Power of FOCAS and MOS

August 30, 2001

Since August, we've been working on re-coating the primary mirror and upgrading some of Subaru Telescope's hardware and software systems at the summit of Mauna Kea. The work will take two months and the progress will be reported in the Subaru Calendar. Further details will appear in "Subaru This Month" for September.

This month, we take a look at the Multi Object Slits (MOS) capability of FOCAS. Attached to the Cassegrain focus, FOCAS is designed to obtain images and spectra in visible light (as described in the March 2000 issue of Subaru This Month). We already have a number of reports on our website regarding observations made using FOCAS (M82 color image; spectra of distant supernovae).

To make spectral observations, we first bring the starlight gathered by the telescope to a focus, as usual. But then we let it continue on: it passes through a narrow slit and into a spectrograph, where it is broken up into the colors of the rainbow. An image of this "spectrum" is then finally captured on the CCDs (charge-coupled devices). With common spectrographs, the number of objects that can be simultaneously observed is generally limited to just one or two, because these spectrographs have just one slit. The spectograph in FOCAS is designed to work with many slits cut into thin carbon fiber sheets, allowing us to observe many objects simultaneously. We call these sheets "Multi Object Slits" (MOS).

Thin carbon fiber sheet (size= 15cm x 15cm, thickness = 0.1mm)

We have to create a new sheet of slits for each field of objects that we wish to observe with MOS. The process is as follows:

  1. We obtain an image of the target objects with FOCAS about one month before the MOS observation;
  2. The observer chooses which objects are to be observed spectroscopically using MOS and makes a list of their positions;
  3. Subaru staff cut slits into a sheet according to the list using a laser processor machine;
  4. The sheet is installed into FOCAS and the MOS observation is made.

The laser processor machine (cutter at center, control system at left and right).

Operating the laser processor machine.

Taking out the sheet after cutting the slits.

Using MOS requires much preparation, including gathering observations with FOCAS in advance. This additional preparation and observing time is amply rewarded by the greatly increased efficiency that results from observing many objects simultaneously with MOS.

The following picture shows a FOCAS image from which target objects were selected in advance of a MOS observation. The black central line is the gap between the two CCDs used in FOCAS.

Image of target objects obtained with FOCAS

The next picture shows a sheet with the slits cut by the laser processor machine. Each narrow line is a slit. We can see that the slits on the sheet agree with the star locations on the previous image.

In addition to the slits, circlar marks seen in this image are holes in the sheet that line up with bright stars. These are used to align the sheet against the sky before making the observation.

With the sheet inserted into FOCAS, we obtain the following spectra of our target objects. Each vertical streak is a spectrum. It is possible to determine the atomic and molecular composition of our objects, their temperature and their velocity from these spectra.

Spectra obtained by the MOS observation.

Illustration showing the preceeding three images combined

It is expected that observational instruments like FOCAS will replace the single-slit spectrograph as the norm on the world's large telescopes. MOS observing with FOCAS begins in October when open-use observations resume.



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