Magnification is generated by the instrument knowing the final lens focal length and adjusting the scan to that appropriate to the selected magnification. When considering x-ray analysis many of the same set up conditions relate.

The variables in the system that may vary the accuracy are:-

1.        High Voltage – stability over time, the general drift as HT tank warms to the point of heat gained is equal to heat lost.

2.        Magnetic history in the final lens and resultant focal values, is this very great and is one able to stabilise it?

3.        The spread of current under a specific WD value (often several turns of the focus control before a change occurs), it is ideal if you are able to measure the current or voltage passing through the final lens.

Tests that may need to be made:-

a)         A range of measurements over a period of one hour with HT on to determine the point of stability.

b)         Experiments with magnetic history, how much variation do repeated out and in focus adjustments make

c)         Experiments with WD factors and determination of a standard procedure to ensure that the specimen is in exactly the same position

d)         Accuracy of the instrument's inbuilt standardisation if such an adjustment is available probably through magnetic history?

Operating Procedures to retain accuracy

 i)          Set the focus at the desired WD or lens current..

 ii)          Bring the specimen height (Z) to enable focus to be attained at the position set in (i).

 iii)         Run a magnetic history correction.

 iv)         Re focus with the specimen height (Z)

 v)          Repeat (iii) and (iv) until the specimen remains in focus.

 vi)         Now during operation DO NOT change the focus other then by adjusting the specimen height (Z).