Large Optics Measurements


The category of “large optics” is somewhat nebulous but generally includes optics greater than 1m in diameter. Such large mirrors and lenses are required for the designs of numerous terrestrial and satellite-based astronomy programs, and by defense and security applications.


Monolithic mirrors are regularly produced with diameters up to 8 meters. The Giant Magellan Telescope now in production will include seven such enormous mirrors, six of which are polished with aspheric surfaces. The primary mirror for the Large Synaptic Survey Telescope (LSST) includes both an 8.4 meter primary and a 5.0 meter tertiary mirror, both manufactured from the same large blank. 


It is not surprising that measuring the shape of these giants presents enormous technical challenges. The measurement system typically must be located many meters, or tens of meters, from the test piece, introducing vibration and air turbulence that renders standard interferometers unusable. Asphericity and steep slopes require high resolution as well as precision alignment.


Virtually every major telescope and satellite-based optics program in recent years has relied upon 4D laser interferometers to ensure its success.

4D PhaseCam Twyman-Green interferometers are the industry choice for measuring large concave spherical optics. PhaseCam systems can measure despite vibration and turbulence to accommodate long stand-off distances. High power allows PhaseCams to measure aspheres with a CGH (computer generated hologram) in the optical path.

AccuFiz Fizeau interferometers can measure large concave and convex spherical optics, with a wide range of reference optics to match test parts. The high resolution system AccuFiz 6MP can measure steep slopes such as those on aspheric surfaces. When used with our very large aperture beam expanders the AccuFiz a collimated beam can measure flat optics up to 800mm in diameter. With a converging optic AccuFiz can measure large convex mirrors and optics as well.

Both PhaseCam and AccuFiz can characterize non-traditional elements, such as testing aspheres and quantifying the modal response of deformable mirrors. 

The NanoCam Sq surface profiler measures the roughness of polished and supersmooth optics. Vibration-insensitive and compact, the NanoCam Sq can be mounted on a gantry or robot arm for measuring waviness and high frequency texture anywhere on large optics, wherever you want to sample the surface.


Added Technical Resources

Optical Metrology for Large Telescope Optics

Requirements for Meter-class metrology, large concave mirrors, and space structural support  hardware. Touching on Mirror Segment alignment, adaptive optics and aspheric optics.

Measuring Large Mirrors with Secondary and Tertiary Support Structures

An analysis system with an extensive tool set for measuring mirrors partially obscured by support structuress.

Products for Large Optics


AccuFiz Fizeau interferometers can measure concave and convex spherical optics, with a wide range of reference optics to match test parts. High resolution systems like the AccuFiz 6MP can measure steep slopes.


4D PhaseCam Twyman-Green interferometers are the industry choice for measuring concave spherical optics, from several millimeters to tens of meters in diameter.

NanoCam Sq 3D profilometer

Measure surface roughness and defects on smooth and supersmooth surfaces. As a dynamic instrument, it can take measurements anywhere on a large optic without worrying about vibration. Highly portable, and pick-and-place automation compatible.

Do you need custom-built metrology?

Get a free consultation. 4D Technology’s mechanical, optical, electrical and software engineering teams are standing by to evaluate what it would take to meet your application’s requirements.

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