Tecnai on-line help manual -- Options - UT Southwestern

<strong>Tecnai</strong> on-line help Options 25Options Software version 2field of a lens can be made reproducible by normalization, a procedure wherein the lens is cycled updown-upand then back to the desired setting. Hysteresis can affect the position of the beam (due tochanges in C1 - spot size and C2 - intensity) as well as the image or diffraction pattern (mostly theprojector system consisting of the diffraction, intermediate and projector 1 and 2 lenses). In order toachieve reproducible switching between the Low Dose states it is necessary to normalize lenses. Inorder to allow full flexibility in this, the user can select which lenses should be normalized during thepossible switches. The default settings are displayed in the panel above. It is not advised to normalizethe objective lens when switching between Focus and Exposure (this can affect the focus).NormalizeNormalizes the lenses selected by the check boxes to the right of the button.Switch Search to Focus via ExposureTo have settings more reproducible (especially when the Objective lens is normalized between Searchand Focus) it may help to proceed from Search to Focus via the Exposure state (the beam will remainblanked during all the transitions). The actual sequence followed is then Search > Focus > Exposure >Focus.User buttonsUnder User buttons, the user defines which <strong>Tecnai</strong> Control Pad User Buttons (L1 .. L3, R1 .. R3) areused by Low Dose and which button does what. If a user button name (L1..R3) is listed in the drop-downlist of a function, that function is assigned to a <strong>Tecnai</strong> User Button (otherwise the list will display None). Ifa new selection is made by selecting another entry in a list, the software will re-assign any functionpreviously assigned to the particular User Button (if present) by shifting it to the next empty User Button.Dose measurementIn the Exposure state you can measure the dose falling on the specimen. The dose is derived from thescreen current falling on the main screen and the magnification. The dose is reported in electons persquare nanometer per second. In order to derive the dose for the actual exposure, multiply by theexposure time used. If the small (focusing) screen is out, the diameter of the beam must be entered(unless it is larger than the main screen). If the small (focusing) screen is in, the beam must be largerthan the small screen for a proper measurement.Dose measurement is continuous if the Continuously check box is checked. However, the Dosemeasurement will only run in the Exposure state (if Low Dose is switched to another state, the dosemeasurement is stopped). You can stop a continuous dose measurement by pressing the (now yellow)button again. If the Continuously check box is unchecked, a single dose measurement will be done.The dose measurement assumes a fudge factor of 1.4 for the loss of backscattered electrons from themain or small screen (determined for 200 kV by comparison of the screen current and currentmeasurement with a proper Faraday cage). Should you find that the dose measurement on yourmicroscope deviates significantly from measurements by other methods, then it is possible to adjust thefudge factor in the registry (a string value in HKEY_LOCAL_MACHINE\Software\Philips\LowDose\BSFactor).