I thought there was a new paper from the Pilatus people saying fine slicing is 
worth it even beyond the original 1/2 mosaicity rule?

I would think, actually, more gains would made by doing light exposures at, 
say, 1/3 mosaicity, collecting 360 deg, then shifting the detector in 2theta by 
a degree or two to shift uniformly the spots to new pixels, maybe accompanied 
by a kappa change. One would have to remember about the two-theta when 
processing, however!

JPK

-----Original Message-----
From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Gerd 
Rosenbaum
Sent: Thursday, July 13, 2017 3:40 PM
To: CCP4BB@JISCMAIL.AC.UK
Subject: Re: [ccp4bb] weird diffraction pattern

Dear Gerard,

    my "sound like a sales person" was meant as poking a little fun - nothing 
serious, of course.

I and our users like our not-so-new-anymore Pilatus3 6M. It's a great detector 
in many ways. But, there is a lot of hype that this detector solves 
all-problem, for instance fine slicing that is claimed to be only possible with 
a pixel array detector. People get carried away and use
0.01 degree slices even as the mosaicity of their sample is, say, 0.3 degree. 
Slicing beyond 1/3 of the mosaicity will gain you very little - only more 
frames, more processing time.

This discourse is already drifting away from the original topic of the thread 
so I will comment on the other arguments  you made like resolution in a private 
e-mail.

Best regards,

Gerd

On 13.07.2017 14:00, Gerard Bricogne wrote:
> Dear Gerd,
>
>       I can assure you that I have no shares in Dectris nor any 
> commecial connections with them. What I do have is a lot of still 
> vivid memories of CCD images, with their wooly point-spread function 
> that was affected by fine-grained spatial variability as well as by 
> irredicible inaccuracies in the geometric corrections required to try 
> and undo the distortions introduced by the fiber-optic taper. By 
> comparison the pixel-array detectors have a very regular structure, so 
> that slight deviations from exact registering of the modules can be 
> calibrated with high accuracy, making it possible to get very small 
> residuals between calculated and observed spot positions. That, I 
> certainly never saw with CCD images.
>
>       I do think that asking for the image width was a highly 
> pertinent question in this case, that had not been asked. As a 
> specialist you might know how to use a CCD to good effect in 
> fine-slicing mode, but it is amazing how many people there are still 
> out there who are told to use 0.5 or even 1.0 degree image widths.
>
>       Compensating the poor PSF of a CCD by fine slicing in the 
> angular dimension is a tall order. With a Pilatus at 350mm from the 
> crystal, the angular separation between 174-micron pixels is 0.5 milliradian.
> To achieve that separation in the angular (rotation) dimension, the 
> equivalent image width would have to be 0.03 degree. For an EIGER the 
> numbers become 75 microns, hence 0.21 milliradian i.e. 0.012 degree.
>
>       Hence my advice, untainted by any commercial agenda :-) .
>       
>       
>       With best wishes,
>       
>            Gerard.
>
> --
> On Thu, Jul 13, 2017 at 01:25:08PM -0500, Gerd Rosenbaum wrote:
>> Dear Gerard,
>>
>> you sound like a sales person for Dectris. Fine slicing is perfectly 
>> fine with CCD detectors - it takes a bit longer because of the step 
>> scan instead of continuous scan. The read noise issue is often 
>> overstated compared to the sample induced scatter background. If for 
>> fine slicing at 0.05 degree or less the diffraction peaks go too 
>> close to the read noise make a longer exposure - signal goes up, 
>> ratio signal to sample-induced-BG less, as for any fine slicing, same read 
>> noise.
>>
>> It would be helpful to analyze the dense spot packing along layer 
>> lines if we knew the wavelength and the sample-to-detector distance 
>> (assuming this is a 300 mm detector) and the rotation width - as you 
>> pointed out. That would help to distinguish between multiple crystals 
>> (my guess) and lattice translocation disorder. Fine slicing is 
>> definitely needed to figure out what the diffraction pattern at 120 
>> degree could tell you in terms of strong anisotropy .
>>
>> Best regard.
>>
>> Gerd
>>
>> On 13.07.2017 08:20, Gerard Bricogne wrote:
>>> Dear Tang,
>>>
>>>       I noticed that your diffraction images seem to have been 
>>> recorded on a 3x3 CCD detector. With this type of detector, fine 
>>> slicing is often discouraged (because of the readout noise), and yet 
>>> with the two long cell axes you have, any form of thick (or only 
>>> semi-fine) slicing would result in spot overlaps.
>>>
>>>       What, then, was your image width? Would you have access to a 
>>> beamline with a Pilatus detector so that you could collect 
>>> fine-sliced data?
>>>
>>>       I would tend to agree with Herman that your crystals might be 
>>> cursed with lattice translocation disorder (LTD), but you might as 
>>> well try and put every chance of surviving this on your side by 
>>> making sure that you collect fine-sliced data. LTD plus thick 
>>> slicing would give you random data along the streaky direction. Use 
>>> an image width of at most 0.1 degree (0.05 would be better) on a 
>>> Pilatus, and use XDS to process your images.
>>>
>>>
>>>       Good luck!
>>>         Gerard
>>>
>>> --
>>> On Thu, Jul 13, 2017 at 01:21:02PM +0100, Tang Chenjun wrote:
>>>> Hi David,
>>>> Thanks for your comments. Although the spots become streaky in certain 
>>>> directions, I have processed the data in HKL3000 and imosflm, which 
>>>> suggested the C2221 space group (66.59, 246.95 and 210.17). The 
>>>> Rmerge(0.14), completeness(94.8%), redundancy(4.6) are OK. When I tried to 
>>>> run Balbes with the solved native structure, the molecular replacement 
>>>> solution was poor. So I ran Balbes with the split domains of the native 
>>>> structure. Although the solutions were also poor, I found the MR score of 
>>>> one solution above 35. On the basis of this solution, I tried to run 
>>>> Buccaneer and the Rfree could be 0.46. Unfortunately, there are four 
>>>> molecules in the asymmetric unit and it is to hard for me to reduce the 
>>>> Rfree further.
>>>>
>>>> All best,
>>>>
>>>> Chenjun Tang

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