Anything looking at the Sun tends to heat up. The SDO instruments have been looking at the Sun for 8.5 years. Although they are designed to look at the Sun for a long time, some changes still happen.
For example, the coatings on the HMI instrument housings and optical surfaces have been degrading slowly but steadily, causing the instrument to absorb more solar radiation and heat up. The front surface of HMIís telescope is now hot enough that the HMI team is having difficulty maintaining proper focus. Without some adjustments, this problem will only get worse as the instrument continues to age.
For this reason, the HMI team is going to increase the operating temperatures of the front of the telescope and adjust the internal focus mechanism. This should give the instrument many more years of precise control and excellent data. The process of increasing the operating temperatures is a slow one — the team will begin heating the front telescope tube up at 1000 UTC on Tuesday, October 16, and will then wait six hours for the temperature to stabilize. The next step is to fine-tune the temperatures. These smaller changes will need about an hour for the temperature to become constant. Science data will not be available during the time it takes to bring the instrument back into focus. That also means there will be no magnetic field, continuum, or Doppler NRT data products.
HMI resumed taking science data before 2130 UTC, October 16. Fine tuning of the focus will occur Wednesday, October 17, but science data will not be interrupted.

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