal beam.
On Nov 23, 2009, at 2:54 PM, Richard Gillilan wrote:
It seems to be widely known and observed that diffuse background
scattering decreases more rapidly with increasing detector-to-sample
distance than Bragg reflections. For example, Jim Pflugrath, in hi
The flux from the spots fall off as the square as well. Assuming that
flux at the detector is linear with respect to measured intensity, I'm
not sure where the benefit would be. I'm also assuming an ideal beam
and ignoring other sources of noise.
James
On Nov 23, 2009, at 2:54 PM, Richar
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It seems to be widely known and observed that diffuse background
scattering decreases more rapidly with increasing detector-to-sample
distance than Bragg reflections. For example, Jim Pflugrath, in his
1999 paper (Acta Cryst 1999 D55 1718-1725) says "Since the X-ray
background falls off as
The BCA Biological Structures Group winter meeting will be held on
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The theme of the meeting is 'Pathological Proteins' - the aim is to
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Sorry I'm not clear exactly what your question is, but it seems to me
that my paper will actually need fewer words than yours, since I can
leave out all occurrences of 'radian' and 'steradian' with no loss of
meaning! This quantity you're talking about presumably has a name
(otherwise how are we g
James,
I could not help typing something!
Consider a circle of radius R, its circumstance L is then 2*Pi*R.
Both R and L have the same unit, the 2*Pi angle is unitless.
SI defines the unit of angle to be Ran just because this unitless
number is different because it is obtained by the length of a
Quoting James Holton :
Now the coefficients of a Taylor polynomial are themselves values of the
derivatives of the function being approximated. Each time you take a
derivative of "f(x)", you divide by the units (and therefore dimensions)
of "x". So, Pete's coefficients below: 1, -1/6, and 1/12
I agree that the official SI documentation has priority, but as I read it there
is no discrepancy between it and Wikipedia. The official SI position (and that
of NIST and IUPAC) is that the radian is a dimensionless unit (i.e., a unit of
dimension 1).
Quoting at length from the SI brochure:
"
I would like to apologize to everyone for creating such a busy thread
(an what could perhaps be construed as an occasionally belligerent
tone), but I really do want to know the right answer to this! I am
trying to model radiation damage from first principles, and in such
models you cannot have
I would believe that the official SI documentation has precedence over
Wikipedia. In the SI brochure it is made quite clear that Radian is
just another symbol for the number one and that it may or may no be
used, as is convenient.
Therefore, stating alpha = 15 (without anything else) is per
Not at all !
If I want to compute the sinus of 15 degrees, using the series
expansion, I write
X = 15 degrees = 15 * pi/180 = 0.2618
because, 1 degree is just a symbol for the unitless, dimensionless
number pi/180.
I plug this X into the series expansion and get the right result.
Marc
James,
I don't think that you are re-phrasing me correctly. At least I can
not understand how your statement relates to mine.
You simply have to tell us whether a value of 27.34 read from the last
column of a PDB file means :
(1) B = 27.34 Å^2 , as I (and hopefully some others) think, or
Nice
Scott
On Mon, Nov 23, 2009 at 1:07 PM, Ed Pozharski wrote:
> Ian,
>
> On Mon, 2009-11-23 at 17:34 +, Ian Tickle wrote:
> > Ed,
> >
> > > For instance, if angles are measured in degrees and x<<1
> > > sin x ~ pi * x / 180
> > > sin x ~ x
> >
> > Your equations cannot simultaneously be tr
Ian,
On Mon, 2009-11-23 at 17:34 +, Ian Tickle wrote:
> Ed,
>
> > For instance, if angles are measured in degrees and x<<1
> > sin x ~ pi * x / 180
> > sin x ~ x
>
> Your equations cannot simultaneously be true & in fact the 1st one is
> obviously wrong, the 2nd is right. In the 1st case I
That's still only by convention. Which was the point of this thread to
begin with: let's settle on a convention.
I'm surprised this is contentious.
phx.
Ian Tickle wrote:
No, just like this: 'solid angle = 1.234' (or whatever its value is).
Since the conversion unit 'steradian' = 1 (i.e.
Dale's assertion that the exponent has units of radians comes from
Euler's formula:
exp(i*x) = cos(x) + i*sin(x)
which does indeed require that "x" has units of radian, or whatever it
is you feed your sin() functions. However, not every exponential has an
"i" in it, and the general complex-n
No, just like this: 'solid angle = 1.234' (or whatever its value is).
