Date: March 11, 2022
After stellar performance since its commissioning back in 2017, ZTF had to be taken out of service in Dec 2021 due to a cryo-cooler malfunction. Since then, my team and I have performed multiple fixes and tests, but let us start from the beginning.
The CCDs in the ZTF camera are cooled to about 170K and this process is achieved without liquid nitrogen but rather by employing two cryocoolers (made by Polycod. These work much like a domestic refrigerator, but to get to much lower temperature, a 3 m long counter-flow heat exchanger is coiled up behind the cold-head and the refrigerant used is a proprietary mixture of liquid propane and gaseous ethane, methane and Argon. Compression to ~375 psi heat the refrigerant which is returned to room temperature by forced air flowing across the heat exchanger preceding a holding tank where oil from the seals is collected and returned to the inlet side.
A charcoal adsorber captures residual oil vapor, but after years of operation, a small amount of this oil makes it into the supply lines, and subsequently to the cold head where it freezes and degrades performance. We have procedures to replace the charcoal adsorber, to change the lines for clean ones, and to warm the system to unblock the capillary tube but by November last year, these measured no longer worked, so we were forced to bring the instrument back on campus, disassemble it and replace the cold heads.
Various option were evaluated and in light of time, we chose to use an ultrasonic cleaner forcing hot water and concentrated detergent through the capillary tube. The results of this process were good enough so we reassembled and re-installed ZTF. In doing so we had to thread newly cleaned refrigerant lines through the telescope cable wraps and back to new compressors in the outhouse.
While dealing with the installation of a pressure gauge, a damage was done to a fine wire mesh container holding zeolite desiccant pellets and some of these pellets appeared in the vaccum chamber. Unfortunately, this meant another de-installation and return to campus for cleaning.
We decided to remove the pellets using tiny vacuum hose without removing the CCDs, then put the instrument on a tilt stand to rotate ±80 degrees. We then clocked 90 degrees and repeated to see if any more would appear from inaccessible cavities. When we could repeat ~15 times without discovering new pellets, we reassembled and returned to the telescope.
Our greatest fear was that the pellets had bent bond wires causing CCDs to malfunction. This did not come to pass. All CCDs worked as did the cooling system so we returned home exhausted, bringing Gattini with us but that’s another story.
Just when we thought we can all head to the beach with ZTF up and running again, we discovered additional pellets inside the camera with some of them already shattered. So here, we are again, back in the cleaning room. Stay tuned…
Roger Smith started out as an electronics engineer designing CCD controllers at the AAO then CTIO. This lead to optimizing the operation of optical and infrared detectors in many instruments, investigating sensor physics and associated calibration issues. He was the lead engineer for ZTF, and is interested in all aspects of astronomical instruments including operation efficiency. Current projects include system architect for NGPS and PI for Cryoscope a pathfinder for imaging in the thermal infrared with field of view comparable to ZTF. In his personal life he enjoys sailing, the outdoors and travel, when possible. He raised 3 children, two of whom graduated from Caltech (both in Mechanical Engineering) and one from UCSB (Physics). For the last 6 years he has been building a house which everyone wishes was finished by now.