如果发现拍的图片有六方形的暗影或者图片有问题的时，点prepare gain reference ，样品远离观察视野，将光斑散开满屏，然后抬屏就可以点了
prepare dark reference，将荧光屏放下，使得CCD无光源信号，即可操作
说明文件的原文如下：
3.2. Prepare dark reference image
This section only applies to DualVision 600 CCD camera which do not have a TEM shutter control. The DualVision 600 camera is shutterless. Since CCD dark current depends upon the temperature, CCD must reach its set target temperature before acquiring the dark reference image. Before starting the preparation procedure, lower the TEM viewing screen (make sure no electron beam illuminates the CCD sensor during the procedure). Under the Camera menu, choose Prepare Dark Reference and follow the screen instructions.
3.3. Prepare gain reference image
It is important to prepare a new gain reference image after the CCD has reached the equilibrium temperature (normally this takes about 1 -2 hours).  The gain reference image should be checked daily. To prepare gain reference image, remove specimen from the field of view, evenly spread the illumination across the CCD sensor; for wide angle CCD cameras, this could mean that the illumination is evenly spread across the entire viewing screen in the TEM. Under the Camera menu, choose Prepare Gain Reference. Set the target intensity to the value of about half the dynamic range. For example, for 14-bit cameras,  set the value to 8000 and 12-bit  camera  to  2500.  Normally  it is sufficient to  set the frames to average to 4. Then  simply follow  the  instructions  on  the  screen.