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INTEGRATE: Difference between revisions
→How does INTEGRATE work?
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'''This article is under construction!''' | '''This article is under construction!''' | ||
== Overview == | |||
INTEGRATE is the most important step ("JOB") of XDS. This steps writes the logfile [[INTEGRATE.LP]]. | INTEGRATE is the most important step ("JOB") of XDS. This steps writes the logfile [[INTEGRATE.LP]]. | ||
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XDS does not adjust the integration boxes such as to center them individually on the observed reflections: it only tries to minimize the deviations between observed and calculated spot coordinates by adjusting about a dozen diffraction parameters (those given by REFINE(INTEGRATE)) for the reflections in a certain range of frames (DELPHI). | XDS does not adjust the integration boxes such as to center them individually on the observed reflections: it only tries to minimize the deviations between observed and calculated spot coordinates by adjusting about a dozen diffraction parameters (those given by REFINE(INTEGRATE)) for the reflections in a certain range of frames (DELPHI). | ||
== How does INTEGRATE work? == | |||
The integration algorithm (see [http://dx.doi.org/10.1107/S0021889888007903]) proceeds along the following lines: | |||
# '''pixel-labelling''': the x,y,z, center of each pixel of the detector (z corresponds to phi, and the z pixelsize is delta-phi) is assigned to its closest (predicted) reflection in reciprocal space. As a consequence, ''each pixel of the detector is used for at most one reflection''. | |||
# '''transformation to local coordinate system''': some of these pixels will mostly allow the background estimation, others will mostly contribute to the integration area (but there is not a 1:1 relationship). | |||
# '''average profile''': the average profile is formed on a grid (using the 3D local coordinate system) from strong reflections. The signal part of the profile is defined by those gridpoints of the average profile that are above a threshold (called "CUT" in XDS.INP). | |||
# '''estimating the intensity''': for each reflection, the background is estimated, and the 3D profile is assembled from the pixels contributing to it. Pixels which are mostly background but whose counts are higher than expected (e.g. due to overlap) are rejected. | |||
# '''handling overlap''': not all pixels of a reflection, which would be required to assemble its full profile (whose shape is given by the average profile), may have been observed due to step 1. Therefore, in another pass, for each reflection, the observed fraction of its theoretical profile is calculated. If this fraction (column "PEAK" in XDS_ASCII.HKL) is less than a threshold (called "MINPK" in XDS.INP), this reflection will be discarded ("too much overlap"). If it is above MINPK, the observed intensity (from the incomplete profile) is scaled up with the inverse of the fraction. | |||
Concerning overlap of reflections, this means that: | |||
* there is ''no conceptual difference'' in XDS ''between overlap in x,y'' (due to too close detector, or smeared spots), ''and overlap by phi rotation'' (due to too large delta-phi, or high mosaicity). | |||
* the program does ''not'' look around each reflection to detect an overlap situation, it just gathers the pixels for each reflection. | |||
* if two reflections differ in phi, but have the same position on the detector, then, as a consequence of step 1 the pixels are assigned to that reflection whose phi-calc is closest to the phi of the frame considered. The relative intensities of these reflections are not taken into account because at this stage they are unknown! Thus, ''no deconvolution is attempted''. | |||
* as a user, when your crystal-detector distance was chosen too low, or the reflections are very broad, or if the crystal has a high mosaicity (all of which result in many overlaps), you may try reducing MINPK down to some percentage between 75 (the default) and (say) 50. This will result in more completeness, ''but you should monitor the quality of the resulting data''. Conversely, if you raise MINPK above 75 you will discard more reflections, but the resulting dataset may be cleaner - again: ''check the statistics''. In particular, the latest versions of [[XDSSTAT]] prints out R_meas as a function of PEAK and intensity. | |||
* this method degrades if the average profiles cannot be completey formed, as the scaling-up relies on their accuracy. This may happen if the reflections are too close in x,y and, at the same time, the mosaicity is high (such that no lunes exist, with edges that help constructing the average profiles). ''It is therefore useful to check the printed profiles in INTEGRATE.LP''. Again, the latest versions of [[XDSSTAT]] help to find the best compromise between data quality and completeness. | |||