Optical Surfaces has delivered five challenging lenses to Amos, which is supplying the Time Delay Integration (TDI) optical corrector unit for the International Liquid Mirror Telescope (ILMT).
Due to be installed at Devasthal in India, the International Liquid Mirror Telescope is made up of three main components: a primary rotating mirror, a focal structure and a CCD camera with optical corrector.
The ILMT uses Liquid Mirror technology whereby the primary mirror of the telescope is a rotating container with reflective liquid mercury in it.
The surface of the spinning liquid takes the shape of a paraboloid.
As the primary mirror of the ILMT is parabolic, the off-axis imaging is distorted by spherical, coma and time delay integration (TDI) aberrations.
In order to correct these off-axis aberrations and to increase as much as possible the field of view of the telescope, an optical corrector must be inserted before the CCD camera.
Lenses manufactured by Optical Surfaces are to be assembled into Amos' TDI optical corrector, which, beside the basic field correction, aims at correcting for the trajectory of stars passing by at the zenith of the ILMT.
The supplied high-precision lenses were of various sizes, with the largest of 465mm diameter steep meniscus.
In manufacturing the lenses, strict tolerances were required in many physical parameters, including centre thickness, radii of curvature, relative positions and references.
In addition, the lenses had to be machined with a wedge and decentre between the mechanical and the optical axis, while an accurately positioned reference slot would allow the precise positioning of the lenses within the system.
The manufacturing tolerances required careful planning and execution from beginning to end in order to ensure that all parameters would converge simultaneously to their target values'.
The size and depth of the larger meniscus lens meant that conventional polishing machines were not adequate to polish the two surfaces.
Optical Surfaces designed and built a purpose-polishing machine to allow multiple accurate and synchronised motions (including an accurately defined swinging motion on the polishing tool) to generate the spherical surfaces with the correct radii of curvature and shape.
After the polishing was complete, the lenses were machined to generate a wedge and decentre between the mechanical and optical axis.
For that reason, the polished lenses were placed accurately in position for the machining and several mechanical and optical reference surfaces were either attached or generated on the substrates.
The latter gave OS the flexibility and the capability to remove and re-place the lenses back at their exact location during machining.
The multiple references gave confidence on the positioning process while eliminating single points of failure.
Finally, it allowed Optical Surfaces to verify the validity of the method, before and after completion, by using the CMM measuring capability at Rutherford Metrology Facility, UK.
