The Coma cluster of galaxies is located at a distance of 100 Mpc (or 326 million light years) which means that its distance modulus is equal to 35. Since the brightest globular clusters seen in the Milky Way and other nearby galaxies have absolute visual magnitudes in the range Mv = -10, the globular cluster systems that surround the giant elliptical galaxies in the Coma cluster become visible only when the limiting magnitude of images reaches fainter than V = 25. Prior to the launch of the Hubble Space Telescope, images of this depth were at the outer limit of what could be achieved from ground-based telescopes.
Globular clusters in the Coma cluster of galaxies were seen for the
first time in the images reproduced here. The wide field photograph,
displayed immediately below, gives a general orientation for this work.
This wide field image shows the inner core of the Coma cluster and the
two giant galaxies that reside there. Superimposed
on the Coma cluster are two boxes where very deep CCD images were obtained.
The stars designated A & B were used to guide the two images
with a high-speed tip-tilt system called ISIS. The box to the
lower left (labeled Field B) was purposely chosen as a "blank field"
with the expectation that it would show few if any images. The other box (Field A)
was chosen as the location where numerous the faint globular clusters should appear.
It is positioned in the halo of the giant galaxy NGC 4874.
Just as expected, the "blank field" image was relatively empty, but the other image -- from the halo of NGC 4874 -- begins to show the extremely rich family of globular clusters that surround NGC 4874. This image is a 30 minute exposure through a B filter. It was obtained with a 500x500 CCD camera at the Cassegrain focus of the 3.6-m Canada France Hawaii Telescope on Mauna Kea and was published in a paper entitled "High-Resolution Imaging from Mauna Kea: Globular Clusters in the Coma Cluster Galaxy NGC 4874" (L. Thompson and F. Valdes 1987 Astrophysical Journal Letters, vol. 315, page L35).