APQ Compensation
To ensure thermal invariance, zero play and high centering accuracy, we developed a thermally compensated optical lens cell (compensation cell) for our Fluorite Quadruplet Polychromats.
The new compensation cell is designed for an operating temperature range from -20 °C to 40 °C (working temperature interval ≥ 60 K) and a storage and transport range from -40 °C to 60 °C.
The design of the cell assembly is outlined below. It was developed by Frank Dionies, Dipl.-Ing. (FH), with technical advice from Prof. Dr.-Ing. Manfred Steinbach (see also the related patent application).
- Requirements
- Objectives
- Compensation cell
In precision instrument manufacturing – for astronomy or space applications, for example – certain components must retain their position and shape within defined limits even under demanding operating conditions.
A refractor objective comprises several components: optical elements made from different materials with defined optical properties, radii and thicknesses, together with retaining elements and the lens cell. Cells for polychromatic objectives are precision assemblies.
Continue readingEvery material has its own coefficient of thermal expansion (CTE). In an assembly made from different materials, the individual components therefore expand and contract by different amounts as temperature changes.
Within the assembly, these differences can produce displacements and stresses that degrade the image quality of the optical system.
The development objective was a lens cell that remains thermally invariant, free of play and accurately centered over substantial temperature changes without unduly restricting the choice of materials.
For our Fluorite Quadruplet Polychromats, this meant specifying a cell capable of supporting a group of four or more optical elements made from materials with different coefficients of thermal expansion.
The elements may be coupled by an intervening optically transparent fluid (oil) and must be positioned and retained by the cell in precisely defined locations relative to one another.
Radial and axial play must be avoided. Neither thermal stresses over a working temperature interval of at least 60 K (operating range -20 °C to 40 °C) nor changing gravitational loads on the elements may produce functionally significant effects, in particular any degradation of the overall optical performance.
Continue readingDifferential expansion during transport and storage must not cause irreversible decentering or damage. During operation, the elements must remain centered and free from harmful stress despite differential expansion within the specified tolerance range.
The optical elements must remain centered throughout the operating temperature range to the accuracy defined by the tolerance analysis.
After exposure to any temperature within the extended storage and transport range (-40 °C to 60 °C), the elements must return to their original positions with the required accuracy once the objective returns to its operating temperature range.
An essential prerequisite was the precise determination of the coefficients of thermal expansion of all materials involved, because manufacturers' catalog values are not sufficiently accurate for the compensation calculation.
We therefore had the expansion coefficients measured by an independent test institute over a range from -100 °C to 100 °C with a resolution of 0.125 nm, covering the relevant terrestrial and atmospheric temperature range.
The new compensation cell offers the following advantages over classic cells:
Each optical element is supported at six or more contact surfaces and held in a stable position by compensation elements as temperature changes.
The optical system reaches thermal equilibrium quickly without compromising image quality.
The elements remain securely located in the cell, preventing movement-related degradation and eliminating the need for readjustment caused by such movement.
The new design fully meets our objective of mounting the Fluorite Quadruplet Polychromats in innovative thermally compensated cells.
We filed a patent application for the new cell design with the German Patent and Trade Mark Office (DPMA) under the title Thermisch kompensierte optische Fassungsbaugruppe, file number 10 2017 008 286.3.
Current development
The compensation lens cell has been further optimized. In addition to another improvement in its mechanical design, the cell weight has been reduced by a further 0.15 kg. The weight reduction does not compromise stability, precision or thermal compensation.