This depends on the size of the beam, relative to either the slit width for "small" beams, or on the aperture diameter for "large" beams. Standard NanoScan apertures are either 3.5mm or 9mm, with 1.8µm, 5µm, 10µm, or 25µm slits. Standard Large Aperture (LA) NanoScan apertures are 12.5mm or 25mm, with 10µm or 25µm slits. (Please note that LA models are no longer available.)
The scanning slit introduces a systematic convolution error in measured spatial beam diameters that depends on the slit width "w" and the beam diameter "d". This error increases as the ratio of the slit width to the beam width increases. However, since the error is systematic it can be corrected, and this is discussed in detail in the NanoScan Manual Section 4.4.9 for TEM00 Gaussian beams.
The NanoScan accuracy specification for measurement of dslit diameter is 3% for all models of scan heads. As a rule-of-thumb, the convolution error only becomes significant for small beams when the 1/e2 beam diameter is of the order of 4 times the slit width, or d≤4w. For a Gaussian beam with d=4w the error is only approximately 3.7%.
The aperture diameter determines the largest beam that can be measured, and this depends on the shape of the beam, say Gaussian or Flat Top. A Flat Top beam can be almost the aperture size. To measure d4 it is necessary to acquire the full profile including the tail out to where the amplitude is <1digital count. For a Gaussian beam with 12-bit digitization, the beam is then limited to ~1/2.1 or ~0.476 × the aperture dimension. To be on the safe side a good rule-of-thumb here is to use a factor of 0.4, so the aperture is 2.5 × the 1/e2 beam diameter. However, if one wants to just measure a clip-level dslit diameter, it is only necessary to acquire the profile out to slightly less than the clip-level diameter. As examples, it is possible to measure a Gaussian beam with 8.9mm 1/e2 diameter using a 9mm aperture scan head. It is also possible to measure the FWHM diameter of a Gaussian beam with 15mm 1/e2 diameter.