What the component does
The disc encodes rotation about its own axis. A telescope crosshair supplies a pointing reference, but does not replace angle encoding.
Surveying Instruments & Angular Measurement
Provide separate angular references for the horizontal and vertical axes of an optical angle-reading surveying instrument.
Where the component works
Horizontal angle: rotation about the upright axis
- Instrument rotates about its upright axis
- Enlarged horizontal angle code disc
- Readhead fixed to the reference structure
The disc follows the rotating structure. Its pattern passes the readhead to provide the horizontal angular reference.
Vertical angle: telescope elevation
- Telescope pivots about the horizontal axis
- Enlarged vertical angle code disc
- Readhead fixed relative to that axis support
A separate disc and readhead measure telescope elevation. The electronic distance measurement system is a separate functional chain.
How the system works together
Incremental divisions need an established reference. Absolute tracks identify coded positions; Gray coding is one compatible absolute-code approach.
What an unsuitable choice changes
Disc eccentricity, bearing motion and readhead alignment affect the complete instrument. More pattern divisions alone do not establish angular accuracy.
Components for this application
Incremental Encoder Discs
Periodic angular reference for instruments with a compatible reference strategy.
Absolute Encoder Discs
Compare absolute code architectures for direct angular identification.
Gray Code Encoder Discs
Parallel Gray-code tracks for a matching angular readhead.
Other relevant products and configurations
Binary Code Encoder Discs
Natural-binary track layouts for compatible decoders.
Crosshair Reticles
Visible pointing reference in a compatible telescope focal plane.
Principles and selection details
System context
A surveying instrument relates a line of sight to measured horizontal and vertical angles. In an optical angle-reading architecture, a patterned disc or ring moves relative to a readhead so the system can determine axis rotation. A telescope reticle supplies a separate visible reference for pointing at the target.
Selection considerations
Choose the disc pattern around the actual readhead, available radius and startup reference method. Absolute coding can identify an angular interval directly; incremental divisions need an established reference. The reticle pattern and its focal-plane position are chosen around the telescope optics.
Two axes, two angular references
Horizontal and vertical angle sensing must each relate the optical pattern to a mechanical axis and a zero convention. The shape of the disc, central clearance and track location follow the instrument structure. A design for a motor encoder is not automatically suitable for a surveying axis.
Where angle errors can enter
Pattern placement, disc eccentricity, bearing motion, readhead alignment and temperature can contribute to the complete measurement. Some instruments use multiple reading positions to manage selected errors. Component geometry and whole-instrument angle accuracy should therefore be evaluated separately.
Aiming and angle readout work together
Crosshairs help the operator establish a repeatable sighting reference, while the encoder reports the rotation of the instrument axis. Both functions matter, but the reticle does not replace angle encoding and the code disc does not establish the telescope sight line by itself.
Related applications
Planning your project
Surveying instruments use encoder discs as angle references for rotating axes and reticles as visible aiming references inside the telescope.
Relate the encoder to the mechanical axis and the reticle to the line of sight; the two components perform different jobs.
Can a reticle replace an encoder disc?
No. A reticle supports observation and aiming. An encoder pattern is read by a head to determine axis position.