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Alignment CGHs

Our HA50D0 and HA6025 Alignment CGHs are used to optically align CGHs to your interferometer test beam and radius slide axis. On replacing the Alignment CGH with a Custom CGH Null, the spherical or collimated test beam is diffracted to produce a prescribed aspheric wavefront.

How They Work

HA50D0 interferogram at Z=+90 mm An Alignment CGH is a reflective zone plate which diffracts your spherical test beam back onto itself to create an interferogram. By turning adjustment screws to remove tilt and focus fringes, the CGH coordinate system is optically aligned to the focus of your interferometer’s spherical test beam.

Easy Optical Alignment

Mount Alignment CGH facing interferometer and with the transmission sphere focus near prescribed distance.
Adjust decenter and focus to acquire and null alignment fringes.
Replace Alignment CGH with custom CGH null.

Each alignment CGH has multiple pie-shaped apertures and acts like a multi-radius return sphere. By using plus and minus diffracted orders, the same alignment CGH works for both real and virtual foci. Grating patterns are “spherical” to about 1 fringe. Diffraction efficiency is approximately 1% in 3rd order.

If space permits, we incorporate retro-reflective alignment features into each custom CGH null. Even then, an Alignment CGH makes alignment easier because of its greater brightness and large aperture.

HA50D0 Alignment CGH

HA50D0 Alignment CGH

We offer the HA50D0 Alignment CGH for use with our small frame CGH Nulls. It works at 632.8 nm wavelength, providing 12 on-axis foci:

HA50D0 Alignment CGH
Focal Distances from HA50D0 Face (mm)
Real: +20 +33 +55 +90 +150 +250
Virtual: −20 −33 −55 −90 −150 −250


For best accuracy in alignment transfer, the HA50D0 and the custom CGH null must be used in the same orientation and the balls which attach to the CGH positioner must be the 3 nearest the notch. Our M506 CGH positioner and M1206R Adapter Plate each include a “key” bolt that can be used to ensure these conditions are met.

HA6025 Alignment CGH

We offer the HA6025 Alignment CGH for use with our 6025-format CGH nulls. It works at 632.8 nm wavelength, providing 16 on-axis foci:

HA6025 Alignment CGH
Focal Distances from HA6025 Face (mm)
Real: +36 +84 +144 +200 +250 +310 +439 +660
Virtual: −36 −84 −144 −200 −250 −310 −439 −660
Focal Distances from CGH Null (mm)
Real: +42 +90 +150 +206 +256 +316 +445 +666
Virtual: −30 −78 −138 −194 −244 −304 −433 −654


The focal distances from the CGH null face are 6 mm greater than from the HA6025 face. This is because the HA6025 faces the interferometer, whereas our 6025-format CGH nulls invariably face the test optic.

The HA6025 Alignment CGH uses flexure corner mounts for best framing accuracy. CGH nulls designed to be used with an HA6025 should also have flexure corner mounts.

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Page updated: 26 September 2018

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