Star Sensor Mask – Carbon Nanotube Mask Technology

New Generation of Star Sensor Baffle—CNT Baffle. A new conception of CNT ( carbon nanometer tube) baffle is presented by increasing the absorptivity of the absorption coat to decrease the baffle outline size and mass，and to enhance the stray light rejection ability．A series of technologies to outgrow CNT coat with absorptivity of 99% more on the Ti alloy base are developed and applied on the new CNT baffle being capable of displacing the current product technology. The space environment applicability of the CNT baffle is demonstrated by a series of ground space environment simulation tests such as atomic oxidation，ultraviolet radiation，high-energy particles radiation，vibrancy mechanics，thermal vacuum，and so on．Comparing the star tracker image values caused by the stray light with the current normal star tracker baffle，the value with the CNT baffle is reduced by 56% ，which indicates the advantages of the CNT baffle in stray light rejection．The innovation success shown in this paper breaks through the bottleneck technologies about the practical application of carbon nanometer tube technology in the stray light rejection technical domain，and has important engineering value for the star tracker technology progress．

Star sensors are currently recognized as having the highest accuracy among spacecraft attitude measurement instruments and play a crucial role in spacecraft flight control. Especially for spacecraft that require high accuracy in attitude pointing and stability, the requirements for star sensor accuracy are more stringent. It is particularly important to eliminate stray light that affects the accuracy of star sensors. As star sensors work on Earth or interstellar flight orbits, Inevitably, it is interfered by all or part of the stray light sources such as the sun, moon, atmospheric light, scattering from the outer surface and components of spacecraft, resulting in an increase in the grayscale of the image surface stray light and an uneven distribution of the image surface illumination, which affects the improvement of signal-to-noise ratio and the correct recognition of star maps. In severe cases, it can cause the star sensor to fail to output the correct attitude, and even lead to satellite failure. Currently, the main means for star sensors to eliminate external stray light is through light shields, It is a method of eliminating external stray light entering the interior of an optical lens by relying on geometric structure occlusion and absorption of an absorbing coating. Theory and design practice have shown that increasing the absorbance of the absorbing coating plays a decisive role in improving the performance of eliminating stray light.

This article focuses on the research results of the research group on the development of a new type of CNT coating sunshade. The light absorption characteristics of carbon nanotube coatings and the design and manufacturing methods of carbon nanotube sunshades are studied, and the process and method of growing carbon nanotube absorbing coatings on metal sunshades are solved. By comparing and testing the stray light performance with existing products, the superiority of the new type of sunshades in eliminating stray light performance is demonstrated, And a validation study was conducted on the applicability of this new type of light shield in space, and it was concluded that the CNT light shield can fully meet the requirements of space applications

Stray light suppression is an important means to improve the accuracy of star sensors, and the design of light masks is the main way to suppress stray light.

1. Point source transmittance (PST)

Star sensor is a weak signal imaging optoelectronic instrument with high requirements for stray light elimination design. Its stray light elimination ability is reflected in the different stray light avoidance angles that different stray light source instruments have for normal operation. The stray light avoidance angle represents the stray light suppression ability of the star sensor, which is widely adopted internationally. The technical indicator of the star sensor’s mask’s ability to eliminate stray light is generally the extinction ratio (or stray light suppression ratio), Defined as the ratio of the parallel stray light energy incident into the hood at a certain angle at the entrance to the energy falling at the exit after passing through the hood. The higher the extinction ratio, the stronger the hood’s suppression ability. In addition, the stray light suppression ability is also evaluated using point source transmission (PST), which is used in practical engineering applications, The residual grayscale output of the detector at the angle of solar stray light suppression can usually be used as the evaluation index for star sensors. The design goal of the sunshade is to achieve its stray light avoidance angle and minimize its size through coating selection and geometric optical design, while ensuring that the sunshade meets the extinction ratio or PST performance

The star sensor hood is mainly used to suppress stray light. Stray light refers to the non target imaging light that scatters into the image plane in an optical system, in addition to the imaging light. It includes two parts: one part is radiation from outside the optical system (such as sunlight, terrestrial light, etc.); The other part is the internal radiation generated by the optical system itself due to changes in temperature and other factors. The impact of stray light on optical systems is very obvious, which generally leads to a decrease in image clarity, seriously affecting the accuracy of star sensor subsequent star point extraction, resulting in errors in star point extraction. Moreover, the chaotic energy distribution caused by stray light can form stray light spots, completely submerging star points by stray light noise. Therefore, in the design of star sensor optical systems, suppressing stray light is very important, and the ability to suppress stray light directly determines whether the optical system can work properly. By setting a light mask in front of the optical system, stray light can be effectively suppressed. Applying a light cancelling coating on the inner wall of the light mask can increase the absorption of stray light, which is conducive to further suppressing stray light.

