This shows the most straightforward way to tell if a NanoScan has a bad slit. Just move the beam across the aperture and see if how the measured beam widths change. They always change a little bit, as the 1/e-squared beam size tolerance is +/- 2% across the aperture. However, if the measured beam width changes by more than this amount as the beam moves through the NanoScan aperture, it’s likely there’s a problem with the slits. Usually at a local area of damage, the beam size becomes larger. That’s because at the damaged area, the local slit width becomes larger and due to the slit convolution effect, the measured beam width increases.
It’s important to note that smaller beam sizes used in this test work better in finding slit damage. A large beam of 3-4 mm will only uncover very large damage, around a millimeter in size. A 100-micron imperfection will largely be missed with a beam larger than 500 microns.
Sometimes it’s difficult to translate the beam in the aperture so another simple test of slit damage is to rotate the NanoScan 90 degrees using the rotation mount. If the beam is truly elliptical, the long axis and short axis should “flip” to the other slit after this rotation. However, if after rotation the same slit measures as large a beam as it did before the 90-degree rotation, it usually means the slit is damaged.
In the example above, we found that when rotating the NanoScan 90 degrees, the yaxis continued to measure 1040 microns; if the beam was truly elliptical, it should have measured 905 microns instead. The x-axis continued to measure around 900 microns after this rotation, owing to the fact this slit was not damaged.
A final signature of slit damage is to look at sensor amplifier gain set by the NanoScan Gain Tracking feature. Usually, the gain value for Axis X and Axis Y are about the same. If one of the NanoScan slits is damaged, the gain values become greatly dissimilar, usually differing by more than 20 due to the fact that one slit is allowing much more light through onto the detector than the other.