Since the conversion unit 'steradian' = 1 (i.e. the dimensionless pure
number 1) identically, 'a solid angle of 1.234 steradians' is identical
to 'a solid angle of 1.234': the unit 'steradian' is redundant.
Cheers
-- Ian
> --
So... how do you measure or report a solid angle without invoking the
steradian? sterdegrees?
Ian Tickle wrote:
James, I think you misunderstood, no-one is suggesting that we can do
without the degree (minute, second, grad, ...), since these conversion
units have considerable practical value.
Argument from authority, from the omniscient Wikipedia:
http://en.wikipedia.org/wiki/Radian
"Although the radian is a unit of measure, it is a dimensionless quantity."
"The radian is a unit of plane angle, equal to 180/pi (or 360/(2 pi)) degrees,
or about 57.2958 degrees, It is the standard
James, I think you misunderstood, no-one is suggesting that we can do
without the degree (minute, second, grad, ...), since these conversion
units have considerable practical value. Only the radian (and
steradian) are technically redundant, and as Marc suggested we would
probably be better off wit
how does the equation
cos(x)= (exp(ix) + exp(-ix))/2
and the sine equivalent fit into this? Clearly exponentials are not restricted
to angles ... indicating that x (and by implication angles) have no dimensions.
Marc Schiltz's previously cited Taylor expansion demonstrates this even bett
Ed,
> For instance, if angles are measured in degrees and x<<1
> sin x ~ pi * x / 180
> sin x ~ x
Your equations cannot simultaneously be true & in fact the 1st one is
obviously wrong, the 2nd is right. In the 1st case I think you meant
(substituting 'x*deg' for 'x' in your correct 2nd equation)
Marc SCHILTZ wrote:
Hi James
I must confess that I do not understand your point. If you read a
value from the last column of a PDB file, say 27.34, then this really
means :
B = 27.34 Å^2
for this atom. And, since B=8*pi^2*U, it also means that this atom's
mean square atomic displacement i
Zitat von marc.schi...@epfl.ch:
Dale Tronrud wrote:
While it is true that angles are defined by ratios which result in
their values being independent of the units those lengths were measured,
common sense says that a number is an insufficient description of an
angle. If I tell you I measure
Did anyone else encounter problems with the
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Once unpacked, the installation starts as normal; after accepting the
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Dale Tronrud wrote:
While it is true that angles are defined by ratios which result in
their values being independent of the units those lengths were measured,
common sense says that a number is an insufficient description of an
angle. If I tell you I measured an angle and its value is "1.5"
Just because something is dimensionless does not mean it is unit-less.
The radian and the degree are very good examples of this. Remember, the
word "unit" means "one", and it is the quantity of something that we
give the value "1.0". Things can only be measured relative to something
else, an
Dear ccp4bb,
I learned that there is a prize for the best page made on the
Wikipedia-style encyclopedia of biological macromolecular structures
Proteopedia ( http://www.proteopedia.org ). For pages made before Dec 31
this year. The prize is a 32Gbyte IPod Touch. What I have done myself
(not b
On Sun, 2009-11-22 at 23:33 -0800, Dale Tronrud wrote:
> I could be describing my angle as
> 1.5 radians, 1.5 degrees, or 1.5 cycles (or 1.5 of the mysterious
> "grad" on my calculator).
I thought that use of degrees is based on dividing a circle into 360
parts - roughly one per day (then in geo
Hi:
I have an old fortran code that I used on an SGI Irix system. i would
like to use it on a linux (Ubuntu).
How to compile this code which uses some ccp4 libraries? What is the
command for this?
Thanks
Subbu
James Holton wrote:
No No No! This is not what I meant at all!
I am not suggesting the creation of a new unit, but rather that we name
a unit that is already in widespread use. This unit is A^2/(8*pi^2)
which has dimensions of length^2 and it IS the unit of B factor. That
is, every PDB file
This is absolutely correct. Radian is in fact just another symbol for 1.
Thus : 1 rad = 1
From the official SI documentation
(http://www.bipm.org/en/si/si_brochure)(section 2.2 - table 3) :
"The radian and steradian are special names for the number one that
may be used to convey information abo
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