The world's first mirror made out of silicon carbide
What specific technical problems do observations in infrared light involve?
Infrared radiation is emitted from any object that has heat. If the telescope and instruments are warm, they emit infrared light that would interfere with the observations of faint objects in the sky. For this reason, both the telescope and all the observational instruments onboard AKARI are housed within a cryostat and are cooled to an absolute temperature of as low as 6K (−267°C) using liquid Helium. Since AKARI was manufactured at normal room temperatures, the actual operating temperatures in space will differ by 300°C. Therefore, the following challenges had to be overcome in order for the telescope to operate properly at these cryogenic temperatures:
- The development of a material that would enable the telescope to observe to a high degree of accuracy, even in harsh environments where the temperature undergoes significant changes and variations.
- Improvements in the efficiency of the cooling equipment so as to maintain these cryogenic temperatures.
- Attitude and orbit control that would avoid heat from the light of the Sun and radiation from the Earth.
Tell us about the new material used in the primary mirror.
Using silicon carbide for the primary mirror of AKARI's telescope fulfilled the first of the three requirements. This was the first time that this material had ever been used in the construction of a mirror for a space telescope. As well as being light and robust, silicon carbide undergoes virtually no thermal expansion with temperature (that is, it has a low coefficient of thermal expansion). However, adopting this new material led to a succession of difficulties. A primary mirror of the same size made out of glass would weigh at least 200 kg. However, for AKARI, the initial target weight was 9 kg, which was relaxed in practice to 11 kg. It proved very challenging to manufacture the silicon carbide (which is a ceramic) to this size. It was also reportedly very difficult to process silicon carbide into a concave mirror.*1 In addition, trouble also arose when the mirror was actually installed.*2 Subsequently, the launch was postponed by approximately 18 months.
Silicon carbide mirrors will likely become the mainstream for future space-borne telescopes, both for astronomical and Earth observation purposes, because of the advantages of their lightweight, robust properties and resistance to temperature change. At present, it is still beyond our technical grasp to manufacture 10 m size mirrors in this material, although a 3.5 m mirror is already being build for the telescope of ESA's Herschel Space Observatory*3, which will be the largest silicon carbide mirror.
- *1Comment by Nikon
The process of refining the mirror was complex and time-consuming, as it involved creating a certain shape (a second-order aspherical surface) and then polishing it using diamond powder. Proper polishing with such a hard and fine substance allows the manufacture of high precision reflecting mirrors that can be used not only for long-wavelength infrared observations, but also for shorter-wavelength observations, such as those used for optical and ultraviolet telescopes. Even for Nikon, the development required for the manufacture of this primary mirror presented an enormous challenge. Nikon will make full use of the experience acquired with AKARI as it engages in the manufacture of even higher-precision optical instruments.
- *2Comment by Nikon
A metal (a special alloy) with a coefficient of thermal expansion similar to that of silicon carbide was used for the component that secures the primary mirror to the structure of the telescope. However, at cryogenic temperatures it displayed properties different from those of ordinary materials, which caused problems in cryogenic temperature experiments. Accordingly, a new metallic material was chosen, and analysis and sample testing was conducted, after which the component for securing the primary mirror was redesigned and remanufactured. Ultra-low-temperature experiments were then carried out once more. Since this component was intended for use in a cryogenic temperature environment, it was necessary to go through the laborious process of cooling it with liquid Helium while measuring its characteristics and strengths and testing the finished item.
- *3The Herschel Space Observatory
The Herschel Space Observatory is a space telescope that the European Space Agency (ESA) is planning to launch in 2008. Equipped with a silicon carbide primary mirror with an aperture of 3.5 m, it will observe the Universe using the far-infrared and sub millimeter wavebands from 60–670 μm.
Was the original plan to conduct one year of observations at cryogenic temperatures?
Cooling space instruments requires a great deal of ingenuity. For example, IRAS used 600 liters of liquid Helium for cooling over the course of its ten-month mission. AKARI only carries 170 liters (roughly a quarter of the IRAS amount). A pair of Stirling cycle cryocoolers, which are capable of cooling down to an absolute temperature of 20K (−253°C) in space, are operated to preserve the liquid Helium.*4 This has enabled an 18-month mission lifetime at the cryogenic temperature (the initial plan called for at least 550 days).
The cryostat is a pressure-tight container and before the launch it is covered by a lid to create a vacuum inside to prevent any heat from the outside reaching the telescope. However, since the lid, made of aluminum, is itself warmer than outer space (which is around 3K), it also emits infrared radiation and thus consumes the liquid Helium. This lid was supposed to have been promptly jettisoned as soon as the satellite was in space.*5 However, due to unforeseen and unconnected difficulties that arose just after launch, the jettisoning of the lid and the commencement of the observations were delayed for about a month.*6 As a result, it was feared that there was significantly less liquid Helium available for the rest of the mission than originally planned. Fortunately, later measurements confirmed that the liquid Helium lifetime was almost the same as originally planned, with even the worst-case prediction concluding that it would be possible to continue observations under cryogenic conditions for at least a year.
Even when the liquid Helium has expired, one of the cameras on the IRC instrument (covering the shortest wavelength range), may be able to continue observations, while being cooled solely by the Stirling cycle cryocoolers alone. It is not known how long AKARI can continue observations with this camera, since it is actually the first ever satellite to be equipped with such mechanical coolers with high cooling power (in fact very few satellites have been fitted with their own cryocoolers). Therefore, almost without realizing it, we have found ourselves at the forefront of space cryogenics technology.
- *4Liquid Helium
The Helium used on AKARI is super-fluid liquid Helium (Helium 4), which has a unique liquid viscosity of zero.
- *5 Comment from Professor Murakami
The section of wire that locked the cryostat lid tight shut was cut using explosives propelling the lid far from the main body of AKARI using spring power. In principle, waste material should not be discarded in space under normal circumstances, as space debris is a problem. However, under the current technical constraints it is extremely difficult to live up to this ideal. I believe that in the future it will be possible to dispense with liquid Helium entirely and to cool using only the cryocoolers. In this case, the lid can be left at a normal temperature on Earth and then cooled once the satellite has entered space. Hence, the lid will simply play the role of a dust cover making it possible to open and close it (obviating the need to discard it).
- *6Comment from Professor Murakami
For reasons that were unclear immediately after the launch, two built in solar sensors, which are designed to maintain the satellite's attitude, were apparently somehow obscured, making safe attitude control difficult. With the lid open, if by any chance, the telescope inadvertently pointed at the Sun, the delicate observational equipment would be destroyed. While AKARI's internal software was rewritten and other steps were taken to address this situation to allow safe operation without the need to use the solar sensors, the lid had to be left in place.