In terms of sunshade design technology, the main approach is to spray black paint inside the baffle type. Even so, there are various techniques and changes in the design of sunshades for different field of view angles and extinction ratio requirements. The design scheme of sunshades can generally be divided into baffle absorption type, reflection type, and reflection absorption mixed type. The baffle absorption type is currently the most widely used, This type of design can be divided into two types: secondary design and primary design. Secondary design is generally larger in size, but its advantage is high extinction ratio. The higher the accuracy requirement of star sensors, the more advantageous this type of application is. Primary design is mainly used for star sensors with a large field of view and accuracy less than 1 “. The advantage is small in size, but the extinction ratio is generally not as high as the secondary hood. Regardless of the type of hood design, The optimization result pursued is to make the size of the light shield as small as possible while meeting performance requirements. The design of reducing the size of the light shield depends on the characteristics of the coating and the design techniques for geometric optics to eliminate stray light. There have been many reports on the design techniques for geometric occlusion to eliminate stray light internationally. Regarding coating technology, the main issue is the selection of coating materials. If a reflective type of light shield is used, the coating with the highest possible reflectivity should be chosen, If the diffuse absorption type is used, choose a coating with the highest possible light absorption rate

At present, there are three types of high absorption coatings used in the sunshade of domestic star sensors: the first type is a super black anodized surface, with a solar absorption ratio of about 97%, which has been applied in the sunshade of China’s “Chang’e-1” lunar probe star sensors; The second type is domestic matte paint SR107-S731 coating, with a solar absorption ratio of 97%, which has been applied in the TG-1 satellite sensor light shield; The third type is PNC black paint developed by MAP company in France, with a solar absorption ratio of up to 98%. It has been applied in the light shields of multiple satellite sensors in China, and the light shields of ASTRO 10 star sensors in Germany also use PNC black paint

2. Experimental analysis

This article provides the principles and methods for determining the structural dimensions of the light shield, derives the formulas for determining the position and height of the horizontal light shield, and proposes a design method for a vertical light shield. Based on this, it is combined with traditional light shields and optimized. Then, through example simulation, the BRDF parameters of the material surface matte paint are set. A total of 16 off axis angles between 1 ° and 75 ° are traced to obtain the point source transmittance (PST) curve of the system. The results show that when the off axis angle is greater than the solar suppression angle of 30 °, the transmittance of the point source reaches the order of 10 ^ -9, and the stray light suppression effect meets the requirements of the system’s stray light suppression ability.

The sunlight in the space environment has a significant impact on the imaging of the star sensor. When installing the star sensor, it should be avoided from being directly illuminated by strong stray light sources as much as possible. According to the environment in which the star sensor is used, a solar suppression angle of 30 ° should be selected. In the design of the sunshade of a star sensor, it is necessary to achieve maximum attenuation of stray light that is incident at an angle suppressed by sunlight. The point source transmittance PST is generally used to evaluate the degree of attenuation, which is the ability of the system to suppress stray light.

The point source transmittance (PST) only reflects the attenuation ability of the optical system itself to point stray light sources, and is independent of the radiation intensity of the stray light sources. It is a very objective quantity when evaluating the system’s ability to suppress stray light. When evaluating the system, a smaller PST indicates a stronger ability to suppress stray light and better system performance. At present, the elimination or suppression of stray light mainly starts from three aspects: optical design, mechanical structure, and surface characteristics. In terms of optical design, starting with optical structure design, mirror transmittance, mirror reflectance, mirror finish, and aperture design, stray light can be effectively suppressed; In terms of surface characteristics, by changing the surface roughness or blackening the surface, the bidirectional scattering distribution function of the structural surface can be reduced to achieve the goal of suppressing stray light; In terms of mechanical structure, a light shield and a light blocking ring structure can be designed to suppress stray light, and extinction lines can also be designed on the inner wall of the light shield to suppress stray light. This article suppresses stray light by designing a light mask, designing a light blocking ring on the inner wall of the mask, and spraying black paint. The design principle of the light shield is to ensure that non target light cannot directly reach the image surface, and it needs to undergo two or more reflections to attenuate before reaching the image surface. This avoids direct irradiation of non imaging light onto the image surface and effectively attenuates the energy of the light reaching the detector.

