Despite what some manufacturers claim, correct overhang length and alignment is critical to obtaining proper audio reproduction without distortion and avoiding unnecessary record and stylus wear. This is a precision task and cannot be done by eye or guess work alone.
While designing our own arms, AVID developed a computer program to calculate optimum alignment, taking into account the arms correct geometry. Every radial tracking arm with differing geometry will have different null points (were distortion is zero) and whilst our universal alignment gauge will be very accurate, the arm specific alignment gauges are 100% precise to the arm involved. Currently we have specific gauges for SME, Rega and Linn arms.
The mirrored surface makes all measurements easier and more accurate. The mirror's parallax effect allows the elimination of sighting errors, and provides better lighting, critical in making fine adjustments on small parts. The two null points on the specific alignment tools are surrounded by parallel lines, making cartridge body alignment, both front and rear very precise and the long 'tail' alignment line is extended so that the stylus itself can be checked and aligned in case of cartridge manufacturing errors.