Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

Instrument Control: Display Control
The display (oscilloscope) controls are usually screwdriver-adjusted, with the
exception of the scale illumination and power on/off. After initial setup and
calibration, the screwdriver-adjusted controls seldom require additional
adjustment. The controls and their functions for the display unit usually
consist of the following:
• Controls for vertical position of the display on the oscilloscope screen.
• Controls for horizontal position of display on the oscilloscope screen.
• Controls for brightness of display.
• Control for adjusting focus of trace on the oscilloscope screen.
• Controls to correct for distortion or astigmatism that may be introduced as
the electron beam sweeps across the oscilloscope screen.
An optical system with astigmatism is one where rays that propagate in two
perpendicular planes have different foci. If an optical system with astigmatism
is used to form an image of a cross, the vertical and horizontal lines will be in
sharp focus at two different distances.