A complete understanding of how an orientation distribution contributes to a cryo-EM reconstruction remains lacking. It is necessary to begin critically assessing the set of views to gain an understanding of its effect on experimental reconstructions. Toward that end, we recently suggested that the type of orientation distribution may alter resolution measures in a systematic manner. We introduced the sampling compensation factor (SCF), which incorporates how the collection geometry might change the spectral signal-to-noise ratio (SSNR), irrespective of the other experimental aspects. We show here that knowledge of the sampling restricted to spherical surfaces of sufficiently large radii in Fourier space is equivalent to knowledge of the set of projection views. Moreover, the SCF geometrical factor may be calculated from one such surface. To aid cryo-EM practitioners, we developed a graphical user interface (GUI) tool that evaluates experimental orientation distributions. The GUI returns plots of projection directions, sampling constrained to the surface of a sphere, the SCF value, the fraction of the empty region of Fourier space, and a histogram of the sampling values over the points on a sphere. Finally, a fixed tilt angle may be incorporated to determine how tilting the grid during collection may improve the distribution of views and Fourier space sampling. We advocate this simple conception of sampling and the use of such tools as a complement to the distribution of views to capture the different aspects of the effect of projection directions on cryo-EM reconstructions.

目前仍缺乏对方向分布如何促进冷冻电镜重建的完整理解。有必要开始批判性地评估这组视图，以了解其对实验重建的影响。为此，我们最近建议方向分布的类型可能会系统地改变分辨率测量。我们引入了采样补偿因子（SCF），它结合了收集几何结构如何改变光谱信噪比（SSNR），而不考虑其他实验方面。我们在这里表明，仅限于傅立叶空间中半径足够大的球面的采样知识相当于投影视图集的知识。此外，SCF几何因子可以从一个这样的表面计算。为了帮助冷冻电镜从业者，我们开发了一种图形用户界面 (GUI) 工具，用于评估实验方向分布。 GUI 返回投影方向图、球体表面的采样约束、SCF 值、傅立叶空间空白区域的分数以及球体上各点采样值的直方图。最后，可以结合固定的倾斜角度来确定在收集期间倾斜网格如何可以改善视图的分布和傅里叶空间采样。我们提倡这种简单的采样概念，并使用此类工具作为视图分布的补充，以捕获投影方向对冷冻电镜重建影响的不同方面。