Joint Work for Spectrograph Development at LAM / マルセイユ天文台での分光器開発強化週間

At Laboratoire d’Astrophysique de Marseille (LAM), PFS Spectrograph Module (SM) is fully integrated and its performance is validated. As a part of these processes, two visible-red camera units are being tested; one is for thermal performance test and the other is for optical performance test.  From 19th to 29th June, we got together at LAM and dedicated ourselves to joint development works and discussions. The key members from John Hopkins University (JHU) and Princeton University (doing camera unit and control software development) and several staffs from the Subaru Telescope observatory and PFS project office participated on site, and not only spent time in the clean room but also had intensive round-table discussions about the strategy and road-map of development toward the delivery of SM to the observatory.

Spectrograph team members discussing spectral image on CCD detectors.

One key objective of this meeting was to discuss the long-standing issue of thermal performance: The cryostat has been designed and built to keep high vacuum (10^{-6} Torr = 2×10^{-8} psi) and low temperature (163 K = -110 degree Celsius) for detector operation inside, but unlike the intentions, cool-down tests revealed difficulties in keeping the detector temperature constantly low for a long time. From many discussions and additional experiments at LAM and JHU, we have internally come to a conclusion that the root cause of this problem is likely the residual small amount of water in the cryostat that condenses on cold surfaces and enhances the emissivity. In one particular session during this period, we yet again reviewed the experiments we had done so far with an invited external reviewer, confirmed our understanding of the root cause, and discussed next step for improvement and operation strategy to maintain the detector temperature. After these discussions, we moved to the clean room and started planned works right away.

Two cryostats in the clean room: The one on the near side is used for thermal performance test, while the other on the far side is for optical performance test.

In addition, we have modified and updated the electronics assemblies to operate the cryostat, and motor assembly to precisely position the focal plane. These assemblies are developed by JHU and Princeton.

Left: The two staffs wearing a white hard hat are Dr. Stephen Smee (on the right) and Dr. Stephen Hope (on the left) from JHU, investigating the wiring of controller assembly.
Right: Dr. Fabrice Madec from LAM is investigating motors for positioning the detector focal plane.

Thermal performance and optical performance tests will be continued at LAM, and other components such as blue and near-infrared camera units will be integrated and tested in the future, envisioning the shipment of the first spectrograph module in the first half of next year.

分光器の最終組上げ・性能評価試験が行われているマルセイユ天文台(LAM)では、現在2台の可視光赤カメラを用いて冷却試験と光学試験が行われています。今回、6月18日から29日にかけて、『開発強化週間』と題してマルセイユ天文台に、カメラユニット部分を担当しているジョンズ・ホプキンス大学(JHU)とプリンストン大学のメンバーに加えすばる望遠鏡やプロジェクトオフィスからの数名が集合し、LAM のメンバーとともに分光器の開発作戦会議、集中作業を行うとともに、今後の開発に向けた議論も行いました。


この期間の最大の目標は、カメラの容器(デュワー)の冷却試験で直面している問題について議論することでした。このデュワーは内部に組み込まれる検出器の運用に必要な高真空(100万分の1 Torr = 約1万分の1 Pa)、低温(163 K = -110 ℃)を長期間実現するためのものですが、試験の結果、温度を期待通りに安定して保持できないことがわかりました。多くの時間を費やしJHUとLAMで追加実験を行いチームで議論を重ねた結果、デュワーから除去しきれない微量の水分子が原因だろう(低温部分に凝結し放射率を上げてしまう)という結論に到達しました。今回は、外部から招いた審査員も交え、これまでの試験結果をおさらいして原因についての理解が正しいかを確認しあったのち、今後の対応策を、検出器の温度を保つための最終的な運用・維持の方針を含め、議論しました。その後、実験室に場所を移し早速作業に取りかかりました。



左:制御系の配線を調べているジョンズ・ホプキンス大学のStephen Smee 氏[白ヘルメット着用、左側]とStephan Hope 氏[白ヘルメット着用、右側]。
右:検出器の位置調整用モーターを調べているマルセイユ天文台のFabrice Madec 氏。


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