Bending mirrors pave way for giant telescopes

A new activity by GSTP with Germany has built a demonstrator model of a  mirror that can be deformed to counteract any decrease in image quality decreases caused by deformations in future large space telescopes by the space environment as well as manufacturing and alignment errors of large optical systems.

The activity, which followed on from a previous GSTP contract, designed and manufactured ten unimorph deformable mirrors, all with an active aperture of 50 mm.

In the future telescopes and optical instruments will need to become larger and larger to gain resolution and sensitivity. Unfortunately, as instruments increase in size it creates issues with mirror manufacturing and opens more possibilities for defects. Additionally, larger instruments are harder to align, more sensitive to the absence of gravity in space and more susceptible to thermoeleastic deformations.

A solution to these issues is to create a mirror in the system that, using actuators, could subtly change its shape to correct for any deformations within the system or for adverse space environment effects. The change in shape is almost invisible to the naked eye, around one micron or less, but it could allow some Earth Observation and Science missions that until now have not been feasible to go ahead.

A solution to these issues is to create a mirror in the system that, using actuators, could subtly change its shape to correct for any deformations within the system or for adverse space environment effects. The change in shape is almost invisible to the naked eye, around one micron or less, but it could allow some Earth Observation and Science missions that until now have not been feasible to go ahead.

Deformable mirrors could mean large enough instruments can be built to gain enough resolution to detect, characterise or even image, Earth-sized exoplanets that are currently too difficult for today’s generation of telescopes to detect. Using deformable mirrors to actively correct the defects of primary mirrors from two to four metres across could also mean satellites are able to guarantee medium to high-quality imaging of Earth down to a few meters  resolution, which could help in disaster monitoring situations.

The ten mirrors manufactured for the activity made improvements on a previous contract where the models were not fully able to survive the harsh mechanical conditions of a launch environment. A successful test campaign was performed on these updated models, partly at the ESTEC Mechanical Testing Laboratory.

The deformation concept was based on piezoelectric material glued below the mirror, which can be actuated due to the wires soldered into this material to locally deform the shape. Early on, the activity encountered  difficulties as the properties of this piezoelectric material can change a lot depending on the supplier and even between the fabrication batches. As such, the activity needed to do a very fine characterisation of the material before it could be implemented.

To carry out this characterisation, four of the ten mirrors were nominally identical and were essentially built retaining the mirror design from the former project but they were made from four different piezo-electric ceramics; two soft piezoceramic materials and two hard piezo-ceramic materials.

The technology is now at a state where, when a mission is presented that requires a deformable mirror, it could be the trigger to prove the mission’s feasibility, but for now it is waiting for its time.

G617-276MM was completed in July 2020.