Suprime-Cam at Prime Focus

July 15, 2000

Acceptance of proposals for Subaru Telescope's very first semester of Open Use ("S00") came to an end on July 3rd with over 100 submissions received. Given that only 36 nights will actually be available for observing this first time around, the Telescope Allocation Committee now has the difficult task of choosing who will be granted time and who will be disappointed. This situation is not at all unusual, with most of the world's telescopes receiving far more requests than they have time available. In fact, this is one of the very reasons why Japanese astronomers have worked so hard to build a large telescope of their own. The world's astronomy community is, of course, grateful that Subaru is offering some of its time for use by non-Japanese astronomers. Semester S00 begins in December of this year.

Two instruments are being offered for use during the first semester: IRCS; and Suprime-Cam, Subaru Telescope's wide-field camera for imaging at visible wavelengths.

At the heart of Suprime-Cam is a cooled vacuum chamber ("dewar") currently filled with eight of the planned ten light detectors called Charge-Coupled Devices ("CCDs"). Each CCD contains a grid of 2048 x 4096 light-sensing areas, giving the finished camera a total of over 80 million resolution elements or "pixels". This is about ten times the number of pixels found in a photograph created using a very good 35mm camera or 40 times the number found in a high-end digital camera.

Assembly of the Suprime-Cam dewar, with first CCD mounted.

Suprime-Cam dewar with eight CCDs mounted, ready for use.

Following Subaru Telescope's First Light in December of 1998, Suprime-Cam was used at the Cassegrain focus of Subaru Telescope to allow an initial detailed testing of both the telescope and instrument. Subaru Telescope can bring light to a focus at any one of four locations. Because each of these foci uses a different combination of mirrors, it was necessary to conduct extensive testing to ensure optimum performance was achieved at each focus. The first to be commissioned was the Cassegrain focus, followed in turn by the Prime, IR Nasmyth and Optical Nasmyth focus. Although not optimized for use at the Cassegrain focus, Suprime-Cam produced some very exciting results during this period.

In July of 1999, Suprime-Cam was finally united with its seven-element, 20-inch diameter corrector lens, essential for use of the camera at Subaru's prime focus. This custom-made lens makes the final subtle adjustments to the converging light beam to ensure images are absolutely sharp all the way to the corners of the camera's large field of view. Instruments placed at the natural ("Prime") focus of Subaru Telescope's 8.3-meter diameter mirror see the sky at much lower magnification and therefore with a much wider field of view than if placed at any of the other foci on Subaru Telescope. From this position 50 feet above the primary mirror, Suprime-Cam can image an area 24 x 30 arcmin in a single exposure (about the size of the full moon), over 30 times the area covered when located at the Cassegrain focus. In prime-focus mode, the telescope is operating at an incredibly fast f/1.9!

Close-up view of the Suprime-Cam Corrector Lens.

Suprime-Cam with Corrector installed in Prime Focus Unit.

The tremendous increase in performance is nicely illustrated by the following figure. The top panel shows a portion of the nearby galaxy M31 (The Andromeda Galaxy) as viewed by Suprime-Cam mounted at the prime focus in August 1999. The blue rectangle indicates the field of view available when the camera was mounted at the cassegrain focus earlier. The lower panels show a region (marked in yellow) common to both the cassegrain-focus (left) and prime-focus (right) images greatly enlarged to show the finest details visible in both images. Note that the prime-focus view shows just as much detail as the cassegrain view, even though the total area imaged has increased by a factor of 30.

The Andromeda Galaxy (M31) imaged with Suprime-Cam:
(top) prime-focus, 14 min. exposure in green light, 24 arcmin sq. field of view; (bottom left) 1 arcmin region of cassegrain-focus image, 15 min. exposure in red light; (bottom right) 1 arcmin region of prime-focus image.

Although the CCDs in Suprime-Cam are sensitive to all colors of visible light, individual exposures made with the instrument are generally taken looking through glass filters to select specific colors (wavelengths) of light. For research purposes, often there is no need for exposures taken with more than just one or two filters. That is why many of the images you'll see showing astronomical discoveries are only available in "black and white". Images gathered for research are generally analyzed using computer software that can precisely measure and catalog details such as location, size, shape and total brightness for each of the multitude of objects found in each image. Studying these cataloged values, astronomers generally can "see" much more subtle and complex relationships that exist between the objects than could ever be found simply by looking at the images. For example, color information can be used to estimate the distance to remote galaxies (determining "photometric redshifts"), and very subtle distortions in the appearance of a large sample of galaxies can be used to map out the distribution of dark matter in the universe (evaluating the "cosmic shear"). We can also use the cataloged values to quickly sift through a multitude of objects to find just those which are unusual and warrant further study.

Suprime-Cam with R band filter seen through the open shutter.

Subaru Telescope is unique among the 6-10 meter class telescopes (operating in the northern hemisphere) in providing such a large field of view at such high angular resolution. This makes it an ideal tool for surveying large swaths of the distant universe, or looking for elusive primordial planetesmals orbiting at the very edge of our solar system.

Suprime-Cam was developed collectively by the University of Tokyo (Graduate School of Science, Institute for Cosmic Ray Research) and NAOJ.

Some members of the Suprime-Cam Instrument Team (clockwise from top left): F. Nakata, Y. Komiyama, Prof. M. Sekiguchi, K. Shimasaku, M. Kimura, S. Miyazaki, Prof. S. Okamura, M. Doi.

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