TriScape100: Surviving the space environment testing
In a previous blog post (Athermal design and analysis using numerical methods) we reported on the challenges when designing an athermal optical system orbiting Earth in heavily fluctuating temperatures. These extreme temperature changes can result in tremendous forces within the glass elements of an optical payload, which could force the glass to crack. This article will explain the approach implemented by Simera Sense to verify that the final manufactured system performs as designed in an extreme thermal and vacuum environment.
The xScape100 series of optical payloads were designed to achieve sub 5-mGSD from a 500km orbit. The quality of the captured images is dependent on the mechanical structure’s ability to keep the relative alignment and surface form of the optical elements within very tight tolerances over a wide range of temperatures. This poses a difficult design challenge during which the displacement of the mechanical structure and optical elements must be matched by considering the different thermal expansion coefficients of the mechanical and optical components.
During the design phase, Simera Sense uses two software packages to analyze structure-opticsinteraction. Finite element analysis software is used to evaluate the effect of thermal loads on the structure, after which optical analysis software is used to calculate the resulting optical performance. Following the design process, thermal tests are conducted to ensure the survivabilityof the designed system as well as to correlate the optical model. The purpose of the thermal functional test is to verify that the electro-optical and optomechanicalsystems maintain their structural integrity and performance after being exposed to extreme temperatures.
Testing the TriScape100 Payload
To simulate the space environment is challenging and requires specialized equipment. Functional tests of the electronic components were performed in a humidity controlled thermal chamber over a temperature range of -20°C to 60°C. The system under test consisted of the xScape100 Optical Front-End, the sensor unit and associated control electronics. Ground support equipment was used to simulate the interface to the payload and test the functionality during the thermal test. Three temperature sensors were used to respectively measure the air temperature inside the chamber, the temperature of the Optical Front-End’s mechanical structure and the temperature of the sensor control unit’s heat sink.
Figure 1 shows the applied temperature profile while Figure 2 shows the measured temperatures. The complete system was successfully exposed to two cycles of the temperature profile with functional tests being performed at each temperature setpoint. A final functional test was performed at 20°C.
Figure 1: The applied temperature profile showing temperature vs time.
Figure 2: Measured temperature data.
The functionality of the electronics was verified during the thermal test, while the performance of the optical system was verified pre and post the thermal test. After the thermal test, visual inspection of the adhesive bonds, optical elements and mechanical structure showed no signs of structural damage. This was followed by optical performance testing, which was performed using Simera Sense’s optical test equipment.
The optical test results showed no deviation in optical performance, either before or after the thermal test. The aforementioned tests, along with the functional test results of the electronics, thus confirmed the structural integrity and performance of the complete payload post thermal testing. Additional vibration testing will confirm the structural integrity of the whole system.
In addition to the thermal functional test described above, one also needs to verify the optical performance of the system when exposed to a given temperature range. This was achieved by using a specially designed thermal chamber that allows for optical measurement of the system under test.
The chamber shown in Figure 3 was used to conduct optical performance measurements at set intervals over a temperature range of 20°C to 50°C. The results obtained showed no change in optical performance over the tested temperature range, thereby confirming that the payload performs in accordance with its specifications in its intended environment.
Figure 3: Thermal vacuum chamber mounted on the large aperture collimator table.
These tests not only confirmed the structural integrity of the TriScape100 under thermal loads but also verified that the optical performance will be maintained during these thermal cycles, a potential shortcoming of many optical payloads.