One of the most important challenges for star trackers and telescopes is eliminating stray light generated by reflections from the walls and other optical elements of the device. Most black coatings are not sufficiently efficient at grazing angles, and the most common solution is the incorporation of blackened vanes in the design. Acktar black coatings exhibit particularly low residual reflectance and have been implemented in various instruments. The new proposed material, commercially known as HexaBlack, exhibits particularly low hemispherical and specular reflectance – especially at grazing angles. This material consists of a 3D-structured substrate coated with a well-known Acktar black coating. This new solution makes new baffle designs possible with a minimum of vanes. In this article the BRDF, specular and diffuse reflectance data and specular simulations for these suggested designs for VIS and IR wavelengths.
Space opto-mechanics require materials with low outgassing. With water vapor being a main component of outgassing, when changing the water adhesion of an optical coating, its outgassing potential changes as well. Acktar developed an ultra-hydrophobic light absorbing optical coating,and put it through the qualification process for space applications. The results were compared with other space qualified Acktar coatings, as they constitute the current industry standard. The new coating showed to have the lowest outgassing amongst Acktar’s coatings, while retaining all other physical and optical qualities. Additional presentation content can be accessed on the supplemental content page.
GHGSat is world’s first nanosatellite dedicated for monitoring greenhouse gas (GHG) and air quality gas (AQG) emissions from any industrial site in the world. The satellite was designed and constructed by MPB Communications to monitor carbon dioxide (CO2) and methane (CH4) from a low earth orbit (about 500 km). To reach the designated resolution of less than 50 meters and high precision of greenhouse gases monitoring, the nanosatellite has large aperture in order to collect enough optical signal. The aperture of about 10cm causes significant stray light issues with limited solution options without adding too much weight to the light weight satellite. As the stray light was the main concern in this project, many precautions was taken. As a solution for blocking unwanted light, baffle with many vanes was used. However, having the vanes is not optimal solution without high absorbing coating applied on it. The coating requirements was low reflectance, no outgassing in space environment and long life stability, being a satellite inaccessible for maintenance. The chosen coating for stray light elimination was Acktar Vacuum BlackTM. Hemispherical reflectance and BRDF data was examined before making the choice. The satellite was launched in June 2016 and the images was analyzed to ensure the aimed resolution and precision of the optical system. After almost two years of activity, no visible changes was discovered in image quality nor stray light issues detected.
One of the largest challenges for optical systems is eliminating stray light generated by reflections off the walls and other optical elements of the device. Most black coatings are not sufficiently effective at grazing angles. Acktar’s black coatings exhibit particularly low residual reflectance and have been implemented in various instruments. The new proposed material exhibits particularly low hemispherical and specular reflectance – especially at grazing angles.
In the context of ESA’s Cosmic Vision 2015-2025 scientific program, Solar Orbiter (SolO) represents the first M-class mission, currently under implementation for a launch planned in October 2018.
One of the SolO solar remote-sensing instruments is the Multi Element Telescope for Imaging and Spectroscopy (METIS), under development by an Italian consortium ATI composed by CGS and TAS-I. METIS is a coronagraph that will perform simultaneously broad-band imaging in visible light and narrow-band imaging in UV of the Sun corona with unprecedented spatial resolution.
The surfaces of several Ti-6Al-4V components of the METIS Optical Unit must have a very low reflectance in order to minimize straylight and therefore ACKTAR Magic BlackTM coating was specified. An example is represented by the internal surface of the Inverted External Occulter (IEO) - the front part of the instrument - which is exposed to the direct solar flux and experiences challenging environmental conditions. Qualification for 450°C was required whereas Acktar's Magic BlackTM qualified maximum temperature was 380°C.
A dedicated thermal-vacuum cycling test (100 total cycles, −120°C − 450°C) was conducted to qualify Magic BlackTM to quantify any performance degradation under these conditions:
• optical (reflectance)
• thermal (∝ and ε values) to allow calculation of the radiative heat transfer of the METIS parts during mission
• mechanical performances
The following success criteria were established:
• total reflectance at wavelengths of 400−1000nm <;5%
• degradation of absorptance ∝ and emissivity ε to be <10%
• surface resistivity <5 kΩ/square
One of the largest challenges for optical systems is eliminating stray light generated by reflections off the walls and other optical elements of the device. Most black coatings are not sufficiently effective at grazing angles. Acktar’s black coatings exhibit particularly low residual reflectance and have been implemented in various instruments. The new proposed material exhibits particularly low hemispherical and specular reflectance – especially at grazing angles.
Light scattering caused by imperfections of optical components can critically affect the performance of optical systems
in terms of losses and image degradation. Because of the numerous potential sources of scattering such as roughness,
surface and sub-surface defects, bulk inhomogeneities, as well as coatings, scattering properties must be carefully
specified and measured at the wavelengths of application. Bidirectional Reflectance and Transmittance Distribution
Functions (BRDF / BTDF) are used to quantify the angle resolved scattering properties. The data can be used as an input
for optical engineering software just as FRED, ASAP, ZEMAX for stray light modeling. In addition, analyzing the
scattered light can provide valuable information about the relevant imperfections. The presentation provides an overview
of instrumentation for light scattering measurements at wavelengths ranging from the visible to the extreme ultraviolet
and the infrared spectral regions. Examples of applications will be discussed ranging from superpolished mirrors to
diffraction gratings, interference coatings, and black absorbing coatings.
Coatings with high absorption or emission of thermal radiation are often required in infrared, space and cryogenic
applications. It is much more difficult to generate a surface which is sufficiently "black" at cryogenic temperatures than
at room or elevated temperatures. An experimental study was carried out to compare the thermal emissivity and
absorptivity of a variety of black coatings on copper and aluminum substrates at temperatures in the range from 10 K to
300 K and the results are presented in this paper. The surfaces tested include paints and thin-film coatings.
A new getter type produced as a vacuum deposited thin-film is under development (patent pending). The film serves as
an efficient pumping substance and also as an efficient absorber of IR stray light. The getter film is activated in place by
heating to 130oC.
The film is very thin and has excellent vibration resistance and temperature stability and is expected to serve as an
advantageous new solution for IR detector dewars and cold shields.
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