ADDRESSING MACROMOLECULAR CRYSTALLOGRAPHY CCD DETECTOR SPATIAL VARIATION

这个子项目是许多研究子项目中利用 资源由NIH/NCRR资助的中心拨款提供。子项目和 调查员(PI)可能从NIH的另一个来源获得了主要资金， 并因此可以在其他清晰的条目中表示。列出的机构是 该中心不一定是调查人员的机构。 在高质量的大分子结晶学数据集中，对称性等价的低分辨率反射通常根据数据冗余度的不同而扩展到2-4%的R-merge值。然而，考虑到典型实验设置产生的随机和系统误差的模拟预测了较低的R-合并。在模拟中没有考虑的一个可能的误差源是探测器的响应中的像素到像素的变化。在SSRL光束线上使用的CCD检测器的情况下，可能会出现这种变化，因为每根光纤具有不同的效率，但是点扩展函数使它们在泛光场中都变得模糊。然而，尖锐的衍射点只在一组给定的纤维上平均。为了验证这一假设，从同一晶体的两个不同探测器位置收集了非常冗余的部分数据集。当合并来自探测器的两个不同部分的图像时，R合并作为冗余的平方根，正如随机噪声所预期的那样。然而，当分别处理来自不同探测器位置的数据集时，发现系统差异为2.5？5%。误差的大小与光点大小相关，这进一步表明探测器的像素到像素的变化(即，当在较大数量的像素上平均时，由该效应引入的误差减小)。计划进行更多的实验，以确定这一效应对SAD和MAD中异常信号测量的影响，以形成软件开发的基础，以减轻任何影响。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Symmetry equivalent low resolution reflections in a good quality macromolecular crystallography data set typically scale to an R-merge value of 2-4 % depending on data redundancy. However, simulations taking into account random and systematic errors arising from typical experimental set-ups predict a lower R-merge. A possible source of error not accounted for in simulations is pixel-to-pixel variation in the response of the detector. In the case of the CCD detectors in use at SSRL beam lines, such variation may arise because each optical fiber has a different efficiency but the point spread function blurs them all together in the flood field. However, sharp diffraction spots only average over a given set of fibers. To test this hypothesis, a very redundant partial data set was collected from the same crystal at two different detector positions. When merging the images from two different part of the detector the R-merge falls as the square root of the redundancy, as expected for random noise. However, when processing the data sets from different detector position separately, a systematic difference of 2.5 ? 5 % was found. The size of the error correlates with the spot size, which further suggests a pixel to pixel variation for the detector (i.e., when the error introduced by this effect decreases when averaged over a larger number of pixels). More experiments to determine the influence of this effect on the measurement of the anomalous signal in SAD and MAD experiments are planned to form the basis for software developments to mitigate any effects.