Cable Wrangling

In previous posts about the DESI spectrograph being installed at Kitt Peak’s Mayall 4-meter telescope, I’ve focused on the 5000 robotic positioners at the telescope’s focal plane, which is up at the top of the telescope, and the ten spectrographs located in a climate controlled room at the telescope’s base. However, I haven’t talked a lot about how the light from the 5000 positioners gets down to those spectrographs. The light travels along optical fibers that run from the telescope’s focal plane down to the room with the spectrographs. The whole distance is roughly 40-meters (or a little less than the length of half a football field).

In the photo to the left, you can see the cables running along the front of the telescope at this angle. They’re draped over the blue horseshoe structure in the foreground. Several of the cables are draped down in the lower left-hand side of the photo. There are ten cables that run from the top of the telescope to the room with the spectrographs. Each cable contains 500 individual optical fibers. Each of these cable bundles feeds one of the spectrographs at the telescope’s base. Since each cable contains 500 optical fibers, they are heavy cables. They’re also very fragile. It would be challenging enough to run these fibers from one point to another if they could be locked down in one position. However, the telescope actually has to move, so we can look at different parts of the sky. This means these heavy, fragile cable bundles have to move too.

Before construction even began on the DESI spectrograph, engineers spent time figuring out the best way to run the cables that minimized how much they had to move. Also, there are devices called e-chains that help assure cables stay nice and neat as the telescope moves. This past week, one of the engineers snapped a photo of me helping to prepare one of the e-chains for installation. He was in a lift up near the telescope’s top and looked down at me and another one of the telescope engineers hard at work. I’m the one in the yellow hard hat.

As I mentioned earlier, these cables are both heavy and fragile. That means there’s been a lot of heavy lifting that requires a great deal of care about where we step and place the cables. We don’t want to bend them too tightly, or they could break. The upshot is that this has been exhausting work. Everyone feels wiped out at the end of the day.

Still, we see the proverbial light at the end of the tunnel, or perhaps that should be the light at the end of the fiber! Once the cables are run, we only need to install the last three spectrographs, then the system will be complete. How soon we’ll start observing with the DESI spectrograph will depend on the results of preliminary testing which has already commenced and will be finished soon after the installation is complete. That said, I am told there’s a very good chance we’ll be pointing DESI at targets on the sky in less than a month. At that point, we may start to understand more about this mysterious thing that astronomers have dubbed dark energy.