A compact detector for far ultraviolet (FUV) spectrograph has been developed and applied to a space observation on a korean micro-satellite, STSAT-1. The dual channel imaging spectrograph utilized two micro-channel plate (MCP) detectors with a single crossed delay line (XDL) anode to record photon arrival events. The unconventional design of sharing the delay on the anode allows for the use of a single set of position encoding electronics for both detector fields, thereby reducing the size, weight, and power of the associated electronics. The ground and on-orbit performance tests proved successful use of the system for astrophysical observations. This paper summarizes the design, the development, and the test results of MCP based delay line detector for FUV spectrograph.
The Far UV Imaging Spectrograph (FIMS), also known as Spectroscopy of Plasma Evolution from Astrophysical Radiation (SPEAR), was developed for a micro-satellite in order to observe far UV emissions emanating from diffuse astronomical targets. The instrument was launched 2003 September 27 on a Korean micro-satellite, STSAT-1 and successfully carried out its observations for nearly two years until the satellite suffered power shortage caused by degradation of its batteries. FIMS employs a dual channel, imaging spectrograph optimized for the detection of faint diffuse emission lines, especially the doublets of O VI (1032, 1038 A) and C IV (1548, 1551A). These two doublets are known to be important diagnostic tools for understanding the cooling mechanism of galactic hot gas. Hence, the two wavelength channels are constructed around these lines, with 900 - 1150 Å and 1350 - 1750 Å bandpasses, respectively. The gap between the two channels, 1150 - 1350 Å, contains strong geocoronal lines 1216 Å and 1304 Å, which are avoided to minimize instrumental contamination. The doublet of O VI resides close to another geocoronal line Lyman β at 1028 Å. One of the technical challenges for the development of FIMS was to discriminate O VI doublet lines from this nearby strong geocoronal line while the allocated space, power, and mass were very limited as the spectrograph was to be flown on a micro-satellite.
The FIMS detecting system is composed of MCP stack, anode, and readout electronics, similar to those flown on previous missions. However, the most important part between them are the anode and CFD in readout electronics. Many anode types are considered and checked for the feasibility using analysis tool, which is also made during the design period. After some concentrated study on this topic, it is known that the unconventional design of sharing the anode can solve the space, power and mass problem given by the satellite by using single set of readout electronics instead of two. For the CFD, instead of power consuming ECL device, ACMOS technology is also applied to additionally reduce the power consumption. Component level test and integrated test has been performed for the flight model.
Overall resolution of the readout electronics is measured to be ~28μm in X and ~83μm in Y. The resolution of the assembled detecting system was evaluated using an electroformed Ni pinhole mask with 10μm holes and 1 mm center spacing. The average spatial resolution of the detector system was ~80μm in X and ~150μm in Y. According to these values, spatial and image resolution of the detector in the rotated optical coordinate is calculated to be ~87μm (X) and ~100μm (Y) each, which meets the requirement derived from the science goals. C IV image and the short band spectrum of Vela supernova remnant taken by FIMS on orbit is shown at last. It can be seen that O VI doublets are separated from the airglow line.
원자외선 분광영상장치(Far Ultraviolet Imaging Spectrograph; FIMS)는 우주 공간에 존재하는 성간물질들로부터 방출되는 원자외선을 관측함으로써 성간에 존재하는 고온 플라즈마의 냉각 메카니즘을 규명하고 은하 생성과 관련된 이론 모델들을 검증하기 위한 목적으로 기획된 국내 최초의 천문 우주관측 탑재체이며, 마이크로 위성인 과학기술위성 1호의 주 탑재체로써 2003년 9월에 발사되어 성공적으로 관측 임무를 수행하였다. 성간의 고온 플라즈마의 냉각 메커니즘을 이해하기 위한 중요한 분광선은 O VI (1032, 1038 A) 와 C IV (1548, 1551A) 방출선이며, 원자외선 분광영상장치에는 900 - 1150 A, 1350 -1750 A의 두 관측대역을 관측할 수 있도록 검출기가 구성되어있다. 이러한 원자외선 분광영상장치의 핵심 부분은 마이크로 위성에서 고 감도의 2차원 분광영상을 획득할 수 있도록 구성된 micro-channel plate(MCP) 기반의 지연선 검출기라 할 수 있으며, 이 연구를 수행하는 과정에서 두 개의 관측 영역에서 지연선 양극판을 공유하는 독창적인 설계 방식을 적용함으로써, 전력 소모를 최소화하고 부피와 무게를 최적화 한 검출기를 성공적으로 개발 하였다.
이 논문에서는 원자외선 분광영상장치에 적용된 MCP 기반의 지연선 검출기의 설계, 제작 및 시험내용들을 정리하였다.