11:00 AM - 1:00 PM
[PCG18-P07] Development of a near-infrared camera TOPICS for planetary atmospheric observations
Keywords:near-infrafed, detector, Cryogenic
In this presentation, We will report on the development of the near-infrared imaging system TOPICS (TOhoku Planetary near-Infrared Camera System). The emission and absorption in the infrared wavelength range have information of physical phenomena, such as the composition and density of the molecular species of the atmosphere, temperature and wind speed. To understand the time and spatial variations in the various ranges, it is necessary to carry out long-term and flexible observations of the planetary atmosphere with ground-based telescopes and planetary probes.
As a specific example, in the Jovian magnetosphere, about 90% of the plasma originates from the volcanic gas of the satellite Io, and the Io volcanic activity greatly affects the variation of Jovian magnetosphere and the magnetosphere-ionosphere coupling. In the ionosphere, the auroral emissions of H2 and H3+ are observed due to the interaction of the descending electrons from the magnetosphere with H2 in the Jovian atmosphere. In order to understand the causal relationship between the volcanic activity of Io and the variations in Jovian magnetosphere and ionosphere, continuous observations of Io's volcanic activity (J-,K-band thermal radiation) and H3+ and H2 auroral emissions (K-,L-band) are important.
The TOPICS detectors are Raytheon InSb 256x256 arrays, which are sensitive to 1-5 µm (including J-, K-, and L-bands). Tohoku University has a 60cm telescope (T60) at the Haleakala Observatory in Hawaii (elevation 3040m), where TOPICS will be installed to demonstrate the operation of the detector drive circuit and conduct test observations. Tohoku University is also developing a near-infrared spectrometer, ESPRIT (Echelle Spectrometer for Planetary research In Tohoku University), which uses the same detector drive circuit as TOPICS.ESPRIT features echelle high-dispersion spectroscopy (λ=20,000) and a slit length of 50 arcsec, which will enable simultaneous polarimetric observations of Jupiter at the time of its opposition (apparent diameter is 42~47 arcsec).
In the future, ESPRIT will be installed in the 1.8m off-axis telescope PLANETS, which is being developed in collaboration with the University of Hawaii and others.
There are two issues that need to be resolved to achieve stable operation of TOPICS. The first is to reduce the temperature of the detectors. In the infrared detector, thermal electrons generated in the photodiode of the detector become dark current noise, so it is important to reduce the temperature of the detector. As of last year, the detector temperature was 44K, but by improving the thermal path, the temperature was reduced to 39K and the dark current was reduced from 2240e-/s to 211e-/s. We are now working on further improvement of the thermal path to achieve the optimum operating temperature of the InSb detector of 30-35 K and to reduce the dark current to a few tens of e-/s.
The second is to reduce the read-out noise of the detector driver circuit. Currently, the readout noise of the entire drive circuit is about 1200 e- RMS in e- RMS terms, which is much higher than the required specification of several tens of e- RMS. As a result of verification under various conditions, it was found that this excessive readout noise is added at the section from the detector to the preamplifier. It was also found that the read-out noise was caused by the electrical noise emitted by the refrigerator used to cool the detector, and we are now trying to reduce the noise effect.
As a specific example, in the Jovian magnetosphere, about 90% of the plasma originates from the volcanic gas of the satellite Io, and the Io volcanic activity greatly affects the variation of Jovian magnetosphere and the magnetosphere-ionosphere coupling. In the ionosphere, the auroral emissions of H2 and H3+ are observed due to the interaction of the descending electrons from the magnetosphere with H2 in the Jovian atmosphere. In order to understand the causal relationship between the volcanic activity of Io and the variations in Jovian magnetosphere and ionosphere, continuous observations of Io's volcanic activity (J-,K-band thermal radiation) and H3+ and H2 auroral emissions (K-,L-band) are important.
The TOPICS detectors are Raytheon InSb 256x256 arrays, which are sensitive to 1-5 µm (including J-, K-, and L-bands). Tohoku University has a 60cm telescope (T60) at the Haleakala Observatory in Hawaii (elevation 3040m), where TOPICS will be installed to demonstrate the operation of the detector drive circuit and conduct test observations. Tohoku University is also developing a near-infrared spectrometer, ESPRIT (Echelle Spectrometer for Planetary research In Tohoku University), which uses the same detector drive circuit as TOPICS.ESPRIT features echelle high-dispersion spectroscopy (λ=20,000) and a slit length of 50 arcsec, which will enable simultaneous polarimetric observations of Jupiter at the time of its opposition (apparent diameter is 42~47 arcsec).
In the future, ESPRIT will be installed in the 1.8m off-axis telescope PLANETS, which is being developed in collaboration with the University of Hawaii and others.
There are two issues that need to be resolved to achieve stable operation of TOPICS. The first is to reduce the temperature of the detectors. In the infrared detector, thermal electrons generated in the photodiode of the detector become dark current noise, so it is important to reduce the temperature of the detector. As of last year, the detector temperature was 44K, but by improving the thermal path, the temperature was reduced to 39K and the dark current was reduced from 2240e-/s to 211e-/s. We are now working on further improvement of the thermal path to achieve the optimum operating temperature of the InSb detector of 30-35 K and to reduce the dark current to a few tens of e-/s.
The second is to reduce the read-out noise of the detector driver circuit. Currently, the readout noise of the entire drive circuit is about 1200 e- RMS in e- RMS terms, which is much higher than the required specification of several tens of e- RMS. As a result of verification under various conditions, it was found that this excessive readout noise is added at the section from the detector to the preamplifier. It was also found that the read-out noise was caused by the electrical noise emitted by the refrigerator used to cool the detector, and we are now trying to reduce the noise effect.