Motion control for optics, photonics and telescopes

In short:Optical motion is about tiny, smooth, repeatable moves: lens groups positioned to a micrometre, stages scanning without vibration, telescope axes tracking to fractions of an arcsecond. Choose the motor for smoothness and holding behaviour (micro steppers, voice coils, piezo or ironless direct drive), put the encoder as close to the optic as you can, and budget for Abbe and thermal errors before resolution.

Typical requirements

Machinery we specify motion for

• Lens focus and zoom drives
• IR and night-vision focus mechanisms
• Microscope and scanning stages
• Surgical microscopes
• Telescope mounts and fibre positioners
• Spectrographs and filter wheels
• Laser processing and beam steering optics
• Laser projection systems
• Machine vision and inspection
• Precision motion stages

What optics and photonics demand from motion

Optical systems move small masses by small amounts, and they notice everything. A lens group that wanders a few micrometres defocuses an image; a stage that vibrates at standstill blurs a scan; a telescope axis that ripples as it tracks smears a long exposure. So motion components here are judged on smoothness, resolution, holding behaviour, heat and vibration, more than on force or speed.

It is a sizeable market. The UK Photonics Leadership Group describes a global photonics market of roughly $1 trillion that has grown by over 6% a year for more than two decades, and puts UK photonics output at £18.5 billion a year. maxon describes the span of optical systems from microscopes to huge space telescopes, plus laser technology, machine vision and sensors.

Where motion components sit in optical systems

FAULHABER’s optics applications illustrate the range: DC motors for focus and zoom in night-vision devices, micro steppers in surgical microscope heads, stepper motors in the MOONS spectrograph and in fibre positioning systems for large telescopes, linear motors in a microscope scanning stage, and brushless motors positioning multispot optics for laser soldering and welding.

Focus, zoom and fibre positioning

Most focus and zoom drives are small steppers or coreless DC motors with a fine lead screw or gearhead. maxon’s camera drive example pairs a 16 mm motor running up to 20,000 rpm with a 16 mm gearhead offering ratios from 4.4:1 to 4,592:1 and up to 0.6 Nm. Voice coils, such as the Akribis AVA flat voice coil, give cog-free short strokes for fast autofocus.

Fibre positioning has become a robotics problem. The SDSS-V focal plane systems each deploy 500 robotic fibre positioners on 2.5 m telescopes, cutting field reconfiguration from about 20 minutes with plug plates to about 3 minutes. Each robot carries science and metrology fibres and must avoid its neighbours, so compact, repeatable two-axis drives matter more than speed.

Worked example: specifying a motorised focus axis

A lens group of 20 g must move over 5 mm with 1 µm resolution and hold position with power removed.

1. Screw: a 0.5 mm lead screw converts one motor revolution into 500 µm.
2. Native step: a 1.8° stepper (200 steps/rev) gives 2.5 µm per full step, too coarse. A 0.72° five-phase stepper such as the Leadshine 5-phase range (500 steps/rev) gives 1 µm per full step.
3. Microstepping: microstepping the 1.8° motor by 8 gives a nominal 0.31 µm, but microstep positions vary with load and detent torque, so do not count on it for accuracy without an encoder.
4. Holding: a fine-lead screw is usually not back-drivable, so the lens holds position unpowered, which also avoids motor heat near the optic.
5. Speed: 5 mm in 0.5 s means 10 rev/s, 5,000 full steps per second on the five-phase motor, which is within reach of small steppers but needs checking on the pull-out torque curve.

If 1 µm must be guaranteed rather than nominal, add a linear encoder on the lens carrier.

Precision stages: accuracy, repeatability and Abbe error

Aerotech separates the terms clearly: resolution is the smallest increment the system can detect or command, repeatability is how closely repeated moves land, and accuracy is closeness to a known standard, verified by laser interferometry to standards such as ISO 230-2. A 1 nm encoder does not make a 1 nm stage.

Abbe error usually dominates. If the scale sits 50 mm from the optical axis and the stage pitches by 10 arcsec (about 48 µrad), the optic moves about 2.4 µm more or less than the scale reads. Mount encoders in line with the work, choose stages with low pitch and yaw, and control temperature, since an aluminium stage grows about 23 µm per metre per degree.

Telescope mounts and direct drive

Large telescopes show what direct drive and modern encoders can do. HEIDENHAIN describes the Gran Telescopio Canarias (10.4 m mirror of 36 segments) with an azimuth scale tape of 48.48 m over about 15 m diameter, giving 0.0003 arcsec resolution and 0.06 arcsec accuracy over 13.2° after compensation. On the 4.3 m Discovery Channel Telescope, stated pointing accuracy is 0.19 arcsec over 360° in azimuth and 0.03 arcsec over 90° in elevation, with the elevation encoders also providing commutation for the motors.

Sidereal tracking is about 15 arcsec per second, so a mount spends most of its life moving very slowly and very smoothly. That favours zero-cogging direct-drive motors such as the Akribis ACD, or curved arc-segment motors like the Akribis ACR built onto a large radius.

Vacuum and cleanroom optics

Photonics, synchrotron and space-optics work often happens in vacuum. Dover Motion prepares stages down to 10⁻⁷ torr with outgassing below 1 × 10⁻⁵ torr·l/s per axis, using vacuum-rated steppers, PTFE-insulated leads, perfluoropolyether lubricant and electroless nickel rather than anodising. Heat is the main limit, so drives cut current when the axis is idle.

In cleanrooms classified to ISO 14644-1, particle generation matters as much as precision. Sealed or bellows-covered stages, cleanroom-grade grease, non-contact encoders and cable carriers chosen for low particle shedding keep optics and wafers clean. Ask suppliers which cleanroom class a stage has been used in, and how it was tested.

Laser systems and beam steering

In laser processing, the stages move the part or the head while the laser fires, so motion quality shows up directly in the cut, weld or mark. Ironless linear motors and direct drives give the smooth velocity needed for consistent energy per millimetre, and controllers that trigger the laser from position rather than time keep spacing even through accelerations. Laser products are classified under IEC 60825-1, so enclosures, interlocks and access doors become part of the motion system’s safety design.

What to send an engineer

• Moving mass, envelope and how the optic is mounted.
• Stroke or angle, and move and settle times.
• Resolution, accuracy and repeatability, and over what travel they apply.
• Holding behaviour required with power off.
• Allowed heat, vibration and stray magnetic field at the optic.
• Environment: cleanroom class, vacuum level, temperature range.
• Feedback and command interface, and production quantities.

Typical pitfalls

1. Buying resolution instead of accuracy, then losing it to Abbe and thermal error.
2. Trusting microsteps for absolute position without an encoder.
3. Putting motor heat next to the optic, causing focus drift as the mount warms.
4. Cogging or gear periodic error on slow tracking and scanning axes.
5. Anodised parts in vacuum that take days to pump down.

Send us the moving mass, travel, accuracy and environment, and our specialist engineering and supply partners can propose motors, stages, encoders and drives for your optical system.

Standards to have in view

• ISO 230-2 (accuracy and repeatability of positioning of numerically controlled axes)
• IEC 60825-1 (safety of laser products)
• EN 60204-1 (safety of machinery: electrical equipment of machines)
• ISO 14644-1 (cleanrooms: classification of air cleanliness by particle concentration)

Which standards apply depends on the end product and its market. Your engineer will confirm what the component itself needs to meet.