![[Open in a new window]](/common/img/icon/window.png){kind=icon}
Located on the summit of Mauna Kea on the island of Hawaii, the National Astronomical Observatory's large-scale optical infrared telescope (the Subaru Telescope) is equipped with several different observing instruments, such as the HDS (High Dispersion Spectrograph), which was introduced in the previous article (Part 1). One of these instruments, FOCAS (the Faint Object Camera and Spectrograph), has proved to be effective in the observation of distant galaxies in the furthest reaches of space. Nikon took charge of the manufacture of the main body and the principal lens assembly of FOCAS. On this occasion, we asked Professor Masanori Iye of the National Astronomical Observatory about the structure of FOCAS, the significance of the detection of distant galaxies, the next-generation observing instruments due to be installed on the Subaru Telescope, and the Extremely Large Telescope (ELT) project that is currently under research.
What is FOCAS installed on the Subaru Telescope?
Where does FOCAS rank among the various observing instruments?
FOCAS mounted at the Cassegrain focus of the Subaru Telescope. FOCAS enables the basic visible-light observation of the Subaru Telescope, such as imaging observation, spectroscopic observation, and polarization observation.
The strategy used for studying the Universe on large is as follows. To start with, survey photographs are taken, and these are then used to search for interesting objects, in our case, distant galaxies in the furthest reaches of space. Only extremely faint light reaches us from these objects; however, what little light there is captured by the 8-m-aperture Subaru Telescope, and analyzed using a spectroscopic technique.
The HDS (High Dispersion Spectrograph) processes light from comparatively close, bright stars that are within our own galactic system (the Milky Way). Since the light from these stars is abundant, the spectrographic resolution is increased, and they can be studied in detail. However, our humble distant galaxies are too dark to observe with the HDS. For this reason, the detailed examination of the spectrum was abandoned, and the idea of FOCAS (the Faint Object Camera and Spectrograph), an observing instrument that can obtain information of faint objects using low-to-medium-dispersion spectroscopy, was conceived.
Prior to the construction of FOCAS, we constructed a similar instrument for use on the 2-m-aperture telescope at the Okayama Astrophysical Observatory. Associate Professor Toshiyuki Sasaki constructed an instrument known as OOPS (the Okayama Optical Polarimetry and Spectrometry system) at the National Astronomical Observatory especially for use on a 91-cm-aperture telescope. OOPS could be described as the prototype for FOCAS. OOPS came onto operation just as computer technology was rapidly developing, and the system allowed computer control of functions ranging from turning observing instruments on and off to saving observation data. We made use of our experience with OOPS when we designed the FOCAS control system. Likewise, since we had expertise on the CCD camera that was to be mounted on FOCAS, we decided to construct it. However, there was no way that we could construct a sophisticated lens system for FOCAS, which is why we asked Nikon to manufacture lens systems and the overall structure.
Research using an optical telescope requires three basic instruments: an observing instrument that takes photographs, a high-dispersion spectroscopic observing instrument, and a high-sensitivity low-dispersion spectroscopic observing instrument. On the Subaru Telescope, the Prime Focus Camera takes photographs, the HDS handles high-dispersion spectroscopy, and FOCAS handles low-dispersion spectroscopy. This three-instrument suite was described in the planning document for putting the concept of the Subaru Telescope into practice (commonly known as the Blue Book), which was drafted in 1989. This is still today believed to have been the correct strategy.
FOCAS is mounted at the Cassegrain focus, on Subaru's hip (or backside) (laughs). FOCAS is one of four instruments at the Cassegrain focus—the others being the Infrared Camera and Spectrograph (IRCS), which was developed jointly with the University of Hawaii, the Cooled Mid-Infrared Camera and Spectrometer (COMICS), used for long-wavelength optical observation, and the Coronagraphic Imager with Adaptive Optics (CIAO), which enables the observation of dark celestial objects by masking out the light from bright celestial objects that are close to them.* Last year, the Multi-Object Infrared Camera and Spectrograph (MOIRCS) also became available for use and IRCS was moved to the Nasmyth focus to be used in conjunction with the new Adaptive Optics system. The Cassegrain instrument flange of the Subaru Telescope is used as a common mechanical interface for all the Cassegrain observational instruments. The instruments can be interchanged semi-automatically by means of a moving wheeled platform that runs on magnetic tape affixed to the floor. Since the interchange takes at least an hour, in principle this operation is not carried out on nights when observation is possible. In the room that houses the observing instruments, there are four standby platforms that are similar to the one at the Subaru's Cassegrain focus. These standby platforms supply electrical power and network connectivity to the observing instruments, and keep the instrument detectors ready in a cooled condition, enabling observation to be carried out at any time.
