Performance of a 2k CCD camera designed for electron crystallography at 400kV

摘要

We discuss the performance of a charge-coupled device CCD camera that has been designed foruse in electron crystallographic studies of proteins.There have been many previous publications describingthe characteristics and performance of CCD-basedcameras in electron microscopy; here we focus oncharacteristics relevant to protein studies at400kV.The low exposure that must be used in such studiesproduces a very poor signal-to-noise ratio, so any lossof signal -to-noise ratio in the recording process mustbe avoided. Images must contain a sufficient number ofmolecules to allow identification of the reciprocallattice, thus requiring a large image format.Electron diffraction patterns may contain some spotswith intensity around 10 -7thimes that of the centralbeam, so the largest possible dynamic range is helpful.Some of the characteristics we discuss are most easilymeasured with crystals, but the conclusions also applyfor other work such as single-particle analyses. Thecamera has been optimized forwork at 400kV with a P43scintillator fiber-optically coupled to a CCD with24um pixels. The scintillator in this camera is thickerthan generally used at lower voltages, which providesan adequate signal level but slightly degrades theresolution. Operation at 400kV leads to a point spreadfunction that is broader than the CCD pixel size. Imagesare thus binned by a factor of two to double theeffective pixel size, with the resulting loss of afactor of two in the size of areas that can be recordedin a single frame. A large CCD with a 2048×2048 pixelarray is used to compensate for this loss and providea sufficient signal for the crystallographic imageprocessing used in this work. Images and electrondiffraction patterns recorded on the CCD arecompared with data recorded on photographic film. Whilethe quality of the images recorded on the CCD at thelow exposures required in protein studies is not quiteas good as that on film, electron diffraction datarecorded on the CCD are superior to hat on film. PACS:87.64D;61.16.B