As the effect is stronger as smaller the focal spot would be, a calculation for a tiny focal spot of nominal 0.2mm [ ] in both directions in the central beam was done. The target angle was 11°, the angle for the test was +/-20° (N->S) +20°/-10° (W->E) and covers the full active area of beam.
A tube similar to the calculation was built and the focal spot size was tested using the ASTM E1165 user method for the calculation of the tiny focal spot. In difference to the post before now the minimum and maximum angles are shown.
The measured values differ from the calculated values. Even if the real values are not as bad as calculated there is a factor of 2 still. Hint: The values are smaller compared to the EN12543-2 - the reasons are explained .
When you measure the focal spot size you should have in your mind that the given focal spot value is for the central beam only.
The good news: A larger angle of the target would reduce the effect and the trend for smaller targets is going back to the target angles of about 20°
.