Thermally Driven Refocus Mechanism

The quality of images acquired with space based telescopes sig­nificantly improves when induced displacements of mirrors caused by mechanical settling effects from the launch, thermal effects or moisture release of the satellite structure are directly compensa­ted in-orbit.

High precision positioning with STI's Thermally Driven Refocus Mechanism

For this purpose, SpaceTech has developed a thermally driven refocus mechanism for spacecraft tele­sco­pes also suitable for all applications where high precision positioning is required to an accu­ra­cy of 1/1000 mm
(1 micrometer). The mechanism is designed to improve in particular the picture quality of earth observation optical instru­ments.

Key features of the STI Thermally Driven Refocus Mechanism

  • Very robust and insensitive design
  • Small envelope and low mass
  • Comfortable torque margin
  • No necessity for a launch lock device
  • Very low but constant power demand
  • No lubricants

Refocus Mechanism Design PrincipleRefocus mechanism design principle

Key characteristics of the Thermally Driven Refocus Mechanism

Since the driving force of the refocus mechanism is the thermal expansion of a metallic piece a torque margin verification is obsolete in case the mechanical strength of the mechanism is proven.
Similarly, in case the mechanical strength of the mechanism is proven a launch lock device is obsolete since there are no gears or motors involved that could change position due to vibration.
This also implies that no lubricants are necessary and consequently no lubricants are used to avoid contamination of sensitive optics and to avoid the need for special maintenance.

Refocus Mechanism Performance SummaryRefocus mechanism performance summary

STI Thermally Driven Refocus Mechanism's robust design

Due to the mechanism's features, SpaceTech adchieved a very robust design based on actively controlled thermal ex­pan­sion of the adjusting elements.

The mechanism can be implemented with minimum effort into the existing design of telescope structures at the mounting interface of the secondary mirror. There it is designed to move the secondary mirror of the optical telescope in order to adjust the focus of the overall optical system.
The adjustment length can be selected by small modifications to the existing design, also causing a similar change in adjustment accuracy.

Functional tests with the Thermally Driven Refocus Mechanism

Functional tests have been performed with the STI Thermally Driven Refocus Mechanism to size and characterize the thermal expansions as a function of applied heater power and thermal environment, proving the desired adjustment capabilities. More profound testing in a thermal vacuum chamber is necessary to finalize the design.

Refocus Mechanism Functional TestingRefocus mechanism functional testing

In order to prove the mechanical strength of the mechanism tensile testing was performed showing excellent capabilities of the mechanism.

Refocus Mechanism Mechanical Strength TestingRefocus mechanism mechanical strength testing

The development of the thermally driven refocus mechanism has been co-funded by the German Aerospace Centre DLR (reference 50EE0805).

Next step in the Thermally Driven Refocus Mechanism project

In a next step the idea is to implement 3 thermally driven refocus mechanisms into a submirror support frame as one purchase part that would be flexible in diameter size and could provide a thermal conditioning and stabilization of the aperture as an additional function to a space based telescope in order to further improve the picture quality.