Wavefront Error Measurement Under Vacuum
About
Artificial satellites and their payloads are subject to comprehensive verification and calibration campaigns prior to their launch. The objective of these campaigns is two-fold:
- To ensure that the integrated system meets the required levels of performance.
- To configure and calibrate the system at an integrated (holistic) level.
In most cases, testing, configuration and calibration is performed in a simulated environment; one which is as cold, dark and empty as space!
Engineers are able to simulate a space environment using purpose-built thermal vacuum chambers. These often large chambers are able to create a little bit of space right here on Earth and facilitate testing and calibration without the need to launch. Earth observation satellites usually incorporate optical instrumentation such as spectrometers; these are special instruments used to measure the characteristics of light. By examining the light reflected from the Earth to the satellite, it is possible to monitor the levels of pollutant gases in Earth’s atmosphere. Given how important Earth sciences are, it is critical that these types of instruments are proven to work correctly in a simulated environment before launch, and to do to this, thermal vacuum chambers need windows!
Precisely engineered windows allow for the calibration of the satellite and its instruments from outside the thermal vacuum chamber; after all, not everything is designed to survive within a space environment. Some of the equipment used simply wouldn’t work if it were inside the chamber.
Project
AEON were asked to design, manufacture and test six optical windows. Of notable challenge, the windows must be strong enough to handle the extreme pressure difference and be highly ‘transmissive’, meaning they do not interfere with the precisely configured light beam that is shone through to the satellite under test.
Part of the project included the measurement of the “Wavefront Error”. Normally, this is not a complex measurement and is reasonably commonplace for optics; the challenge was to ensure that there was no change to the Wavefront Error when each the window was subject to vacuum. Wavefront Error measurement looks to characterise any difference between the ‘ideal’ wavefront and the real-world wavefront. Usually, the wavefront is affected by irregularities in geometry and material imperfections.
(Above) Example of a wave aberration contour map
That’s all well and good, but what does this actually mean?
Well, if you consider the Point Spread Function (PSF); Modulation Transfer Function (MTF) and Phase Transfer Function (PTF), these aberrations (abnormalities) become quite important because they affect the resolution of the transmitted light.
Putting this into a day-to-day situation, consider how a human eye works: the pupil opens and closes to let light in, light passes through the pupil, through a lens and hits the optic nerve in a focal point at the back of the eye. The lens directs all the light to a very small area, creating a clear and focussed image. If the lens is uneven and scatters light in a non-uniform way over a larger area, there will be a reduction in the image quality or resolution. The level of lens aberrations dictates how much correction would be required to rectify the image to the ideal.
Similarly, the glass in the vacuum chamber windows needs to be extremely free of any aberrations which may otherwise affect the highly calibrated light passing through it to/from the satellite in the chamber.
Normally, wavefront error measurement is performed to derive Zernike polynomials, which are a mathematical interpretation of test data; these are used as a ‘best fit’ representation to understand the apparent distortions depending on the term, e.g. tilt, focus, astigmatism, coma and tilt, spherical and defocus.
To overcome the requirement to test under vacuum conditions, AEON designed and manufactured a novel (fully adjustable) vacuum chamber capable of measuring the wavefront error of glass up to 300mm diameter, to a rough vacuum of 2×10-3 mbar. The chamber is suspended, allowing for directional adjustment in 5-axis for the alignment of the housed laser reflector flag with respect to the light source.
The measurement is performed using a laser interferometer emitting a calibration photon through the window and onto a laser reflector flag. The flag would reflect the emitted photon and allow the test technician to align the beam with the laser interferometer emitter. The interferometer can accurately plot any changes to the beam as a consequence of passing through the window. Notionally these changes are due to aberrations.
Results
AEON successfully characterised six different windows (made of two different materials). A detailed report of the results from each test included:
- Atmospheric conditions of the test environment using AEON’s proprietary instrumentation.
- Illustrations and videos of the wavefront error measurement testing conditions, both pre- and post-test.
- Illustrations and videos of the novel vacuum chamber and test fixtures used.
- Zernike polynomials for each window under ambient and vacuum conditions.
Characterisation of the glass windows was successful in characterising all windows to <30nm RMS. This provides our client with a high level of assurance that the windows will not adversely affect satellite calibration activities.
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