The redshift 0.4 supercluster MS0302+17 has been the subject of a deep multicolor imaging observation by University of Hawaii Institute for Astronomy researchers using the Canada-France-Hawaii Telescope on Mauna Kea. The CFH telescope has a 3.6m diameter mirror which gives it great light gathering power, and also has correcting optics which enable it to make sharp images over a very wide field of view. The images were captured by a large and highly sensitive electronic camera; the UH8K `mosaic' camera built by Gerard Luppino at U. Hawaii. This consists of eight of the largest CCD chips available laid out in a grid to cover most of the focal plane of the telescope. The field of view with this instrument is 1/2 degree on a side; roughly the size of the moon. This image was constructed from a set of about 17 exposures each of about 20 minutes duration, or about 6 hours in all. The images were taken through both a red filter and a blue filter, and with small positional offsets between exposures. This enables us to determine the colors of objects and construct this seamless color image.
The brightest objects visible in this image are foreground stars living in our galaxy. The great majority of the visible objects, however, are distant galaxies, and it is these galaxies that were the target of the observation.
The field contains three massive clusters of galaxies. The first of these to be discovered was found by Alan Dressler and Jim Gunn using a conventional optical telescope. This cluster, known as CL0303+1706, lies on the left (Eastern) edge of the field and is conspicuous as a concentration of reddish galaxies, all with essentially the same color. Galaxy clusters are also often detectable by satellite borne X-ray telescopes - the X-rays being emitted by extremely hot gas trapped in the gravitational potential wells of the clusters - and the Dressler-Gunn cluster was the target of an observation by the Einstein X-ray Observatory. This observation revealed two neighboring clusters, MS0302+1659 and MS0302+1717, which lie close to the lower (Southern) and upper (Northern) edges of the field. The X-ray observations were part of a survey, the `Einstein Medium Sensitivity Survey'. The MS in MS0302+17 stands for `Medium Sensitivity' and the numbers are the celestial coordinates of the object. An interesting feature of the lower cluster is the pair of `giant arcs' which can be seen near the bright central galaxies. These arcs are the images of a more distant background galaxy that is being gravitational lensed by the cluster.
By measuring the redshifting of spectral lines in the optical light one can determine the distance to the supercluster. This supercluster lies at redshift z = 0.42 which puts it at about 109 parsec, or about 3 billion light-years, distance from us. From the angular size of the supercluster (about 0.5 degrees on a side - or about the size of the moon) one can also tell how big it is; about 6,000,000 parsec across. In addition to the supercluster, the image also reveals a huge number of much fainter and smaller objects. These are also galaxies, but generally more distant than the supercluster. On this image we can detect about 40,000 such objects. For more information see http://www-nk.ifa.hawaii.edu/ ~ kaiser.