The Process of Cassegrain Instrument Exchange at Subaru Telescope
April 27, 2016
How do you exchange delicate and heavy instruments that require accurate alignment with the telescope? There are robots working at the Subaru Telescope to ensure safe operation of such tasks. There is a suite of observational instruments dedicated to the specific wavelength ranges from optical to mid-infrared. Of them there are three imaging/spectrograph instruments for the Cassegrain focus below the primary mirror. A robot cart exchanges Cassegrain instruments during the daytime according to the observation schedule at night.
Each Cassegrain focus instrument is in a 2 x 2 x 2-meter cube, weighs two tons, and has a center of gravity in middle of the cube. An instrument rotator between this cube and the telescope compensates the rotation of the field of view during the tracking of an object.
The three Cassegrain instruments each cover a specific wavelength range. The Faint Object Camera and Spectrograph (FOCAS) is optimized for visible-light observations. Near-infrared observations are done by the Multi-Object InfraRed Camera and Spectrograph (MOIRCS), and the Cooled Mid-Infrared Camera and Spectrograph (COMICS) is for the mid-infrared range of between 7.5 - 25 microns. Because of the capabilities of this versatile suite of instruments, the competition for their use on the telescope is serious.
Swapping Out Instruments
Thanks to the Cassegrain Instrument Auto eXchanger (CIAX), day crews are able to securely transport each instrument between the Cassegrain focus and the standby room. They can do this by using a yellow robot that looks very much like one of the good Transformer characters in the popular entertainment.
During the daylight hours, crews set up the observation instrument that is to be used each night. Exchange of the Cassegrain instrument takes place about once per week. CIAX enables this delicate task to be accomplished in several hours. The timelapse video above shows the crew detaching the FOCAS instrument and attaching COMICS. FOCAS has one part extending below the main cube, so there is a special jig on CIAX used during FOCAS's transportation. COMICS requires another maneuver to handle its cryogenic plumbing, since keeping the low temperature even during transportation is critical for observation after it is installed.
The CIAX cart functions in several dimensions: it moves back and forth, and jacks up and down. Here's how the exchange process works. First, the cart goes to the Cassegrain focus and receives the instrument from the telescope. Then, CIAX brings the instrument back to its standby station. Next, CIAX picks up another instrument and brings it to the telescope. The video shows the cart raising and lowering with the instrument. It may not be clear, but the telescope side has motor-driven bolts help to lower or raise the instrument and enable precise alignment of the instrument with the telescope's optical axis. A set of automatic connector system hooks up the power cables, signal lines, and plumbing for the cryogenics.
There is one very cool gadget associated with the cart. In order not to disturb the sensitive equipment at the neighboring radio telescopes, there is no Wi-Fi system at the Subaru Telescope. So, communication between the CIAX cart and the telescope is done through a wire, and a fishing pole assists the line's moving about. This very flexible pole helps keep the communication line from getting tangled with the transporter.
The manufacturer of this equipment - Craft Corporation - worked with the engineers of the National Astronomical Observatory of Japan. The company is based near Nagoya and specializes in automated transporters. The first CIAX unit was delivered almost 20 years ago. The president of the company himself came to Maunakea for the CIAX commissioning and his firm continues to provide upgrades. The stable and safe exchange of the scientific instruments is due to the dedication of the company, and Mr. Shigeru Sawada remains very proud that the product is still used at the Subaru Telescope.