The first fiber cable unit has been installed on Subaru telescope

pfs_project 2021-02-19 Comments Off

We PFS project passed two big milestones recently. We will talk about them in this post and the next. The fist milestone is the first fiber cable unit, which was delivered to Hawaii Observatory, National Astronomical Observatory of Japan in the end of the last year, has been installed on Subaru telescope at last!

Left: Fiber cable unit attached on the spider arm of Subaru telescope
Right: Fiber cable unit connected Spectrograph Module integrated in December 2019

We observe about 2400 spectra of celestial objects using 4 identical spectrograph modules, by paving the fibers equipped with tiny (size of pens) actuators on the prime focus of Subaru telescope. “Fiber Cable Unit” is the 55m-long cable to deliver the light from the prime focus to spectrograph modules in the dome building.

Sketch of fiber cable unit

If the cables are twisted, fibers got stress, which degrades the image quality of lights coming out from the fibers. Besides, if stress changes while the telescope is moving, the light into spectrograph cannot have stable image quality. These points prevent us from processing the obtained spectra robustly. Therefore, it is important to route the fiber cable unit avoiding twist as much as possible.

We studied how to route the cable avoiding the twist using the prototype cable. We put marks to check the orientation of the cable at each point. Installation was done with 2-day work in February 2021. On the first day, the fiber cable routed from the spider arm at the top of Subaru telescope to the floor of the dome building ((1)-(4) in the below figure). On the second day, the cable routed up in the dome floor to the floor where spectrograph modules are accommodated ((5)-(7) in the below figure). We monitored the image quality to see if the fibers had got stressed.

Snapshots of installation. On the first day, (1) we lifted up the half of the cable, (2) routed the cable on the top of the telescope, (3) routed the cable on the side of the telescope, and (4) passed the rest of half to the dome building. On the second day, (5) we pulled up the cable from the bottom (6) to the above floor, and (7) routed cable into the room for Spectrograph Modules.

Thanks to wonderful team-play of staff of Hawaii observatory, installation went well and the fiber cable has been connected to spectrograph module which was delivered in December 2019. After Prime Focus Instrument comes later this year, we will start the system test connecting end to end.

In fact, there is one more instrument installed on the Subaru telescope in the same period. We will talk about it in the next article.

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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。

　