The design of the light shield was carried out based on the horizontal and vertical light shields, and the external parameters of the light shield, as well as the position and height of the light shield, were calculated using Matlab software. The structure of the light shield was modeled and drawn in SolidWorks, and then imported into TracePro software for simulation analysis. By setting the bidirectional scattering distribution function of the structural surface and the number of light traces, a total of 16 off axis angles between 1 ° and 75 ° were traced at different azimuth angles, Based on the obtained data, the point source transmittance (PST) of the system was calculated, and a PST curve was plotted. From the curve, it can be seen that the overall PST of the system shows a downward trend, and the stray light suppression ability of the improved structure of the light shield reaches the order of 10 ^ -9 at an off axis angle of 30 °, greatly improving the level of stray light suppression.

Carbon nanotube materials have not been widely used in star sensor masks, mainly due to the unresolved growth process of carbon nanotubes on titanium alloy substrates. If the absorption rate of the coating is increased by one percentage point, then for a second absorption mask, considering the attenuation of spatial propagation between baffles, the extinction ratio will be increased by more than one order of magnitude, or the extinction ability will be improved by more than one order of magnitude. Therefore, Improving the absorption rate of the coating is a prerequisite factor for the extinction ability of the light shield, and on this basis, geometric extinction design can play a role

In terms of design methods and tools, there is no essential difference between the design of carbon nanotube masks and existing technology masks. The difference lies in the fact that the light absorbing coating of carbon nanotube masks is carbon nanotubes, with a light absorption rate of over 99% in the entire spectral range, which is more than 1% higher than existing technology. This difference brings about significant changes in the structure of masks, The requirement for 3 and 4 diffuse reflections to reach the lens can be reduced to 2 or 3 times to meet the requirements, thus greatly reducing the size of the light shield

Carbon nanotube materials, as a coating to eliminate stray light, significantly improve the single absorption ability of the light shield, bringing huge optimization conditions to the design of the light shield. Regarding the process used to ensure that carbon nanotubes grow firmly on the inner surface of the light shield, the research team has conducted a lot of research work and ultimately determined a comprehensive implementation plan with the best performance, And develop the manufacturing process for CNT masks. The overall technical strategy adopted is to first solve the feasibility problem of titanium alloy sample growth of CNT, obtain the growth process, and then promote it to the CNT growth of complex parts, adjust the process, and promote it to the CNT growth of masks, adjust the process, Finally, a stable CNT mask production process has been formed. Currently, a set of techniques for growing carbon nanotubes to eliminate stray light and ultra-high absorption coatings suitable for space environments in titanium alloy based masks has been mastered, and has been verified through space environment experiments. The technical flowchart for growing CNTs in titanium alloy masks is shown in Figure 12

The development process diagram of the CNT hood is shown in Figure 13

The concept of carbon nanotube masks introduced in this article has successfully developed innovative products, breaking through the process technology of growing carbon nanotubes inside complex and large-sized titanium alloy based masks. It has solved key technologies such as new mask design, thin-walled processing, carbon nanotube coating growth, spatial environment verification, and stray light design testing, making this new mask significantly reduce its volume and weight compared to existing mask technologies This new type of light shield has significantly improved its performance in eliminating stray light. It has undergone research in accordance with the requirements of current space sensor products for space environment testing, proving that it has the state of space application technology and can become a replacement product of existing technology. Moreover, due to the full spectral absorption characteristics of carbon nanotubes, the growth process technology of carbon nanotubes on titanium alloys can achieve target stealth, standard radiation sources The internal elimination of stray light in instruments and equipment is widely used in various fields.