The third fiber cable was installed on Subaru Telescope

PFS Project 2023-03-06 Comments Off

Time flies: it has been a few months since the last article. In the last a few months, the integration of the PFS Spectrograph Modules (SMs) and cameras has been proceeding at LAM and JHU. At Subaru Telescope, the second SM (SM3) experienced the first light in the engineering run in November 2022 just after shipping. However, the two successive engineering runs were cancelled due to the bad weather. Meanwhile, ASIAA visited Subaru Telescope and modified Prime Focus Instrument and Metrology Camera.

We have various stories as above, and among them one of the big stories is the installation of the third fiber cable unit on the telescope.

The optical fiber cable of the PFS instrument is about 60 m long in total. The fiber cable consists of three sections to deal with instrument exchange and to enable the development of the systems more efficiently. The main and the longest cable is about 55 m long, and delivers the light from celestial objects caught on the prime focal plane of the Subaru telescope to the Spectrograph Modules on the 4th floor in the dome building.

PFS developed 4 fiber cable units, among which the first and the second cables were installed on the telescope in 2021 and 2022, respectively.

The third cable was installed in February 2023.

Since the fiber cable is 55 m long, it is quite tough to unpack it from the shipping crate and to route on the floor. Several staffs started installation work with removing the cable coiled up on the drum in the crate, laying down on the floor as loosely as possible, and testing the performance before installation.

Winter bad weather prevented us from starting installation on the originally scheduled day, but thanks to great efforts of DayCrews and PFS team, installation was completed in two-day period.

We have been monitoring the performance by measuring the value called “FRD” (※) to see whether the fiber cables has got stressed during installation.

※ FRD: Focal Ratio Degradation.
FRD is a phenomenon where output light gets enlarged compared to input light when the fiber gets stressed. If FRD gets worse and/or changes while telescope is moving, observed spectra will be affected.

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Fiber positioner Cobra / ファイバー配置用アクチュエータ『コブラ』

PFI accommodates about 2400 fibers and their positioners for scientific use, which are divided into 42 units of 57 fibers each. The fiber unit contains the fibers, fiber positioners, and electronic boards to control them. Therefore, each 42 rail has 57 holes, as the first picture.

Other than the rails with the science fibers, the bench accommodates a lot of different things such as 6 cameras for acquisitions and guiding of a target field, and fixed fibers as a reference during the fiber configuration of the astronomical object on the focal plane.

主焦点装置には約2400本の観測用ファイバーが設置されますが、57本ずつをまとめた42ユニットに分けられます。ユニットはファイバーと配置用のアクチュエータ、アクチュエータを制御する為の電子基板から構成されます。

土台の大きさは直径約70cm、観測用ファイバーが付いたレールの他にも、これらのファイバーを焦点面の天体に配置するときに位置の参照として用いる固定ファイバーや、目的の視野に望遠鏡を導入し保持するために使用する6つのカメラを載せます。

Spectrographs / 分光器

We will observe ~2400 objects using four identical spectrograph modules, each of which acquires ~600 spectra of astronomical sources and calibration sources. Each module covers wide wavelength range on visible and near-infrared band (380 — 1260 nm). To achieve this, the module has 3 camera units; visible-blue, visible-red and near-infrared. The light of each band is introduced by using two “dichiroic mirrors”, which split the light at a given wavelength.

PFSでは約2400個の天体を観測しますが、4つの同型の分光器を用いてそれぞれ600本のスペクトルを取得します。各分光器は可視光域から近赤外域までの広い波長範囲 (380 から1260 nm) をカバーする為、「可視光域青用」、「可視光域赤用」、「近赤外域用」の3つのカメラユニットを持っています。3つのカメラには、特定の波長で光を分けることのできるダイクロイックミラーを2つ用いて、光を導入します。

　

Metrology Camera System / メトロロジカメラシステム

To measure the fibers position on the focal plane, “Metrology Camera System” is attached on the Cassegrain focus of Subaru telescope. Looking up to the prime focus (see this image from HSC blog for clear image), this camera takes a image of back-illuminated fibers. Thanks to a large format of ~50 M pixels CMOS sensor, this camera can acquire all ~2400 fiber spots in a single exposure. The camera system is also designed to execute the process from exposure to measurement of fiber centroids within 5 seconds. Effective measurement of the fibers position enables us to shorten overhead for fiber configuration and execute large statistical survey observation.
The Metrology Camera System will be delivered to Subaru in early 2018.

焦点面上のファイバーの位置を測定する為に、『メトロロジカメラシステム』をすばる望遠鏡のカセグレン焦点に取り付けます。このカメラは焦点面を見上げるようにして後ろ側から照らされたファイバーの画像を取得します(イメージ的にはこちらの画像 (HSCブログより) のような感じです)。メトロロジカメラには50Mピクセルの大判のCMOSセンサが搭載されているため、約2400本のファイバー像をたった一回の露出で得ることができます。また、画像の取得から各ファイバーの位置測定までを5秒以内に行うように設計されています。このようにファイバーの効率よく位置を測定することで、ファイバーの配置など観測準備にかかる時間をなるべく短くし、大規模サーベイ観測を行うことができるのです。
メトロロジカメラシステムは2018年4月にすばる望遠鏡へ搬送されるました。

　

Spectrograph Clean Room / 分光器設置室

The Spectrograph Clean Room is a room whose size is 6.7 m wide, 8.6 m long ,and 3.6 m high. Four PFS spectrographs will be accommodated in the clean room. To stabilize the spectrograph performance, the temperature in the room will be controlled at 5 +- 1 degree Celsius. Besides, a crane with capability of 500 kg is installed for assembly and maintenance of the Spectrograph.

分光器設置室は、すばる望遠鏡ドーム棟4階に建設された部屋で、幅6.7m × 奥行8.6m × 高さ 3.6 mの大きさです。 PFSの4台の分光器がこの部屋に設置されます。分光器の安定性を図るために、部屋全体が5±1℃の範囲で温度管理されます。また、分光器の設置やメンテナンス用に室内には耐荷重500kgのクレーンが設置されています。

　