- * The IRCS was modified for use at the infrared Nasmyth focus in combination with newly developed adaptive optics, and CIAO will be decommissioned in the near future. Currently, FOCAS, MOIRCS, and COMICS are the principal observing instruments used at the Cassegrain focus.
Currently FOCAS, COMICS, MOIRCS, and CIAO are operated at the Subaru Telescope's Cassegrain focus.
Please tell us about the structure and workings of FOCAS.
First, there were some limitations imposed by the Subaru Telescope on observing instruments so that they can be mounted at its Cassegrain focus. Instruments could be no more than 2 meters in diameter, 2 meters high, and 2 tons in weight. These limitations bounded the design of FOCAS. One outstanding feature of the FOCAS structure is the pipes that are formed into a truss shape. These lightweight trusses are made of carbon fiber reinforced plastic (CFRP) and are designed to support the entire FOCAS assembly at its center of gravity, to minimize the mechanical flexure of FOCAS under its own weight. If the main optics of FOCAS were supported directly by the telescope flange plate with which it is attached to the Subaru Telescope, it would flex considerably as the orientation of the telescope changes to follow the target. In the initial planning stage, there was a proposal to install a system to automatically regulate this flexion; however, this truss system to support the main FOCAS modules at its center of gravity has successfully reduced the degree of flexion to approximately one-tenth. Nikon took charge of this structural design, the simulation-based optimization for reducing strain, and the main optical system for FOCAS, which is described below.
The main body of FOCAS is supported at its center of gravity by a truss made of CFRP to reduce the degree of flexion. Although the light travels in a simple, straight line, the internal mechanism is complex, and multiple optical components such as filters and grisms can be interchanged according to observational requirements.
The FOCAS control screen: the status of each observational mechanism can be monitored and controlled using the GUI.
The remote monitoring room at the National Astronomical Observatory in Mitaka, Tokyo. The status of the entire Subaru Telescope, the status of each instrument, and observation data can be monitored in real time.
The internal structure of FOCAS is designed to allow a straight light path without using a mirror to avoid any loss of the precious photons captured by the Subaru Telescope. The multi-slit mechanism of FOCAS first extracts only the light from the target objects. The light introduced to FOCAS is then expended and transformed into a parallel beam using a set of collimator lenses. The optical elements that can be inserted in the collimated beam are filters that only extract light of certain wavelengths, dispersive grisms that are a combination of a diffraction grating and a prism, and polarizers that analyze the light in terms of its polarization components. This is where the light from a celestial object is processed according to the objectives of observation. Finally, the treated light is gathered by a set of camera lens and is photographed by a CCD camera.
One distinctive feature is the internal turret structure. In accordance with the observation program, the timely interchange of various optical components such as filters and grisms can be carried out in under a minute. The system is controlled by computer in combination with sensors. The graphic user interface on the control screen helps observers to recognize which filters are currently in place and which set of optical elements are to be used next. This control system was developed on the OOPS system.
Incidentally, the operational status of FOCAS and the entire Subaru Telescope can be monitored from here at the National Astronomical Observatory in Mitaka. In addition, actual observation data is transmitted to Mitaka in real time and archived. Since the Pacific Ocean communication lines linking Hawaii and Japan are still technically insecure, we do not issue commands for moving the Subaru Telescope or the observing instruments from here. However, since we are continuously monitoring the parameters necessary for maintaining the instruments, when there is a problem of some kind, we can resolve it from here.