2:45 PM - 3:00 PM
[PCG18-11] Specifications and development of Hydrogen Imager for Comet Interceptor mission
Keywords:Lyman alpha line, comet, absorption filter
Comet Interceptor (CI) is a mission to observe long-period comets, which are hardly affected by space weathering, and is aimed to unravel the formation process of our solar system. The Hydrogen Imager (HI) is one of the instruments onboard the CI probe B1 and is responsible for observing the Lyman-alpha emission line (121.6 nm) of hydrogen to determine the spatial distribution of hydrogen coma, D/H ratio, and water production rate. In particular, the D/H ratio is a parameter that characterizes the properties of water, and is one of the indicators that may reveal the source that brought water to Earth in the early stages of planet formation.
The HI is a Cassegrain-typed telescope, and images are obtained through two types of resonant absorption filters and a single interferometric bandpass filter; the two absorption filters contain molecular hydrogen and deuterium, respectively, which are thermally dissociated into atoms by heating the filament inside the cell. Since the wavelengths of the Lyman-alpha emission lines of hydrogen and deuterium are slightly different, two types of absorption filters can be used to absorb only the Lyman-alpha emission lines of hydrogen or deuterium atoms, respectively. Therefore, the D/H ratio can be derived indirectly by switching on and off the two filters and comparing the transmission intensities. However, the lifetime of the filament is shortened in some cases, which will be a problem. This is thought to be due to the fact that the tungsten used as the filament is oxidized when oxygen molecules or water molecules are contained in the cell, causing chemical change to tungsten oxide, which has a low sublimation point, and ultimately burns out the filament.
In this study, we conducted experiments to evaluate the optical performance of the system using a prototype and to extend the life of the filament. In the former experiment, collimated light was generated from a point light source and injected into the optical system, and the changes in the point spread function (PSF) were examined by gradually changing the positions of the primary mirror and secondary mirror in the optical axis direction with shims. In the latter experiment, the lifetime of filaments were measured while changing conditions – hydrogen pressure in the cell, baking of the vacuum system, etc. – in order to find out the factors that determine the lifetime of the filament and to extend its life.
The HI is a Cassegrain-typed telescope, and images are obtained through two types of resonant absorption filters and a single interferometric bandpass filter; the two absorption filters contain molecular hydrogen and deuterium, respectively, which are thermally dissociated into atoms by heating the filament inside the cell. Since the wavelengths of the Lyman-alpha emission lines of hydrogen and deuterium are slightly different, two types of absorption filters can be used to absorb only the Lyman-alpha emission lines of hydrogen or deuterium atoms, respectively. Therefore, the D/H ratio can be derived indirectly by switching on and off the two filters and comparing the transmission intensities. However, the lifetime of the filament is shortened in some cases, which will be a problem. This is thought to be due to the fact that the tungsten used as the filament is oxidized when oxygen molecules or water molecules are contained in the cell, causing chemical change to tungsten oxide, which has a low sublimation point, and ultimately burns out the filament.
In this study, we conducted experiments to evaluate the optical performance of the system using a prototype and to extend the life of the filament. In the former experiment, collimated light was generated from a point light source and injected into the optical system, and the changes in the point spread function (PSF) were examined by gradually changing the positions of the primary mirror and secondary mirror in the optical axis direction with shims. In the latter experiment, the lifetime of filaments were measured while changing conditions – hydrogen pressure in the cell, baking of the vacuum system, etc. – in order to find out the factors that determine the lifetime of the filament and to extend its life.