Preparing the Instrument for Parallax-free Measurements A cross-hair parallax is an error that affects optical and electro-optical instruments such as levels and total stations. The error occurs when the plane of the cross-hair reticle does not coincide with the image plane of the focused object, i.e. the level staff or the prism. This can be easily recognized by moving your head slightly up/down or left/right in front of the eyepiece. The reticle appears to move and does not stay in line with the optical axis. If this error is not corrected the readings of the level 6 | Preparing to Measure staff or the aiming of the prism is incorrect and consequently leads to wrong results. Everytime before you start measuring check the parallax and eliminate as follows, if required: aim the telescope at a high-contrast or bright background (e.g. a piece of paper) focus the cross-hairs by turning the eyepiece drive now focus on the staff or the prism The image plane of both the cross-hairs and the object aimed at now coincide.
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