As announced on September 20, 2023 (first report) and September 27, 2023 (second report), due to the detection of abnormal values in the primary mirror support mechanism, the Subaru Telescope has suspended its night observation since September 15. We have been continuing to investigate the cause and examine the effects on the telescope. We would like to report what we have confirmed to date as follows.
1. The event that occurred:
① Abnormal value of primary mirror support mechanism (fixed point) (September 15 local time)
As part of the maintenance work, an operational test was conducted in which the telescope was tilted to a near-horizontal position, and when the telescope was returned to the vertical position, values indicating excessive loads were detected at three fixed points. After that, when we turned on the power to the actuator that supports the telescope and checked, it continued to show an abnormal load at the fixed point (a value lower than the normal value).
② Damages on primary mirror (September 19 local time)
During the above operational test, a part of the primary mirror cover opened under its own weight and was damaged. At a later date, while repairing the cover, a metal part (gear rail) fell and damaged the main mirror surface. There are two damaged areas that are thought to have occurred this time. There have been cases in the past where the primary mirror was similarly damaged, but they have already been repaired. We also found minor scratches, which are thought to be caused by something other than the current incident.
Although it is possible to repair the damaged areas, since the primary mirror is a brittle material, we will carefully proceed with technical studies such as the repair method, the finished shape of the repaired areas, and the strength analysis of the primary mirror after repair. The repair work must be carried out safely on-site on Hawai`i Island.
2. Status (as of October 16th)
The work has been underway for about two weeks since the second report, and the status is as follows:
Regarding the abnormal value of the primary mirror support mechanism (fixed point), we have confirmed by today that there is no deformation of the primary mirror itself using an optical system detection device (Shack-Hartmann camera). Based on this, we decided to investigate whether the cause of the abnormal value was a failure of the force sensor body at the fixed point or some other failure.
While confirming that the supporting force of the primary mirror support mechanism, including the fixed points, does not impose a load on the primary mirror, we plan to proceed with analysis work such as comparative evaluation with model calculations of the primary mirror shape.
If the force sensor at the fixed point is out of order, it is necessary to procure the sensor, consider and implement a replacement method, and complete the restoration through inspection. On the other hand, if the failure occurred in something other than the force sensor itself at the fixed point, it is necessary to identify the failure location. It may take several months for recovery to occur.
As for the damage on the primary mirror, as a result of detailed confirmation, the impact cracks thought to have occurred this time are 26mm x 19mm in size (the estimated maximum depth of the cracks is about 9mm) and an oval shape of about 10mm x 7mm (the cracks could be shallower than the ones mentioned above, but the detailed depth will be examined later on).
In preparation for repair, we will closely examine the depth of the cracks, reduce the load on the primary mirror support mechanism, etc. We will work with optical manufacturers to consider in detail on how to repair cracks, which can be safely performed on-site on Hawai`i Island, and determine.
From now on, information will be updated on the website approximately once a month, in addition to when the repair policy and the expected time for resuming observation are known. We sincerely apologize for the long-term inconvenience this will cause to those planning to observe.
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