The first fiber cable unit has been installed on Subaru telescope

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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.